Ask the root the way every other widget is asked

The root had a layout path of its own: `root_layout` read its declared
lengths against the window, while every other widget's box came of
`Painter::widget_at`, where a rule of the widget's own -- a share with pixels
or a fraction beside it -- is compared against the offer and can take the box
past it. So a share on the root was the window whatever it asked for, which
`docs/LAYOUT_LOG.md` recorded as a gap rather than fixing, and any later rule
that reads the offer would have had to be written twice.

There is one box nobody drew, and that is the whole of what the root is
asked in. `Placing::WINDOW` says it -- the full output, fractions of the full
output, no move entry and no mask -- and `Placing::ask` is then the one place
a box is decided, called by the painter, by a local redraw, and by the root's
first draw. The root's own path is what is left of it: a widget with no
parent keeps different bookkeeping, not a different layout.

Measured in a 400 px window, a probe under each of three parents, which now
agree on every row where two of them agreed before:

    rule                      as root   wrapped   in a span
    leftover(1)                   400       400         400
    px(50) + leftover(1)          400       400         400
    px(500) + leftover(1)         500       500         500   (was 400 as root)
    rel(0.5) + leftover(1)        400       400         400
    rel(2.0) + leftover(1)        800       800         800   (was 400 as root)
    px(500)                       500       500         500
    rel(0.5)                      200       200         200

The comparison is kept on the widget asked about rather than on the asker,
which is what makes the root need nothing of its own: a window range means
the same thing at either end of an ask, `in_parent` passes one up unchanged,
and the asker ends up holding it through the child's drawing exactly as it
did when `longer_than` narrowed the asker directly. The root has no asker, so
its own record is the only place that range can live -- and `resize` already
checks that record, so a share crossing its length is caught with no new
code. `a_share_past_the_box_is_decided_again_on_either_side_of_the_crossing`
now runs at the root too: 500 at a 400 window, 900 at 900, 500 again at 400.

Two things this changes beyond the share. `DrawInfo::asked` is now the place
the parent offered rather than the place the ask came to, so a local redraw
re-decides the rule instead of re-reading the decision -- the two were the
same until a rule could move the box. And the root's `is_region_node` is
read, where the old path passed `false`: a region-node root now gets its
entry, whose translation is the identity, pinned by
`a_region_node_root_is_a_region_node`.

Format, clippy with and without layout-diagnostics, and the suite (136 + 19 +
13 + 4) are clean. The cold dump over 400 depth-5 trees is byte-identical to
`2dba90b` across all 34,571 boxes, and the three seed scans pass: 400 at
depth 5 (61s), 1,000 at depth 6 (155s), 2,000 at depth 4 (291s).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Cargo.toml

@@ -3,6 +3,9 @@ name = "iris"
 version.workspace = true
 edition.workspace = true
 
+[features]
+layout-diagnostics = ["iris-core/layout-diagnostics"]
+
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
@@ -20,7 +23,19 @@ tokio = { workspace = true, features = ["sync", "rt", "rt-multi-thread"] }
 tokio = { workspace = true, features = ["sync", "rt", "rt-multi-thread", "time"] }
 
 [workspace]
-members = ["core", "macro"]
+members = ["core", "macro", "rig-input"]
+
+# Full debug info was the bulk of what the linker wrote here and almost none of
+# what anything read. `dev` keeps line tables and scopes, which is what stepping
+# through an example wants; the tests keep the line tables alone, which is what
+# a backtrace reads. Measured when the tests became one target: relinking them
+# went from 9.8 s to 7.7 s with these, and target/ from 45 GB to 13 GB with the
+# two changes together.
+[profile.dev]
+debug = 1
+
+[profile.test]
+debug = "line-tables-only"
 
 [workspace.package]
 version = "0.1.0"
@@ -29,9 +44,9 @@ edition = "2024"
 [workspace.dependencies]
 pollster = "0.4.0"
 winit = "0.30.12"
-wgpu = "28.0.0"
+wgpu = "30.0.1"
 bytemuck = "1.23.1"
-image = "0.25.6"
+image = "0.25.10"
 parley = "0.11.1"
 swash = "0.2.10"
 fxhash = "0.2.1"
@@ -40,3 +55,5 @@ arboard = "3.6.1"
 iris-core = { path = "core" }
 iris-macro = { path = "macro" }
 tokio = "1.49.0"
+wayland-client = "0.31.15"
+wayland-protocols-wlr = { version = "0.3.12", features = ["client"] }

TODO

@@ -14,3 +14,27 @@ WidgetRef<W> or smth instead of Id
 vecs for each widget type?
 
 POTENTIAL BUG: closures that store IDs will not decrement the id!!! need to not increment id if moved into closure somehow??? wait no, need to decrement ID every time an event fn is added...... only if the id is used in it..??
+
+transforms on a move entry (scale + rotation)
+    an entry is a translation today; composing through one scales the rel
+    part and passes px through untouched, so fixed-size content and glyphs
+    do not follow a shortened entry
+    want a real transform per entry, resolved in resolve_move the way the
+    translation already is, so a whole subtree transforms with one buffer
+    write and no redraw
+    wanted for compose-style stretch at the end of a scroll area, and for
+    rotation generally
+
+a prepare stage on Event, so Data has no placeholder field
+    run_sensors builds one CursorData per widget and has to put something in
+    `sense` before anything knows which sense matched, so it writes
+    CursorSense::Hovering and says in place that it means nothing;
+    should_run then clones the whole thing to overwrite that one field
+    the state is representable only because the type lets the caller say it:
+    what the caller supplies and what matching adds are two different things
+    wearing one struct
+    the awkward part is doing it without the generics getting annoying --
+    Data<'a> is already a GAT with a default, and splitting it in two adds
+    another associated type to every Event impl for the sake of one field
+    (Bryan, 2026-09-20; low priority, he wants a good answer rather than a
+    quick one)

core/Cargo.toml

@@ -3,6 +3,9 @@ name = "iris-core"
 version.workspace = true
 edition.workspace = true
 
+[features]
+layout-diagnostics = []
+
 [dependencies]
 wgpu = { workspace = true }
 bytemuck ={ workspace = true }

core/src/event/manager.rs

@@ -79,6 +79,7 @@ type EventData<Rsc, E> = (E, Rc<dyn for<'a> EventFn<Rsc, <E as Event>::Data<'a>>
 pub struct TypeEventManager<Rsc: HasEvents, E: Event> {
     // TODO: reduce visiblity!!
     pub active: HashMap<LayerId, HashMap<WidgetId, E::State>>,
+    pub global: E::Global,
     map: HashMap<WidgetId, Vec<EventData<Rsc, E>>>,
 }
 
@@ -107,6 +108,7 @@ impl<Rsc: HasEvents, E: Event> Default for TypeEventManager<Rsc, E> {
     fn default() -> Self {
         Self {
             active: Default::default(),
+            global: Default::default(),
             map: Default::default(),
         }
     }
@@ -138,11 +140,13 @@ impl<Rsc: HasEvents + 'static, E: Event> TypeEventManager<Rsc, E> {
     pub fn run_fn<'a>(
         &mut self,
         id: impl IdLike,
-    ) -> impl for<'b> FnOnce(EventCtx<'_, Rsc, E::Data<'b>>, &mut Rsc) + 'a {
+    ) -> impl for<'b> FnOnce(EventCtx<'_, Rsc, E::Data<'b>>, &mut Rsc) -> bool + 'a {
         let fs = self.map.get(&id.id()).cloned().unwrap_or_default();
         move |ctx, rsc| {
+            let mut consumed = false;
             for (e, f) in fs {
                 if let Some(data) = e.should_run(&ctx.data) {
+                    consumed |= e.consumes(&data);
                     f(
                         EventCtx {
                             state: ctx.state,
@@ -152,6 +156,7 @@ impl<Rsc: HasEvents + 'static, E: Event> TypeEventManager<Rsc, E> {
                     )
                 }
             }
+            consumed
         }
     }
 }

core/src/event/mod.rs

@@ -9,10 +9,19 @@ pub use rsc::*;
 pub trait Event: Sized + 'static + Clone {
     type Data<'a>: Clone = ();
     type State: Default = ();
+    /// State the whole event type keeps, rather than one copy per widget.
+    type Global: Default = ();
     #[allow(unused_variables)]
     fn should_run<'a>(&self, data: &Self::Data<'a>) -> Option<Self::Data<'a>> {
         Some(data.clone())
     }
+
+    /// Whether having run on this data uses up whatever triggered it, so
+    /// nothing further should see it.
+    #[allow(unused_variables)]
+    fn consumes(&self, data: &Self::Data<'_>) -> bool {
+        false
+    }
 }
 
 pub trait EventLike {

core/src/event/rsc.rs

@@ -21,12 +21,13 @@ pub trait HasEvents: Sized + UiRsc + HasState {
 }
 
 pub trait RunEvents: HasEvents {
+    /// Whether anything that ran used up what triggered it.
     fn run_event<E: EventLike>(
         &mut self,
         id: impl IdLike,
         data: <E::Event as Event>::Data<'_>,
         state: &mut Self::State,
-    ) {
+    ) -> bool {
         let f = self.events_mut().get_type::<E>().run_fn(id);
         f(EventCtx { state, data }, self)
     }

core/src/fixed.rs

@@ -0,0 +1,547 @@
+use crate::{UiNum, util::Vec2};
+use std::{
+    fmt::{Debug, Display, Formatter},
+    ops::{Add, AddAssign, Div, Mul, Neg, Sub, SubAssign},
+};
+
+/// A number held as a whole count of `1 / 2^SHIFT`.
+///
+/// Layout reaches one place by more than one route -- a box composed down the
+/// chain, and the same box summed from what its children asked for -- and has
+/// to decide whether the two are the same place. In floats they land a few
+/// bits apart, which is a defect wherever the answer changes what is drawn
+/// rather than where. Here adding and subtracting are exact, a multiply
+/// drops to the step below, and a conversion between grids takes the nearest
+/// one, so two routes to one place land on one number and everything
+/// downstream compares for equality instead of for nearness.
+///
+/// `SHIFT` is the number of fractional bits, which is what makes the steps
+/// divide a whole number: a power of two also converts to `f32` without
+/// rounding while the value fits in its mantissa.
+///
+/// Arithmetic wraps at the ends of the range, the way the `i32` underneath
+/// does. Saturating instead was measured at a twelfth of layout's
+/// instructions -- five per add against one -- to keep the ordering of
+/// coordinates two million pixels out, where nothing draws anyway. A value
+/// off the end is a defect either way; wrapping makes it an obvious one.
+/// Only [`Self::from_f32`] clamps, since a float has further to come from.
+#[repr(transparent)]
+#[derive(
+    Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Default, bytemuck::Pod, bytemuck::Zeroable,
+)]
+pub struct Fixed<const SHIFT: u32>(i32);
+
+/// A length or a coordinate in pixels, in steps of `1/1024`. Finer than
+/// anything a display can show, and exact in `f32` up to 16,384 px, which is
+/// what lets the same number reach the GPU.
+pub type Px = Fixed<PX_SHIFT>;
+
+/// How many bits of a pixel a [`Px`] keeps. One place, because [`PxVec2`]
+/// and the shader's own decoding are the same grid or nothing lines up.
+pub const PX_SHIFT: u32 = 10;
+
+/// A share of what a box has left over, which is a weight beside its
+/// siblings rather than a fraction of anything: a list divides its room by
+/// the total of these, so the range has to hold a whole list's worth and the
+/// precision only has to tell two weights apart.
+pub type Weight = Fixed<16>;
+
+/// A fraction of a box. Twenty-four bits of it, which matches `f32` around a
+/// half and beats it above one -- where anchors actually sit -- and leaves
+/// +/-128 of range, enough to sum a hundred children each asking for a whole
+/// box. A `leftover` weight is not one of these: it is a share of what is
+/// left rather than a fraction of anything, and it sums over a whole list.
+pub type Rel = Fixed<REL_SHIFT>;
+
+/// How many bits of a box a [`Rel`] keeps, beside [`PX_SHIFT`] and for the
+/// same reason.
+pub const REL_SHIFT: u32 = 24;
+
+impl<const SHIFT: u32> Fixed<SHIFT> {
+    pub const ZERO: Self = Self(0);
+    pub const ONE: Self = Self::one();
+    /// The gap between neighbouring values, which is also how far apart two
+    /// numbers can be and still mean the same place.
+    pub const STEP: Self = Self(1);
+    /// Also what stands in for an unbounded end: compared against, never
+    /// added to, since arithmetic wraps past it.
+    pub const MIN: Self = Self(i32::MIN);
+    pub const MAX: Self = Self(i32::MAX);
+
+    const fn one() -> Self {
+        assert!(SHIFT < 31, "a Fixed needs a bit for the whole part");
+        Self(1 << SHIFT)
+    }
+
+    pub const fn from_raw(raw: i32) -> Self {
+        Self(raw)
+    }
+
+    /// The count of steps, for a caller that needs the representation rather
+    /// than the number.
+    pub const fn raw(self) -> i32 {
+        self.0
+    }
+
+    pub const fn from_int(v: i32) -> Self {
+        Self(v.wrapping_mul(Self::one().0))
+    }
+
+    /// Rounds to the nearest step, and clamps to the ends of the grid rather
+    /// than wrapping: this is where a number from outside arrives, and a float
+    /// has the range to be anywhere. A NaN has no nearest step and becomes
+    /// zero, which is a caller's mistake rather than a value worth carrying.
+    ///
+    /// Half-away is written out rather than called through `f32::round`,
+    /// which is not `const`: a layout constant has to stay a constant.
+    pub const fn from_f32(v: f32) -> Self {
+        debug_assert!(!v.is_nan(), "a NaN has no place on the grid");
+        let scaled = v * Self::one().0 as f32;
+        // Above 2^23 an `f32` has no fractional part left to round, and
+        // adding a half there rounds the number itself up instead. The cast
+        // saturates at both ends and sends NaN to zero, which is the
+        // behaviour wanted at both.
+        const WHOLE: f32 = (1 << 23) as f32;
+        Self(match (scaled >= WHOLE, scaled <= -WHOLE, scaled < 0.0) {
+            (true, _, _) | (_, true, _) => scaled as i32,
+            (_, _, true) => (scaled - 0.5) as i32,
+            _ => (scaled + 0.5) as i32,
+        })
+    }
+
+    /// The first step at or above `v`, where [`Self::from_f32`] takes the
+    /// nearest one and is below it half the time. For a bound that has to
+    /// admit the value it came from: a measurement rounded down is a bound
+    /// that leaves out the thing it was measured from.
+    pub const fn ceil_from_f32(v: f32) -> Self {
+        let nearest = Self::from_f32(v);
+        match nearest.to_f32() < v {
+            true => nearest.next_up(),
+            false => nearest,
+        }
+    }
+
+    /// From a number as it is written in source -- `16`, `1.5` -- which is
+    /// the other place a value enters the grid.
+    pub fn from_num(v: impl UiNum) -> Self {
+        Self::from_f32(v.to_f32())
+    }
+
+    pub const fn to_f32(self) -> f32 {
+        self.0 as f32 / Self::one().0 as f32
+    }
+
+    pub const fn add(self, rhs: Self) -> Self {
+        Self(self.0.wrapping_add(rhs.0))
+    }
+
+    pub const fn sub(self, rhs: Self) -> Self {
+        Self(self.0.wrapping_sub(rhs.0))
+    }
+
+    pub const fn neg(self) -> Self {
+        Self(self.0.wrapping_neg())
+    }
+
+    /// Scaled by a number on any grid, which is how a length takes a fraction
+    /// of itself and keeps being a length: the product is measured in the
+    /// receiver's steps.
+    ///
+    /// Dropped to the step below rather than taken to the nearest one
+    /// (Bryan, 2026-09-16), which costs a share a thousandth of a pixel of
+    /// its row -- less than an even number of pixels draws. Toward negative
+    /// infinity on both sides of zero, since that is a shift and nothing
+    /// else: a value and its negation therefore land different distances
+    /// from where they came, so a flipped span can sit a step from its
+    /// mirror image.
+    pub const fn mul<const BY: u32>(self, by: Fixed<BY>) -> Self {
+        Self(((self.0 as i64 * by.0 as i64) >> BY) as i32)
+    }
+
+    /// Repeated a whole number of times, which no grid rounds.
+    pub const fn mul_int(self, by: i32) -> Self {
+        Self(self.0.wrapping_mul(by))
+    }
+
+    /// Divided into a whole number of parts, rounded to the nearest step.
+    pub const fn div_int(self, by: i32) -> Self {
+        debug_assert!(by != 0, "no part of nothing");
+        if by == 0 {
+            return Self::ZERO;
+        }
+        Self(div_round(self.0 as i64, by as i64) as i32)
+    }
+
+    /// Divided by a number on any grid. A zero divisor is a caller bug -- a
+    /// box of no length has no fraction of itself -- and answers with the end
+    /// of the range so that a release build lays out something absurd rather
+    /// than dying.
+    pub const fn div<const BY: u32>(self, by: Fixed<BY>) -> Self {
+        debug_assert!(by.0 != 0, "dividing by a length of zero");
+        if by.0 == 0 {
+            return match self.0 < 0 {
+                true => Self::MIN,
+                false => Self::MAX,
+            };
+        }
+        Self(div_round((self.0 as i64) << BY, by.0 as i64) as i32)
+    }
+
+    /// `num / den` on *this* grid rather than on theirs, for weights coarser
+    /// than the share they divide.
+    pub const fn ratio<const OF: u32>(num: Fixed<OF>, den: Fixed<OF>) -> Self {
+        debug_assert!(den.0 != 0, "no part of a whole of nothing");
+        if den.0 == 0 {
+            return Self::ZERO;
+        }
+        Self(div_round((num.0 as i64) << SHIFT, den.0 as i64) as i32)
+    }
+
+    /// `from` and `to` a fraction of the way apart, the fraction being the
+    /// receiver -- the argument order [`crate::util::LerpUtil`] already uses.
+    pub const fn lerp<const OF: u32>(self, from: Fixed<OF>, to: Fixed<OF>) -> Fixed<OF> {
+        from.add(to.sub(from).mul(self))
+    }
+
+    pub const fn min(self, other: Self) -> Self {
+        match self.0 < other.0 {
+            true => self,
+            false => other,
+        }
+    }
+
+    pub const fn max(self, other: Self) -> Self {
+        match self.0 > other.0 {
+            true => self,
+            false => other,
+        }
+    }
+
+    pub const fn abs(self) -> Self {
+        Self(self.0.wrapping_abs())
+    }
+
+    pub const fn clamp(self, lo: Self, hi: Self) -> Self {
+        debug_assert!(lo.0 <= hi.0, "an empty clamp has no answer");
+        self.max(lo).min(hi)
+    }
+
+    /// The next value along, for an interval that must not admit its own
+    /// boundary. The step is the whole gap, so there is nothing to exclude
+    /// between this and the boundary itself.
+    pub const fn next_up(self) -> Self {
+        Self(self.0.wrapping_add(1))
+    }
+
+    pub const fn next_down(self) -> Self {
+        Self(self.0.wrapping_sub(1))
+    }
+}
+
+const fn div_round(num: i64, den: i64) -> i64 {
+    let (q, rem) = (num / den, num % den);
+    match rem.unsigned_abs() * 2 >= den.unsigned_abs() {
+        true => match (num < 0) == (den < 0) {
+            true => q + 1,
+            false => q - 1,
+        },
+        false => q,
+    }
+}
+
+/// Toward positive infinity when `up`, toward negative infinity otherwise.
+pub(crate) const fn div_toward(num: i64, den: i64, up: bool) -> i64 {
+    let (q, rem) = (num / den, num % den);
+    if rem == 0 {
+        return q;
+    }
+    match (rem < 0) == (den < 0) {
+        true => q + up as i64,
+        false => q - !up as i64,
+    }
+}
+
+/// Clamped to the ends, unlike a [`Fixed`]'s own arithmetic: a range of box
+/// lengths that runs past `i32` really is unbounded.
+pub(crate) const fn narrow(v: i64) -> i32 {
+    if v > i32::MAX as i64 {
+        return i32::MAX;
+    }
+    if v < i32::MIN as i64 {
+        return i32::MIN;
+    }
+    v as i32
+}
+
+const impl<const SHIFT: u32> Add for Fixed<SHIFT> {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        Fixed::add(self, rhs)
+    }
+}
+
+const impl<const SHIFT: u32> Sub for Fixed<SHIFT> {
+    type Output = Self;
+
+    fn sub(self, rhs: Self) -> Self {
+        Fixed::sub(self, rhs)
+    }
+}
+
+const impl<const SHIFT: u32> Neg for Fixed<SHIFT> {
+    type Output = Self;
+
+    fn neg(self) -> Self {
+        Fixed::neg(self)
+    }
+}
+
+const impl<const SHIFT: u32> AddAssign for Fixed<SHIFT> {
+    fn add_assign(&mut self, rhs: Self) {
+        *self = Fixed::add(*self, rhs);
+    }
+}
+
+const impl<const SHIFT: u32> SubAssign for Fixed<SHIFT> {
+    fn sub_assign(&mut self, rhs: Self) {
+        *self = Fixed::sub(*self, rhs);
+    }
+}
+
+const impl<const SHIFT: u32, const BY: u32> Mul<Fixed<BY>> for Fixed<SHIFT> {
+    type Output = Self;
+
+    fn mul(self, rhs: Fixed<BY>) -> Self {
+        Fixed::mul(self, rhs)
+    }
+}
+
+const impl<const SHIFT: u32, const BY: u32> Div<Fixed<BY>> for Fixed<SHIFT> {
+    type Output = Self;
+
+    fn div(self, rhs: Fixed<BY>) -> Self {
+        Fixed::div(self, rhs)
+    }
+}
+
+impl<const SHIFT: u32> Display for Fixed<SHIFT> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        Display::fmt(&self.to_f32(), f)
+    }
+}
+
+/// Prints the number rather than the count of steps: a failing layout test
+/// reports boxes, and `1126` is not a height anybody can read.
+impl<const SHIFT: u32> Debug for Fixed<SHIFT> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        Display::fmt(&self.to_f32(), f)
+    }
+}
+
+/// Two of them, for the places a size or a position needs both axes: a
+/// window, a box in pixels, a pointer. Held apart from [`crate::util::Vec2`]
+/// because that one is what the GPU and the platform speak.
+#[repr(C)]
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Default)]
+pub struct FixedVec2<const SHIFT: u32> {
+    pub x: Fixed<SHIFT>,
+    pub y: Fixed<SHIFT>,
+}
+
+pub type PxVec2 = FixedVec2<PX_SHIFT>;
+
+impl<const SHIFT: u32> FixedVec2<SHIFT> {
+    pub const ZERO: Self = Self::splat(Fixed::ZERO);
+
+    pub const fn new(x: Fixed<SHIFT>, y: Fixed<SHIFT>) -> Self {
+        Self { x, y }
+    }
+
+    pub const fn splat(v: Fixed<SHIFT>) -> Self {
+        Self { x: v, y: v }
+    }
+
+    pub fn from_f32(v: Vec2) -> Self {
+        Self::new(Fixed::from_f32(v.x), Fixed::from_f32(v.y))
+    }
+
+    /// The first step at or above each part, for a measurement reported as a
+    /// box: what it occupies is not less than what was measured.
+    pub fn ceil_from_f32(v: Vec2) -> Self {
+        Self::new(Fixed::ceil_from_f32(v.x), Fixed::ceil_from_f32(v.y))
+    }
+
+    pub fn to_f32(self) -> Vec2 {
+        Vec2::new(self.x.to_f32(), self.y.to_f32())
+    }
+
+    pub const fn div_int(self, by: i32) -> Self {
+        Self::new(self.x.div_int(by), self.y.div_int(by))
+    }
+
+    pub const fn min(self, other: Self) -> Self {
+        Self::new(self.x.min(other.x), self.y.min(other.y))
+    }
+
+    pub const fn max(self, other: Self) -> Self {
+        Self::new(self.x.max(other.x), self.y.max(other.y))
+    }
+}
+
+// `impl_op!` names one concrete type, and this one is generic.
+const impl<const SHIFT: u32> Add for FixedVec2<SHIFT> {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        Self::new(self.x.add(rhs.x), self.y.add(rhs.y))
+    }
+}
+
+const impl<const SHIFT: u32> Sub for FixedVec2<SHIFT> {
+    type Output = Self;
+
+    fn sub(self, rhs: Self) -> Self {
+        Self::new(self.x.sub(rhs.x), self.y.sub(rhs.y))
+    }
+}
+
+const impl<const SHIFT: u32> AddAssign for FixedVec2<SHIFT> {
+    fn add_assign(&mut self, rhs: Self) {
+        *self = Add::add(*self, rhs);
+    }
+}
+
+const impl<const SHIFT: u32> SubAssign for FixedVec2<SHIFT> {
+    fn sub_assign(&mut self, rhs: Self) {
+        *self = Sub::sub(*self, rhs);
+    }
+}
+
+impl<const SHIFT: u32> Debug for FixedVec2<SHIFT> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(f, "({}, {})", self.x, self.y)
+    }
+}
+
+impl<const SHIFT: u32> Display for FixedVec2<SHIFT> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(f, "({}, {})", self.x, self.y)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn a_sum_of_steps_does_not_drift() {
+        let mut at = Px::ZERO;
+        for _ in 0..20_000 {
+            at += Px::from_raw(3);
+        }
+        assert_eq!(at, Px::from_raw(60_000));
+        for _ in 0..20_000 {
+            at -= Px::from_raw(3);
+        }
+        assert_eq!(at, Px::ZERO);
+    }
+
+    #[test]
+    fn a_pixel_survives_the_trip_through_f32() {
+        for raw in [0, 1, -1, 64, -1000, 16_777_215, -16_777_215] {
+            let px = Px::from_raw(raw);
+            assert_eq!(Px::from_f32(px.to_f32()), px);
+        }
+    }
+
+    #[test]
+    fn a_fraction_of_a_length_is_a_length() {
+        let half = Px::from_int(100) * Rel::from_f32(0.5);
+        assert_eq!(half, Px::from_int(50));
+        assert_eq!(Px::from_int(100) * Rel::ONE, Px::from_int(100));
+        assert_eq!(Px::from_int(100) * Rel::ZERO, Px::ZERO);
+    }
+
+    /// Toward negative infinity on both sides of zero, which is what makes
+    /// it a shift rather than a shift and a sign branch -- and what makes a
+    /// value and its negation land different distances from where they came,
+    /// so a flipped span can sit a step from its mirror image.
+    #[test]
+    fn a_multiply_drops_to_the_step_below_on_both_sides_of_zero() {
+        // A step and a half of one, which has no step of its own.
+        let step_and_a_half = Rel::from_f32(1.5).div_int(Px::ONE.raw());
+        assert_eq!(Px::ONE * step_and_a_half, Px::from_raw(1));
+        assert_eq!(Px::ONE.neg() * step_and_a_half, Px::from_raw(-2));
+    }
+
+    /// A division rounds to the nearest step, so it cannot put back the
+    /// steps a truncating multiply dropped: a round trip comes back short,
+    /// never long, and by the few steps the two operations gave up.
+    #[test]
+    fn dividing_by_a_fraction_cannot_undo_a_truncating_multiply() {
+        let third = Rel::ONE / Rel::from_int(3);
+        let len = Px::from_int(300);
+        let back = len * third / third;
+        assert!(back <= len, "{back:?} is longer than {len:?}");
+        assert!(len - back <= Px::from_raw(3), "{back:?} against {len:?}");
+        assert_eq!(Px::from_int(100) / Rel::from_f32(0.5), Px::from_int(200));
+    }
+
+    /// The bound a greedy line break needs: the width it was measured at is
+    /// not on the grid, and the narrowest box the break still holds for is
+    /// the step at or above it, never the one below.
+    #[test]
+    fn a_ceiling_never_lands_below_the_number_it_came_from() {
+        let step = 1.0 / (1 << PX_SHIFT) as f32;
+        for n in 0..64 {
+            let v = 189.0 + n as f32 * step / 3.0;
+            let up = Px::ceil_from_f32(v);
+            assert!(up.to_f32() >= v, "{up:?} is below {v}");
+            assert!(
+                up.to_f32() - v < step,
+                "{up:?} is more than a step above {v}"
+            );
+        }
+        // An exact step is its own ceiling.
+        assert_eq!(Px::ceil_from_f32(189.5), Px::from_f32(189.5));
+    }
+
+    #[test]
+    fn a_number_from_outside_is_clamped_to_the_grid() {
+        assert_eq!(Px::from_f32(1e12), Px::MAX);
+        assert_eq!(Px::from_f32(-1e12), Px::MIN);
+    }
+
+    #[test]
+    fn lerp_takes_the_fraction_as_the_receiver() {
+        let (from, to) = (Px::from_int(10), Px::from_int(20));
+        assert_eq!(Rel::ZERO.lerp(from, to), from);
+        assert_eq!(Rel::ONE.lerp(from, to), to);
+        assert_eq!(Rel::from_f32(0.5).lerp(from, to), Px::from_int(15));
+        assert_eq!(Rel::from_f32(0.5).lerp(to, from), Px::from_int(15));
+    }
+
+    #[test]
+    fn a_ratio_is_finer_than_the_weights_it_divides() {
+        let (one, three) = (Weight::ONE, Weight::from_int(3));
+        // A third, which the weights' own grid could only hold to 1/65536.
+        assert_eq!(Rel::ratio(one, three), Rel::from_raw(5592405));
+        assert_eq!(Rel::ratio(three, three), Rel::ONE);
+        assert_eq!(Rel::ratio(Weight::ZERO, three), Rel::ZERO);
+    }
+
+    #[test]
+    fn nothing_sits_between_a_value_and_the_next_one() {
+        let at = Px::from_int(3);
+        assert_eq!(at.next_up().next_down(), at);
+        assert_eq!(at.next_up().raw() - at.raw(), 1);
+        assert!(at.next_down() < at && at < at.next_up());
+    }
+
+    #[test]
+    fn it_prints_the_number_rather_than_the_steps() {
+        assert_eq!(format!("{:?}", Px::from_f32(17.59375)), "17.59375");
+        assert_eq!(format!("{}", Px::from_int(-2)), "-2");
+    }
+}

core/src/layout_diagnostics.rs

@@ -0,0 +1,493 @@
+//! Opt-in counters and coarse timers for explaining CPU layout cost.
+//!
+//! Enable the `layout-diagnostics` feature. With it disabled, none of the
+//! instrumentation is compiled into Iris. The retained rig in
+//! `tests/layout_diagnostics.rs` is the ordinary entry point.
+//!
+//! Timers are inclusive: `update total` contains `full layout` or
+//! `incremental layout`, and `text render` contains shaping and glyph
+//! placement. They locate cost within one instrumented run and must not be
+//! added together. Use an uninstrumented build under `perf` for final CPU
+//! totals; counting every primitive and distinct widget deliberately perturbs
+//! the instrumented run.
+//!
+//! Call [`trace_widget`] before a frame to retain the ordered constraint,
+//! reuse, size, placement, and text events for one suspicious widget. The
+//! selection is a set and survives [`take`] until cleared.
+
+use crate::{Axis, LayoutHolds, LayoutLen, PxVec2, Size, UiRegion, UiVec2, WidgetId};
+use std::{
+    cell::RefCell,
+    collections::{HashMap, HashSet},
+    fmt::Write,
+    time::Instant,
+};
+
+/// Declares a counter or timer kind beside the name its report prints. Two
+/// lists in the same order was one list too many: a variant inserted without
+/// its label moving with it renames every total after it, and nothing says
+/// so.
+macro_rules! labelled {
+    ($(#[$meta:meta])* $vis:vis enum $Name:ident { $($variant:ident = $label:literal,)* }) => {
+        $(#[$meta])*
+        #[derive(Clone, Copy)]
+        $vis enum $Name { $($variant,)* }
+
+        impl $Name {
+            const COUNT: usize = [$($label,)*].len();
+            const NAMES: [&'static str; Self::COUNT] = [$($label,)*];
+        }
+    };
+}
+
+labelled! {
+    pub(crate) enum Counter {
+        Updates = "updates",
+        DrawRequests = "draw requests",
+        WidgetDraws = "widget draws",
+        RegionNodeDraws = "region-node draws",
+        SizeReads = "draw-result size reads",
+        HintHits = "hint hits",
+        HintMisses = "hint misses",
+        ReuseAttempts = "reuse attempts",
+        ReuseExact = "reuse exact",
+        ReuseMoved = "reuse moved",
+        ReuseDirty = "reuse: dirty",
+        ReuseUndrawn = "reuse: nothing drawn to keep",
+        ReuseWrongParent = "reuse: wrong parent",
+        ReuseRemapped = "reuse remapped",
+        ReuseOutside = "reuse: outside what it holds for",
+        ReuseWrongLayer = "reuse: another layer",
+        ReuseWrongNode = "reuse: region-node choice changed",
+        ReuseWrongMask = "reuse: a different inherited mask",
+        QueuePops = "redraw queue pops",
+        DepthReads = "depth reads",
+        LocalRedraws = "local redraws",
+        SizeChanges = "size changes",
+        ReaderEdges = "reader edges",
+        PrimitiveWrites = "primitive writes",
+        TextRenders = "text renders",
+        TextShapeHits = "text shape hits",
+        TextShapes = "text shapes",
+        TextBreaks = "text line breaks",
+        GlyphPlacements = "glyph placements",
+        OutsidePinnedLen = "reuse outside: the length it was pinned to",
+        OutsideWindow = "reuse outside: this window",
+        OutsideRelBase = "reuse outside: a rel base",
+        OutsideRegion = "reuse outside: a region length",
+    }
+}
+
+labelled! {
+    pub(crate) enum TimerKind {
+        Update = "update total",
+        FullLayout = "full layout",
+        IncrementalLayout = "incremental layout",
+        TextRender = "text render",
+        TextShape = "text shape",
+        TextBreak = "text line break",
+        GlyphPlacement = "glyph placement",
+    }
+}
+
+#[derive(Clone)]
+pub struct Report {
+    counters: [u64; Counter::COUNT],
+    nanos: [u64; TimerKind::COUNT],
+    distinct_widgets: usize,
+    distinct_text_widgets: usize,
+    hot_widgets: Vec<Callsite>,
+    hot_text: Vec<Callsite>,
+    traces: Vec<TraceEvent>,
+}
+
+impl Default for Report {
+    fn default() -> Self {
+        Self {
+            counters: [0; Counter::COUNT],
+            nanos: [0; TimerKind::COUNT],
+            distinct_widgets: 0,
+            distinct_text_widgets: 0,
+            hot_widgets: Vec::new(),
+            hot_text: Vec::new(),
+            traces: Vec::new(),
+        }
+    }
+}
+
+impl Report {
+    pub fn counters(&self) -> impl Iterator<Item = (&'static str, u64)> + '_ {
+        Counter::NAMES.into_iter().zip(self.counters)
+    }
+
+    /// Inclusive elapsed time accumulated for each targeted operation.
+    pub fn timings_ns(&self) -> impl Iterator<Item = (&'static str, u64)> + '_ {
+        TimerKind::NAMES.into_iter().zip(self.nanos)
+    }
+
+    pub fn distinct_widgets(&self) -> usize {
+        self.distinct_widgets
+    }
+
+    pub fn distinct_text_widgets(&self) -> usize {
+        self.distinct_text_widgets
+    }
+
+    pub fn hot_widgets(&self) -> &[Callsite] {
+        &self.hot_widgets
+    }
+
+    pub fn hot_text(&self) -> &[Callsite] {
+        &self.hot_text
+    }
+
+    /// Ordered layout events for widgets selected with [`trace_widget`].
+    pub fn traces(&self) -> &[TraceEvent] {
+        &self.traces
+    }
+
+    /// Formats nonzero totals divided by `frames`.
+    pub fn per_frame(&self, frames: usize) -> String {
+        let divisor = frames.max(1) as f64;
+        let mut out = String::new();
+        for (name, value) in self.counters() {
+            if value != 0 {
+                let _ = writeln!(out, "  {name:<27} {:>12.2}", value as f64 / divisor);
+            }
+        }
+        if self.distinct_widgets != 0 {
+            let _ = writeln!(
+                out,
+                "  {:<27} {:>12}",
+                "distinct widgets", self.distinct_widgets
+            );
+        }
+        if self.distinct_text_widgets != 0 {
+            let _ = writeln!(
+                out,
+                "  {:<27} {:>12}",
+                "distinct text widgets", self.distinct_text_widgets
+            );
+        }
+        for (name, nanos) in self.timings_ns() {
+            if nanos != 0 {
+                let ms = nanos as f64 / divisor / 1_000_000.0;
+                let _ = writeln!(out, "  {name:<27} {ms:>12.3} ms");
+            }
+        }
+        if !self.hot_widgets.is_empty() {
+            let _ = writeln!(out, "  hottest widget draws:");
+            for callsite in &self.hot_widgets {
+                let calls = callsite.calls as f64 / divisor;
+                let _ = writeln!(
+                    out,
+                    "    {calls:>9.2}  {:?} {}",
+                    callsite.id, callsite.label
+                );
+            }
+        }
+        if !self.hot_text.is_empty() {
+            let _ = writeln!(out, "  hottest text renders:");
+            for callsite in &self.hot_text {
+                let calls = callsite.calls as f64 / divisor;
+                let _ = writeln!(
+                    out,
+                    "    {calls:>9.2}  {:>3} widths  {:?} {}",
+                    callsite.distinct_widths, callsite.id, callsite.label
+                );
+            }
+        }
+        if !self.traces.is_empty() {
+            let _ = writeln!(out, "  targeted layout trace:");
+            for event in &self.traces {
+                let _ = writeln!(out, "    {event:?}");
+            }
+        }
+        out
+    }
+}
+
+#[derive(Clone)]
+pub struct Callsite {
+    pub id: WidgetId,
+    pub label: String,
+    pub calls: u64,
+    pub distinct_widths: usize,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum ReuseOutcome {
+    Exact,
+    Moved,
+    Dirty,
+    WrongParent,
+    WrongLayer,
+    WrongMask,
+    WrongNode,
+    Remapped,
+    Outside,
+    Undrawn,
+}
+
+/// One targeted layout event. Events are retained in execution order, making
+/// repeated constraint paths visible without logging every widget globally.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum TraceEvent {
+    DrawRequest {
+        id: WidgetId,
+        parent: Option<WidgetId>,
+        region: UiRegion,
+        region_px: PxVec2,
+        region_node: bool,
+    },
+    Reuse {
+        id: WidgetId,
+        outcome: ReuseOutcome,
+    },
+    SizeReported {
+        id: WidgetId,
+        size: Size,
+    },
+    RegionNode {
+        id: WidgetId,
+        parent: WidgetId,
+        region: UiRegion,
+    },
+    SizeRead {
+        id: WidgetId,
+        reader: WidgetId,
+        size: Size,
+    },
+    HintRead {
+        id: WidgetId,
+        reader: WidgetId,
+        axis: Axis,
+        hint: Option<LayoutLen>,
+    },
+    TextRendered {
+        id: WidgetId,
+        width: Option<f32>,
+    },
+}
+
+#[derive(Default)]
+struct Calls {
+    label: String,
+    count: u64,
+    widths: HashSet<Option<u32>>,
+}
+
+#[derive(Default)]
+struct Current {
+    report: Report,
+    widgets: HashMap<WidgetId, Calls>,
+    text_widgets: HashMap<WidgetId, Calls>,
+    traced: HashSet<WidgetId>,
+}
+
+thread_local! {
+    static CURRENT: RefCell<Current> = RefCell::new(Current::default());
+}
+
+pub(crate) fn bump(counter: Counter) {
+    CURRENT.with_borrow_mut(|current| current.report.counters[counter as usize] += 1);
+}
+
+pub(crate) fn draw_widget(id: WidgetId, label: &str) {
+    CURRENT.with_borrow_mut(|current| {
+        let calls = current.widgets.entry(id).or_default();
+        if calls.label.is_empty() {
+            calls.label = label.to_owned();
+        }
+        calls.count += 1;
+    });
+}
+
+/// Adds a widget to the targeted trace set. Selection survives [`take`]
+/// until explicitly removed or cleared.
+pub fn trace_widget(id: impl Into<WidgetId>) {
+    CURRENT.with_borrow_mut(|current| {
+        current.traced.insert(id.into());
+    });
+}
+
+pub fn untrace_widget(id: impl Into<WidgetId>) {
+    CURRENT.with_borrow_mut(|current| {
+        current.traced.remove(&id.into());
+    });
+}
+
+pub fn clear_traced_widgets() {
+    CURRENT.with_borrow_mut(|current| current.traced.clear());
+}
+
+fn trace(id: WidgetId, event: TraceEvent) {
+    CURRENT.with_borrow_mut(|current| {
+        if current.traced.contains(&id) {
+            current.report.traces.push(event);
+        }
+    });
+}
+
+pub(crate) fn draw_request(
+    id: WidgetId,
+    parent: Option<WidgetId>,
+    region: UiRegion,
+    region_px: PxVec2,
+    region_node: bool,
+) {
+    trace(
+        id,
+        TraceEvent::DrawRequest {
+            id,
+            parent,
+            region,
+            region_px,
+            region_node,
+        },
+    );
+}
+
+pub(crate) fn reuse(id: WidgetId, outcome: ReuseOutcome) {
+    trace(id, TraceEvent::Reuse { id, outcome });
+}
+
+/// A drawing that cannot be reused because the box on offer is outside what
+/// it holds for, and which of the four contracts said so. They overlap: a
+/// drawing can be outside two of them at once, and counting each is what
+/// says where a rel base redrawing more than it should is coming from.
+pub(crate) fn outside(
+    id: WidgetId,
+    holds: LayoutHolds,
+    region: UiRegion,
+    rel_base: UiVec2,
+    window: PxVec2,
+) {
+    for axis in Axis::BOTH {
+        let holds = holds[axis];
+        let len = region[axis].len();
+        let window = window[axis];
+        if holds.region_len.is_some_and(|pinned| pinned != len) {
+            bump(Counter::OutsidePinnedLen);
+        }
+        if !holds.window.contains(window) {
+            bump(Counter::OutsideWindow);
+        }
+        if holds
+            .rel_base
+            .is_some_and(|pinned| pinned != rel_base[axis])
+        {
+            bump(Counter::OutsideRelBase);
+        }
+        if !holds.region.contains(len.to_px(window)) {
+            bump(Counter::OutsideRegion);
+        }
+    }
+    bump(Counter::ReuseOutside);
+    reuse(id, ReuseOutcome::Outside);
+}
+
+pub(crate) fn size_reported(id: WidgetId, size: Size) {
+    trace(id, TraceEvent::SizeReported { id, size });
+}
+
+pub(crate) fn region_node(id: WidgetId, parent: WidgetId, region: UiRegion) {
+    trace(id, TraceEvent::RegionNode { id, parent, region });
+}
+
+pub(crate) fn size_read(id: WidgetId, reader: WidgetId, size: Size) {
+    trace(id, TraceEvent::SizeRead { id, reader, size });
+}
+
+pub(crate) fn hint_read(id: WidgetId, reader: WidgetId, axis: Axis, hint: Option<LayoutLen>) {
+    trace(
+        id,
+        TraceEvent::HintRead {
+            id,
+            reader,
+            axis,
+            hint,
+        },
+    );
+}
+
+pub(crate) fn render_text(id: WidgetId, label: &str, width: Option<f32>) {
+    CURRENT.with_borrow_mut(|current| {
+        let calls = current.text_widgets.entry(id).or_default();
+        if calls.label.is_empty() {
+            calls.label = label.to_owned();
+        }
+        calls.count += 1;
+        calls.widths.insert(width.map(f32::to_bits));
+        if current.traced.contains(&id) {
+            current
+                .report
+                .traces
+                .push(TraceEvent::TextRendered { id, width });
+        }
+    });
+}
+
+pub(crate) struct Timer {
+    kind: TimerKind,
+    start: Instant,
+}
+
+pub(crate) fn timer(kind: TimerKind) -> Timer {
+    Timer {
+        kind,
+        start: Instant::now(),
+    }
+}
+
+impl Drop for Timer {
+    fn drop(&mut self) {
+        let nanos = self.start.elapsed().as_nanos().min(u64::MAX as u128) as u64;
+        CURRENT.with_borrow_mut(|current| current.report.nanos[self.kind as usize] += nanos);
+    }
+}
+
+/// Takes all diagnostics accumulated on this thread and resets them.
+pub fn take() -> Report {
+    CURRENT.with_borrow_mut(|current| {
+        current.report.distinct_widgets = current.widgets.len();
+        current.report.distinct_text_widgets = current.text_widgets.len();
+        current.report.hot_widgets = hottest(&current.widgets);
+        current.report.hot_text = hottest(&current.text_widgets);
+        let report = std::mem::take(&mut current.report);
+        current.widgets.clear();
+        current.text_widgets.clear();
+        report
+    })
+}
+
+fn hottest(calls: &HashMap<WidgetId, Calls>) -> Vec<Callsite> {
+    let mut calls: Vec<_> = calls
+        .iter()
+        .map(|(&id, calls)| Callsite {
+            id,
+            label: calls.label.clone(),
+            calls: calls.count,
+            distinct_widths: calls.widths.len(),
+        })
+        .collect();
+    calls.sort_by(|a, b| b.calls.cmp(&a.calls).then_with(|| a.label.cmp(&b.label)));
+    calls.truncate(8);
+    calls
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn taking_a_report_resets_its_counters() {
+        let _ = take();
+        bump(Counter::Updates);
+        bump(Counter::Updates);
+
+        let report = take();
+        assert_eq!(report.counters().next(), Some(("updates", 2)));
+        assert!(take().counters().all(|(_, count)| count == 0));
+    }
+}

core/src/lib.rs

@@ -9,9 +9,14 @@
 #![feature(unsize)]
 #![feature(coerce_unsized)]
 #![feature(option_into_flat_iter)]
+#![feature(const_index)]
+
+#[cfg(feature = "layout-diagnostics")]
+pub mod layout_diagnostics;
 
 mod attr;
 mod event;
+mod fixed;
 mod num;
 mod orientation;
 mod primitive;
@@ -23,6 +28,7 @@ pub mod util;
 
 pub use attr::*;
 pub use event::*;
+pub use fixed::*;
 pub use num::*;
 pub use orientation::*;
 pub use primitive::*;

core/src/orientation/align.rs

@@ -1,8 +1,9 @@
-use crate::vec2;
+use crate::util::impl_axis_index;
+use crate::{Px, Rel};
 
 use super::*;
 
-#[derive(Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Align {
     pub x: Option<AxisAlign>,
     pub y: Option<AxisAlign>,
@@ -30,20 +31,32 @@ impl Align {
     }
 }
 
-#[derive(Clone, Copy, PartialEq, Eq)]
+/// Where a widget sits in a box longer than it is. The default is the middle,
-pub enum AxisAlign {
+/// because the two edges are the ones that assume a direction: which of them
-    Neg,
+/// is the near one depends on the writing system and on which way a container
-    Center,
+/// runs, and the middle is the same either way.
-    Pos,
+#[derive(Debug, Clone, Copy, PartialEq)]
-}
+pub struct AxisAlign(Rel);
 
 impl AxisAlign {
-    pub const fn rel(&self) -> f32 {
+    pub const NEG: Self = Self::new(0.0);
-        match self {
+    pub const CENTER: Self = Self::new(0.5);
-            Self::Neg => 0.0,
+    pub const POS: Self = Self::new(1.0);
-            Self::Center => 0.5,
+
-            Self::Pos => 1.0,
+    pub const fn new(rel: f32) -> Self {
+        Self(Rel::from_f32(rel))
     }
+
+    /// A fraction of the room left over, which is what the layout reads: the
+    /// three constants are the familiar places along it, not the only ones.
+    pub const fn rel(&self) -> Rel {
+        self.0
+    }
+}
+
+impl Default for AxisAlign {
+    fn default() -> Self {
+        Self::CENTER
     }
 }
 
@@ -53,41 +66,38 @@ pub struct CardinalAlign {
 }
 
 impl CardinalAlign {
-    pub const LEFT: Self = Self::new(Axis::X, AxisAlign::Neg);
+    pub const LEFT: Self = Self::new(Axis::X, AxisAlign::NEG);
-    pub const H_CENTER: Self = Self::new(Axis::X, AxisAlign::Center);
+    pub const H_CENTER: Self = Self::new(Axis::X, AxisAlign::CENTER);
-    pub const RIGHT: Self = Self::new(Axis::X, AxisAlign::Pos);
+    pub const RIGHT: Self = Self::new(Axis::X, AxisAlign::POS);
-    pub const TOP: Self = Self::new(Axis::Y, AxisAlign::Neg);
+    pub const TOP: Self = Self::new(Axis::Y, AxisAlign::NEG);
-    pub const V_CENTER: Self = Self::new(Axis::Y, AxisAlign::Center);
+    pub const V_CENTER: Self = Self::new(Axis::Y, AxisAlign::CENTER);
-    pub const BOT: Self = Self::new(Axis::Y, AxisAlign::Pos);
+    pub const BOT: Self = Self::new(Axis::Y, AxisAlign::POS);
 
     pub const fn new(axis: Axis, align: AxisAlign) -> Self {
         Self { axis, align }
     }
 }
 
-#[derive(Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
 pub struct RegionAlign {
     pub x: AxisAlign,
     pub y: AxisAlign,
 }
 
 impl RegionAlign {
-    pub const TOP_LEFT: Self = Self::new(AxisAlign::Neg, AxisAlign::Neg);
+    pub const TOP_LEFT: Self = Self::new(AxisAlign::NEG, AxisAlign::NEG);
-    pub const TOP_CENTER: Self = Self::new(AxisAlign::Center, AxisAlign::Neg);
+    pub const TOP_CENTER: Self = Self::new(AxisAlign::CENTER, AxisAlign::NEG);
-    pub const TOP_RIGHT: Self = Self::new(AxisAlign::Pos, AxisAlign::Neg);
+    pub const TOP_RIGHT: Self = Self::new(AxisAlign::POS, AxisAlign::NEG);
-    pub const CENTER_LEFT: Self = Self::new(AxisAlign::Neg, AxisAlign::Center);
+    pub const CENTER_LEFT: Self = Self::new(AxisAlign::NEG, AxisAlign::CENTER);
-    pub const CENTER: Self = Self::new(AxisAlign::Center, AxisAlign::Center);
+    pub const CENTER: Self = Self::new(AxisAlign::CENTER, AxisAlign::CENTER);
-    pub const CENTER_RIGHT: Self = Self::new(AxisAlign::Pos, AxisAlign::Center);
+    pub const CENTER_RIGHT: Self = Self::new(AxisAlign::POS, AxisAlign::CENTER);
-    pub const BOT_LEFT: Self = Self::new(AxisAlign::Neg, AxisAlign::Pos);
+    pub const BOT_LEFT: Self = Self::new(AxisAlign::NEG, AxisAlign::POS);
-    pub const BOT_CENTER: Self = Self::new(AxisAlign::Center, AxisAlign::Pos);
+    pub const BOT_CENTER: Self = Self::new(AxisAlign::CENTER, AxisAlign::POS);
-    pub const BOT_RIGHT: Self = Self::new(AxisAlign::Pos, AxisAlign::Pos);
+    pub const BOT_RIGHT: Self = Self::new(AxisAlign::POS, AxisAlign::POS);
 
     pub const fn new(x: AxisAlign, y: AxisAlign) -> Self {
         Self { x, y }
     }
-    pub const fn rel(&self) -> Vec2 {
-        vec2(self.x.rel(), self.y.rel())
-    }
 }
 
 impl UiVec2 {
@@ -140,16 +150,17 @@ impl Vec2 {
     }
 }
 
-impl UiScalar {
+impl Len {
+    /// This length placed in the box it is measured in: the alignment names a
+    /// point along that box, and the two ends are that point less the part of
+    /// the length falling before it and plus the part falling after.
     pub const fn align(&self, align: AxisAlign) -> UiSpan {
         let rel = align.rel();
-        let mut start = UiScalar::rel(rel);
+        let at = Len::from_parts(rel, Px::ZERO);
-        start.abs -= self.abs * rel;
+        UiSpan {
-        start.rel -= self.rel * rel;
+            start: at - self.scale(rel),
-        let mut end = UiScalar::rel(rel);
+            end: at + self.scale(Rel::ONE.sub(rel)),
-        end.abs += self.abs * (1.0 - rel);
+        }
-        end.rel += self.rel * (1.0 - rel);
-        UiSpan { start, end }
     }
 }
 
@@ -165,8 +176,8 @@ impl From<RegionAlign> for Align {
 impl From<Align> for RegionAlign {
     fn from(align: Align) -> Self {
         Self {
-            x: align.x.unwrap_or(AxisAlign::Center),
+            x: align.x.unwrap_or(AxisAlign::CENTER),
-            y: align.y.unwrap_or(AxisAlign::Center),
+            y: align.y.unwrap_or(AxisAlign::CENTER),
         }
     }
 }
@@ -189,7 +200,10 @@ impl From<CardinalAlign> for Align {
 
 const impl From<RegionAlign> for UiVec2 {
     fn from(align: RegionAlign) -> Self {
-        Self::rel(align.rel())
+        Self::new(
+            Len::from_parts(align.x.rel(), Px::ZERO),
+            Len::from_parts(align.y.rel(), Px::ZERO),
+        )
     }
 }
 
@@ -198,3 +212,6 @@ impl RegionAlign {
         UiVec2::from(self)
     }
 }
+
+impl_axis_index!(RegionAlign => AxisAlign);
+impl_axis_index!(Align => Option<AxisAlign>);

core/src/orientation/axis.rs

@@ -1,11 +1,18 @@
 use super::*;
+use crate::util::impl_axis_index;
+use crate::{Fixed, FixedVec2};
 
-#[derive(Copy, Clone, Eq, PartialEq)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub enum Axis {
     X,
     Y,
 }
 
+impl Axis {
+    /// Both of them, for the layout code that asks the same question of each.
+    pub const BOTH: [Self; 2] = [Self::X, Self::Y];
+}
+
 impl std::ops::Not for Axis {
     type Output = Self;
 
@@ -40,21 +47,16 @@ pub enum Sign {
     Pos,
 }
 
+impl<const SHIFT: u32> FixedVec2<SHIFT> {
+    pub const fn from_axis(axis: Axis, aligned: Fixed<SHIFT>, ortho: Fixed<SHIFT>) -> Self {
+        match axis {
+            Axis::X => Self::new(aligned, ortho),
+            Axis::Y => Self::new(ortho, aligned),
+        }
+    }
+}
+
 impl Vec2 {
-    pub fn axis(&self, axis: Axis) -> f32 {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut f32 {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-
     pub const fn from_axis(axis: Axis, aligned: f32, ortho: f32) -> Self {
         Self {
             x: match axis {
@@ -69,47 +71,5 @@ impl Vec2 {
     }
 }
 
-pub const trait AxisT {
+impl_axis_index!({const SHIFT: u32} FixedVec2<SHIFT> => Fixed<SHIFT>);
-    fn get() -> Axis;
+impl_axis_index!(Vec2 => f32);
-}
-
-pub struct XAxis;
-const impl AxisT for XAxis {
-    fn get() -> Axis {
-        Axis::X
-    }
-}
-
-pub struct YAxis;
-const impl AxisT for YAxis {
-    fn get() -> Axis {
-        Axis::Y
-    }
-}
-
-#[derive(Clone, Copy, Debug, Default)]
-pub struct BothAxis<T> {
-    pub x: T,
-    pub y: T,
-}
-
-impl<T> BothAxis<T> {
-    pub const fn axis<A: const AxisT>(&mut self) -> &mut T {
-        match A::get() {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-    pub fn take_axis<A: const AxisT>(self) -> T {
-        match A::get() {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-    pub fn axis_dyn(&mut self, axis: Axis) -> &mut T {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-}

core/src/orientation/len.rs

@@ -1,22 +1,31 @@
 use super::*;
-use crate::{UiNum, util::impl_op};
+use crate::util::impl_axis_index;
+use crate::{Px, PxVec2, Rel, UiNum, Weight, util::impl_op};
 
 #[derive(Debug, Default, Clone, Copy, PartialEq)]
 pub struct Size {
-    pub x: Len,
+    pub x: LayoutLen,
-    pub y: Len,
+    pub y: LayoutLen,
 }
 
-#[derive(Debug, Clone, Copy, PartialEq)]
+/// What a widget asks for along one axis: a [`Len`] -- pixels and a fraction
-pub struct Len {
+/// of the box it is given -- plus a share of whatever is left over once
-    pub abs: f32,
+/// everything fixed has been taken. The parts add up rather than choosing
-    pub rel: f32,
+/// between one another.
-    pub rest: f32,
+///
+/// Only a container dividing its room can answer a share, so a length nobody
+/// divides is a `Len`: a position, a padding, a cap, anything already
+/// resolved.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct LayoutLen {
+    pub px: Px,
+    pub rel: Rel,
+    pub leftover: Weight,
 }
 
-impl<N: UiNum> From<N> for Len {
+impl<N: UiNum> From<N> for LayoutLen {
     fn from(value: N) -> Self {
-        Len::abs(value.to_f32())
+        LayoutLen::px(value.to_f32())
     }
 }
 
@@ -29,52 +38,76 @@ impl<Nx: UiNum, Ny: UiNum> From<(Nx, Ny)> for Size {
     }
 }
 
-impl From<Len> for Size {
+/// A length with no share in it is a length a container does not have to
-    fn from(value: Len) -> Self {
+/// divide, which is one it can always give.
+impl From<Len> for LayoutLen {
+    fn from(len: Len) -> Self {
+        Self {
+            px: len.px,
+            rel: len.rel,
+            leftover: Weight::ZERO,
+        }
+    }
+}
+
+impl From<LayoutLen> for Size {
+    fn from(value: LayoutLen) -> Self {
         Self { x: value, y: value }
     }
 }
 
 impl Size {
     pub const ZERO: Self = Self {
-        x: Len::ZERO,
+        x: LayoutLen::ZERO,
-        y: Len::ZERO,
+        y: LayoutLen::ZERO,
     };
 
-    pub const REST: Self = Self {
+    pub const LEFTOVER: Self = Self {
-        x: Len::REST,
+        x: LayoutLen::LEFTOVER,
-        y: Len::REST,
+        y: LayoutLen::LEFTOVER,
     };
 
-    pub fn abs(v: Vec2) -> Self {
+    /// From something measured outside layout -- a texture, a shaped line --
+    /// which is where a size in floats comes from.
+    pub fn px(v: Vec2) -> Self {
+        Self::from_px(PxVec2::from_f32(v))
+    }
+
+    pub const fn from_px(v: PxVec2) -> Self {
         Self {
-            x: Len::abs(v.x),
+            x: LayoutLen {
-            y: Len::abs(v.y),
+                px: v.x,
+                ..LayoutLen::ZERO
+            },
+            y: LayoutLen {
+                px: v.y,
+                ..LayoutLen::ZERO
+            },
         }
     }
 
     pub fn rel(v: Vec2) -> Self {
         Self {
-            x: Len::rel(v.x),
+            x: LayoutLen::rel(v.x),
-            y: Len::rel(v.y),
+            y: LayoutLen::rel(v.y),
         }
     }
 
-    pub fn rest(v: Vec2) -> Self {
+    pub fn leftover(v: Vec2) -> Self {
         Self {
-            x: Len::rest(v.x),
+            x: LayoutLen::leftover(v.x),
-            y: Len::rest(v.y),
+            y: LayoutLen::leftover(v.y),
         }
     }
 
     pub fn to_uivec2(self) -> UiVec2 {
         UiVec2 {
-            x: self.x.apply_rest(),
+            x: self.x.apply_leftover(),
-            y: self.y.apply_rest(),
+            y: self.y.apply_leftover(),
         }
     }
 
-    pub fn from_axis(axis: Axis, aligned: Len, ortho: Len) -> Self {
+    pub fn from_axis(axis: Axis, aligned: LayoutLen, ortho: LayoutLen) -> Self {
         match axis {
             Axis::X => Self {
                 x: aligned,
@@ -86,54 +119,88 @@ impl Size {
             },
         }
     }
-
-    pub fn axis(&self, axis: Axis) -> Len {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
 }
 
-impl Len {
+impl LayoutLen {
     pub const ZERO: Self = Self {
-        abs: 0.0,
+        px: Px::ZERO,
-        rel: 0.0,
+        rel: Rel::ZERO,
-        rest: 0.0,
+        leftover: Weight::ZERO,
     };
 
-    pub const REST: Self = Self {
+    pub const LEFTOVER: Self = Self {
-        abs: 0.0,
+        px: Px::ZERO,
-        rel: 0.0,
+        rel: Rel::ZERO,
-        rest: 1.0,
+        leftover: Weight::ONE,
     };
 
-    pub fn apply_rest(&self) -> UiScalar {
+    /// The whole of what is left over counts as the whole box, which is what
-        UiScalar {
+    /// a length means to something that is not dividing a box between
-            rel: self.rel + if self.rest > 0.0 { 1.0 } else { 0.0 },
+    /// siblings -- a scroll asking how long its content is.
-            abs: self.abs,
+    pub fn apply_leftover(&self) -> Len {
-        }
+        let share = match self.leftover > Weight::ZERO {
+            true => Rel::ONE,
+            false => Rel::ZERO,
+        };
+        Len::from_parts(self.rel.add(share), self.px)
     }
 
-    pub fn abs(abs: impl UiNum) -> Self {
+    /// Only pixels: the same number of them whatever box it lands in, and
+    /// whatever anyone else in the row asks for. A length that is any part
+    /// of a box or of what is left over is not one.
+    pub fn is_px(&self) -> bool {
+        self.rel == Rel::ZERO && self.leftover == Weight::ZERO
+    }
+
+    /// Nothing but a claim on what is left over, so there is no length here
+    /// at all where nothing is.
+    pub fn is_only_leftover(&self) -> bool {
+        self.leftover > Weight::ZERO && self.without_leftover() == Len::ZERO
+    }
+
+    /// This as a length of a box, where it is one. `leftover` is not: a
+    /// share of what is left over is a length only to whoever divides one,
+    /// so it passes up in the reported size instead and is resolved there.
+    pub fn declared(&self) -> Option<Len> {
+        (self.leftover == Weight::ZERO).then(|| self.without_leftover())
+    }
+
+    /// What this takes whatever is left over: the reading of a length for
+    /// anyone not dividing a box between siblings, where a share is a claim
+    /// on someone else's room rather than a length of its own.
+    /// [`Self::apply_leftover`] is the opposite reading of the same value.
+    pub const fn without_leftover(&self) -> Len {
+        Len::from_parts(self.rel, self.px)
+    }
+
+    /// This length, given as a part of a box `len` long, as a part of the
+    /// box `len` is itself a part of. The share is untouched: it is a claim
+    /// on whoever divides the room, not a fraction of anything.
+    pub const fn within_len(self, len: Len) -> Self {
+        let part = self.without_leftover().within_len(len);
         Self {
-            abs: abs.to_f32(),
+            px: part.px,
-            rel: 0.0,
+            rel: part.rel,
-            rest: 0.0,
+            leftover: self.leftover,
+        }
+    }
+
+    pub fn px(px: impl UiNum) -> Self {
+        Self {
+            px: Px::from_num(px),
+            ..Self::ZERO
         }
     }
     pub fn rel(rel: impl UiNum) -> Self {
         Self {
-            abs: 0.0,
+            rel: Rel::from_num(rel),
-            rel: rel.to_f32(),
+            ..Self::ZERO
-            rest: 0.0,
         }
     }
-    pub fn rest(ratio: impl UiNum) -> Self {
+    pub fn leftover(ratio: impl UiNum) -> Self {
         Self {
-            abs: 0.0,
+            leftover: Weight::from_num(ratio),
-            rel: 0.0,
+            ..Self::ZERO
-            rest: ratio.to_f32(),
         }
     }
 }
@@ -141,38 +208,26 @@ impl Len {
 pub mod len_fns {
     use super::*;
 
-    pub fn abs(abs: impl UiNum) -> Len {
+    pub fn px(px: impl UiNum) -> LayoutLen {
-        Len {
+        LayoutLen::px(px)
-            abs: abs.to_f32(),
-            rel: 0.0,
-            rest: 0.0,
     }
+    pub fn rel(rel: impl UiNum) -> LayoutLen {
+        LayoutLen::rel(rel)
     }
-    pub fn rel(rel: impl UiNum) -> Len {
+    pub fn leftover(ratio: impl UiNum) -> LayoutLen {
-        Len {
+        LayoutLen::leftover(ratio)
-            abs: 0.0,
-            rel: rel.to_f32(),
-            rest: 0.0,
-        }
-    }
-    pub fn rest(ratio: impl UiNum) -> Len {
-        Len {
-            abs: 0.0,
-            rel: 0.0,
-            rest: ratio.to_f32(),
-        }
     }
 }
 
-impl_op!(Len Add add; abs rel rest);
+impl_op!(same LayoutLen Add add; px rel leftover);
-impl_op!(Len Sub sub; abs rel rest);
+impl_op!(same LayoutLen Sub sub; px rel leftover);
 
-impl_op!(Size Add add; x y);
+impl_op!(same Size Add add; x y);
-impl_op!(Size Sub sub; x y);
+impl_op!(same Size Sub sub; x y);
 
-impl Default for Len {
+impl Default for LayoutLen {
     fn default() -> Self {
-        Self::rest(1.0)
+        Self::leftover(1.0)
     }
 }
 
@@ -182,17 +237,19 @@ impl std::fmt::Display for Size {
     }
 }
 
-impl std::fmt::Display for Len {
+impl std::fmt::Display for LayoutLen {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        if self.abs != 0.0 {
+        if self.px != Px::ZERO {
-            write!(f, "{} abs;", self.abs)?;
+            write!(f, "{} px;", self.px)?;
         }
-        if self.rel != 0.0 {
+        if self.rel != Rel::ZERO {
             write!(f, "{} rel;", self.rel)?;
         }
-        if self.rest != 0.0 {
+        if self.leftover != Weight::ZERO {
-            write!(f, "{} rest;", self.rest)?;
+            write!(f, "{} leftover;", self.leftover)?;
         }
         Ok(())
     }
 }
+
+impl_axis_index!(Size => LayoutLen);

core/src/orientation/pos.rs

@@ -1,41 +1,47 @@
-use std::{fmt::Display, hash::Hash, marker::Destruct};
+use crate::util::impl_axis_index;
+use std::{fmt::Display, marker::Destruct};
 
 use super::*;
-use crate::{
+use crate::{Px, PxVec2, Rel, UiNum, util::impl_op};
-    UiNum,
-    util::{LerpUtil, impl_op},
-};
 
 #[repr(C)]
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, bytemuck::Pod, bytemuck::Zeroable, Default)]
 pub struct UiVec2 {
-    pub x: UiScalar,
+    pub x: Len,
-    pub y: UiScalar,
+    pub y: Len,
 }
 
 impl UiVec2 {
     pub const ZERO: Self = Self {
-        x: UiScalar::ZERO,
+        x: Len::ZERO,
-        y: UiScalar::ZERO,
+        y: Len::ZERO,
     };
 
-    pub const fn new(x: UiScalar, y: UiScalar) -> Self {
+    pub const fn new(x: Len, y: Len) -> Self {
         Self { x, y }
     }
 
-    pub const fn abs(abs: impl const Into<Vec2>) -> Self {
+    pub const fn px(px: impl const Into<Vec2>) -> Self {
-        let abs = abs.into();
+        let px = px.into();
         Self {
-            x: UiScalar::abs(abs.x),
+            x: Len::px(px.x),
-            y: UiScalar::abs(abs.y),
+            y: Len::px(px.y),
+        }
+    }
+
+    /// From lengths already on the grid, with no fraction of a box.
+    pub const fn from_px(px: PxVec2) -> Self {
+        Self {
+            x: Len::from_parts(Rel::ZERO, px.x),
+            y: Len::from_parts(Rel::ZERO, px.y),
         }
     }
 
     pub const fn rel(rel: impl const Into<Vec2>) -> Self {
         let rel = rel.into();
         Self {
-            x: UiScalar::rel(rel.x),
+            x: Len::rel(rel.x),
-            y: UiScalar::rel(rel.y),
+            y: Len::rel(rel.y),
         }
     }
 
@@ -56,37 +62,15 @@ impl UiVec2 {
         }
     }
 
-    pub const fn outside(&self, region: &UiRegion) -> UiVec2 {
+    /// Resolved against a box of `size`, which is where a fraction stops
-        UiVec2 {
+    /// being one and becomes a place.
-            x: self.x.outside(&region.x),
+    pub fn to_px(&self, size: PxVec2) -> PxVec2 {
-            y: self.y.outside(&region.y),
+        PxVec2::new(self.x.to_px(size.x), self.y.to_px(size.y))
-        }
-    }
-
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut UiScalar {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-
-    pub fn axis(&self, axis: Axis) -> UiScalar {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-
-    pub fn to_abs(&self, rel: Vec2) -> Vec2 {
-        Vec2 {
-            x: self.x.to_abs(rel.x),
-            y: self.y.to_abs(rel.y),
-        }
     }
 
     pub const FULL_SIZE: Self = Self::rel(Vec2::ONE);
 
-    pub const fn from_axis(axis: Axis, aligned: UiScalar, ortho: UiScalar) -> Self {
+    pub const fn from_axis(axis: Axis, aligned: Len, ortho: Len) -> Self {
         match axis {
             Axis::X => Self {
                 x: aligned,
@@ -99,34 +83,27 @@ impl UiVec2 {
         }
     }
 
-    pub fn get_abs(&self) -> Vec2 {
+    pub fn get_px(&self) -> Vec2 {
-        (self.x.abs, self.y.abs).into()
+        (self.x.px.to_f32(), self.y.px.to_f32()).into()
     }
 
     pub fn get_rel(&self) -> Vec2 {
-        (self.x.rel, self.y.rel).into()
+        (self.x.rel.to_f32(), self.y.rel.to_f32()).into()
-    }
-
-    pub fn abs_mut(&mut self) -> Vec2View<'_> {
-        Vec2View {
-            x: &mut self.x.abs,
-            y: &mut self.y.abs,
-        }
     }
 }
 
 impl Display for UiVec2 {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "rel{};abs{}", self.get_rel(), self.get_abs())
+        write!(f, "rel{};px{}", self.get_rel(), self.get_px())
     }
 }
 
-impl_op!(UiVec2 Add add; x y);
+impl_op!(same UiVec2 Add add; x y);
-impl_op!(UiVec2 Sub sub; x y);
+impl_op!(same UiVec2 Sub sub; x y);
 
 const impl From<Vec2> for UiVec2 {
-    fn from(abs: Vec2) -> Self {
+    fn from(px: Vec2) -> Self {
-        Self::abs(abs)
+        Self::px(px)
     }
 }
 
@@ -134,133 +111,137 @@ const impl<T: const UiNum, U: const UiNum> From<(T, U)> for UiVec2
 where
     (T, U): const Destruct,
 {
-    fn from(abs: (T, U)) -> Self {
+    fn from(px: (T, U)) -> Self {
-        Self::abs(abs)
+        Self::px(px)
     }
 }
 
+/// A length along one axis: a fraction of the box it is measured in plus an
+/// offset, `rel * box + px`. A position is the same number -- the length from
+/// the start of the box to the point -- which is why a [`UiSpan`] is two of
+/// these. Both parts are fixed point, so composing one through a chain of
+/// boxes rounds only where it multiplies, and lands on the same number as any
+/// other route to the same place.
+///
+/// It carries no claim on what a container has left over. That is
+/// [`crate::LayoutLen`], which is this plus a weight, and which means nothing
+/// to anyone but whoever divides the room.
 #[repr(C)]
-#[derive(Debug, Copy, Clone, PartialEq, bytemuck::Pod, Default, bytemuck::Zeroable)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, bytemuck::Pod, Default, bytemuck::Zeroable)]
-pub struct UiScalar {
+pub struct Len {
-    pub rel: f32,
+    pub rel: Rel,
-    pub abs: f32,
+    pub px: Px,
 }
 
-impl Eq for UiScalar {}
+impl_op!(same Len Add add; rel px);
-impl Hash for UiScalar {
+impl_op!(same Len Sub sub; rel px);
-    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+
-        state.write_u32(self.rel.to_bits());
+impl Len {
-        state.write_u32(self.abs.to_bits());
+    pub const ZERO: Self = Self {
-    }
+        rel: Rel::ZERO,
+        px: Px::ZERO,
+    };
+    pub const FULL: Self = Self {
+        rel: Rel::ONE,
+        px: Px::ZERO,
+    };
+
+    pub const fn new(rel: f32, px: f32) -> Self {
+        Self::from_parts(Rel::from_f32(rel), Px::from_f32(px))
     }
 
-impl_op!(UiScalar Add add; rel abs);
+    /// From parts already on the grid, rather than numbers to be put on it.
-impl_op!(UiScalar Sub sub; rel abs);
+    pub const fn from_parts(rel: Rel, px: Px) -> Self {
-
+        Self { rel, px }
-impl UiScalar {
-    pub const ZERO: Self = Self { rel: 0.0, abs: 0.0 };
-    pub const FULL: Self = Self { rel: 1.0, abs: 0.0 };
-
-    pub const fn new(rel: f32, abs: f32) -> Self {
-        Self { rel, abs }
     }
 
     pub const fn rel(rel: f32) -> Self {
-        Self { rel, abs: 0.0 }
+        Self::from_parts(Rel::from_f32(rel), Px::ZERO)
     }
 
-    pub const fn abs(abs: f32) -> Self {
+    pub const fn px(px: f32) -> Self {
-        Self { rel: 0.0, abs }
+        Self::from_parts(Rel::ZERO, Px::from_f32(px))
-    }
-
-    pub const fn rel_min() -> Self {
-        Self::new(0.0, 0.0)
-    }
-
-    pub const fn rel_max() -> Self {
-        Self::new(1.0, 0.0)
     }
 
     pub const fn max(&self, other: Self) -> Self {
         Self {
             rel: self.rel.max(other.rel),
-            abs: self.abs.max(other.abs),
+            px: self.px.max(other.px),
         }
     }
 
     pub const fn min(&self, other: Self) -> Self {
         Self {
             rel: self.rel.min(other.rel),
-            abs: self.abs.min(other.abs),
+            px: self.px.min(other.px),
         }
     }
 
-    pub const fn offset(mut self, amt: f32) -> Self {
+    /// Both parts by the same fraction, which is what a part of a length
-        self.abs += amt;
+    /// means when the length is part pixels and part a fraction of a box.
+    pub const fn scale(&self, by: Rel) -> Self {
+        Self {
+            rel: self.rel.mul(by),
+            px: self.px.mul(by),
+        }
+    }
+
+    pub const fn offset(mut self, amt: Px) -> Self {
+        self.px = self.px.add(amt);
         self
     }
 
     pub const fn within(&self, span: &UiSpan) -> Self {
-        let anchor = self.rel.lerp(span.start.rel, span.end.rel);
-        let offset = self.abs + self.rel.lerp(span.start.abs, span.end.abs);
         Self {
-            rel: anchor,
+            rel: self.rel.lerp(span.start.rel, span.end.rel),
-            abs: offset,
+            px: self.px.add(self.rel.lerp(span.start.px, span.end.px)),
         }
     }
 
-    pub const fn outside(&self, span: &UiSpan) -> Self {
+    pub const fn within_len(&self, len: Len) -> Self {
-        let rel = self.rel.lerp_inv(span.start.rel, span.end.rel);
-        let abs = self.abs - rel.lerp(span.start.abs, span.end.abs);
-        Self { rel, abs }
-    }
-
-    pub fn within_len(&self, len: UiScalar) -> Self {
         self.within(&UiSpan {
-            start: UiScalar::ZERO,
+            start: Len::ZERO,
             end: len,
         })
     }
 
-    pub fn select_len(&self, len: UiScalar) -> Self {
-        len.within_len(*self)
-    }
-
     pub const fn flip(&mut self) {
-        self.rel = 1.0 - self.rel;
+        self.rel = Rel::ONE.sub(self.rel);
-        self.abs = -self.abs;
+        self.px = self.px.neg();
     }
 
     pub const fn to(&self, end: Self) -> UiSpan {
         UiSpan { start: *self, end }
     }
 
-    pub const fn to_abs(&self, rel: f32) -> f32 {
+    /// Resolved against a box of `len`, which is the only place a fraction
-        self.rel * rel + self.abs
+    /// becomes a number of pixels.
+    pub const fn to_px(&self, len: Px) -> Px {
+        self.px.add(len.mul(self.rel))
     }
 }
 
 #[repr(C)]
 #[derive(Debug, Copy, Clone, PartialEq, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct UiSpan {
-    pub start: UiScalar,
+    pub start: Len,
-    pub end: UiScalar,
+    pub end: Len,
 }
 
 impl UiSpan {
     pub const FULL: Self = Self {
-        start: UiScalar::ZERO,
+        start: Len::ZERO,
-        end: UiScalar::FULL,
+        end: Len::FULL,
     };
 
     pub const fn rel(rel: f32) -> Self {
         Self {
-            start: UiScalar::rel(rel),
+            start: Len::rel(rel),
-            end: UiScalar::rel(rel),
+            end: Len::rel(rel),
         }
     }
 
-    pub const fn new(start: UiScalar, end: UiScalar) -> Self {
+    pub const fn new(start: Len, end: Len) -> Self {
         Self { start, end }
     }
 
@@ -268,14 +249,19 @@ impl UiSpan {
         self.start.flip();
         self.end.flip();
         std::mem::swap(&mut self.start.rel, &mut self.end.rel);
-        std::mem::swap(&mut self.start.abs, &mut self.end.abs);
+        std::mem::swap(&mut self.start.px, &mut self.end.px);
     }
 
-    pub const fn shift(&mut self, offset: UiScalar) {
+    pub const fn shift(&mut self, offset: Len) {
         self.start += offset;
         self.end += offset;
     }
 
+    /// Composing a box through the one it sits in, and the hottest line in
+    /// layout. It used to skip the multiplies where a span was the whole of
+    /// its parent or the parent the whole of its own; both come out of the
+    /// multiply unchanged anyway, and the body those comparisons cost was
+    /// what kept the inliner from taking this at all.
     pub const fn within(&self, parent: &Self) -> Self {
         Self {
             start: self.start.within(parent),
@@ -283,14 +269,16 @@ impl UiSpan {
         }
     }
 
-    pub const fn outside(&self, parent: &Self) -> Self {
+    /// A box `len` long inside this one, on the side `align` says. Both must
-        Self {
+    /// be lengths of the same rel base: it subtracts one from the other
-            start: self.start.outside(parent),
+    /// rather than composing it in, which is what keeps a fraction the same
-            end: self.end.outside(parent),
+    /// fraction however long this box turns out to be.
-        }
+    pub const fn place(self, len: Len, align: AxisAlign) -> Self {
+        let start = self.start + (self.len() - len).scale(align.rel());
+        Self::new(start, start + len)
     }
 
-    pub const fn len(&self) -> UiScalar {
+    pub const fn len(&self) -> Len {
         self.end - self.start
     }
 }
@@ -324,27 +312,6 @@ impl UiRegion {
             y: self.y.within(&parent.y),
         }
     }
-    pub const fn outside(&self, parent: &Self) -> Self {
-        Self {
-            x: self.x.outside(&parent.x),
-            y: self.y.outside(&parent.y),
-        }
-    }
-
-    pub const fn axis(&mut self, axis: Axis) -> &UiSpan {
-        match axis {
-            Axis::X => &self.x,
-            Axis::Y => &self.y,
-        }
-    }
-
-    pub const fn axis_mut(&mut self, axis: Axis) -> &mut UiSpan {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-
     pub const fn flip(&mut self, axis: Axis) {
         match axis {
             Axis::X => self.x.flip(),
@@ -363,10 +330,10 @@ impl UiRegion {
         self
     }
 
-    pub fn to_px(&self, size: Vec2) -> PixelRegion {
+    pub fn to_px(&self, size: PxVec2) -> PixelRegion {
         PixelRegion {
-            top_left: self.top_left().get_rel() * size + self.top_left().get_abs(),
+            top_left: self.top_left().to_px(size),
-            bot_right: self.bot_right().get_rel() * size + self.bot_right().get_abs(),
+            bot_right: self.bot_right().to_px(size),
         }
     }
 
@@ -421,21 +388,21 @@ impl Display for UiRegion {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct PixelRegion {
-    pub top_left: Vec2,
+    pub top_left: PxVec2,
-    pub bot_right: Vec2,
+    pub bot_right: PxVec2,
 }
 
 impl PixelRegion {
-    pub fn contains(&self, pos: Vec2) -> bool {
+    pub fn contains(&self, pos: PxVec2) -> bool {
         pos.x >= self.top_left.x
             && pos.x <= self.bot_right.x
             && pos.y >= self.top_left.y
             && pos.y <= self.bot_right.y
     }
 
-    pub fn size(&self) -> Vec2 {
+    pub fn size(&self) -> PxVec2 {
         self.bot_right - self.top_left
     }
 }
@@ -446,19 +413,5 @@ impl Display for PixelRegion {
     }
 }
 
-pub struct Vec2View<'a> {
+impl_axis_index!(UiVec2 => Len);
-    pub x: &'a mut f32,
+impl_axis_index!(UiRegion => UiSpan);
-    pub y: &'a mut f32,
-}
-
-impl Vec2View<'_> {
-    pub fn set(&mut self, other: Vec2) {
-        *self.x = other.x;
-        *self.y = other.y;
-    }
-
-    pub fn add(&mut self, other: Vec2) {
-        *self.x += other.x;
-        *self.y += other.y;
-    }
-}

core/src/primitive/layer.rs

@@ -1,7 +1,7 @@
 use std::ops::{Index, IndexMut};
 
 use crate::{
-    render::{MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst, Primitives},
+    render::{LayerDraws, MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst},
     util::to_mut,
 };
 
@@ -39,7 +39,7 @@ struct Child {
     tail: usize,
 }
 
-pub type PrimitiveLayers = Layers<Primitives>;
+pub type DrawLayers = Layers<LayerDraws>;
 
 impl<T: Default> Layers<T> {
     pub fn new() -> Layers<T> {
@@ -119,7 +119,11 @@ impl<T: Default> Layers<T> {
     }
 }
 
-impl PrimitiveLayers {
+impl DrawLayers {
+    /// Inlined on purpose: it is one call per glyph, the innermost thing a
+    /// frame does, and whether the inliner takes it turns out to depend on
+    /// unrelated code elsewhere in the crate -- 12% of a resize frame.
+    #[inline]
     pub fn write<P: Primitive>(
         &mut self,
         layer: LayerId,

core/src/primitive/text.rs

@@ -1,12 +1,17 @@
+#[cfg(feature = "layout-diagnostics")]
+use crate::layout_diagnostics::{self as diag, Counter, TimerKind};
 use crate::{
-    Align, GlyphAtlas, GlyphEntry, GlyphKey, PlacedGlyph, RegionAlign, Textures, UiColor,
+    Align, GlyphAtlas, GlyphEntry, GlyphKey, PlacedGlyph, Px, PxVec2, RegionAlign, UiColor,
     util::Vec2,
 };
 use parley::{
     Alignment, AlignmentOptions, FontContext, FontFamily, FontFamilyName, GenericFamily, Layout,
     LayoutContext, LineHeight, PositionedLayoutItem, StyleProperty,
 };
-use std::hash::{DefaultHasher, Hash, Hasher};
+use std::{
+    collections::VecDeque,
+    hash::{DefaultHasher, Hash, Hasher},
+};
 use swash::{
     FontRef,
     scale::{Render, ScaleContext, Source, StrikeWith},
@@ -18,8 +23,32 @@ pub struct TextData {
     pub layout_ctx: LayoutContext<UiColor>,
     scale_ctx: ScaleContext,
     pub atlas: GlyphAtlas,
+    spare: VecDeque<Placed>,
 }
 
+/// The glyphs of one text at one width. A buffer holds the ones it is drawn
+/// as; these are the ones it had before, kept because a container measures a
+/// child by drawing it in a box it may not keep, and so comes back to widths
+/// it has already asked for.
+struct Placed {
+    /// Where the glyphs land is a function of these three and nothing else,
+    /// so no widget or buffer identity is involved and two texts of the same
+    /// words share an answer.
+    text: String,
+    key: LayoutKey,
+    glyphs: RenderedText,
+}
+
+/// How many to keep. Bounding the whole store rather than each buffer is what
+/// makes this a fixed cost instead of one a tree of ten thousand texts pays
+/// ten thousand times; the re-asks come from laying out one subtree, so they
+/// are close together and few are needed. Instructions over 500 resize frames
+/// of `tests/revision_cost.rs`, both the repeating widths and the sweep that
+/// cannot hit across frames: 13.7B at 32, 12.1B at 64, 10.4B and 12.1B at 128,
+/// and nothing past that -- so 128, which is no worse in the case that never
+/// repeats and better in the one that does.
+const SPARE_PLACED: usize = 128;
+
 impl Default for TextData {
     fn default() -> Self {
         Self {
@@ -27,6 +56,7 @@ impl Default for TextData {
             layout_ctx: LayoutContext::new(),
             scale_ctx: ScaleContext::new(),
             atlas: GlyphAtlas::default(),
+            spare: VecDeque::new(),
         }
     }
 }
@@ -82,6 +112,9 @@ pub struct TextBuffer {
     text: String,
     layout: Layout<UiColor>,
     layout_key: Option<LayoutKey>,
+    /// The glyphs placed from `layout`, so drawing this text again at the
+    /// width it already has places them once.
+    placed: Option<RenderedText>,
 }
 
 #[derive(PartialEq)]
@@ -96,13 +129,10 @@ impl TextBuffer {
             text: text.into(),
             layout: Layout::new(),
             layout_key: None,
+            placed: None,
         }
     }
 
-    pub fn new_empty() -> Self {
-        Self::new("")
-    }
-
     pub fn text(&self) -> &str {
         &self.text
     }
@@ -120,15 +150,45 @@ impl TextBuffer {
         if text != self.text {
             self.text = text;
             self.layout_key = None;
+            self.placed = None;
         }
     }
 
     /// Invalidates the layout and returns the underlying string for editing.
     pub fn edit(&mut self) -> &mut String {
         self.layout_key = None;
+        self.placed = None;
         &mut self.text
     }
 
+    /// The glyphs of the shaping it is drawn as, once they are placed.
+    pub fn rendered(&self) -> Option<&RenderedText> {
+        self.placed.as_ref()
+    }
+
+    /// The width its shaping wraps at, and `None` where it does not wrap or
+    /// has not been shaped.
+    pub fn wrap_width(&self) -> Option<f32> {
+        self.layout_key.as_ref()?.max_width
+    }
+
+    /// Widths covered by the current line breaks, including a wider shaping
+    /// retained when a later draw requested a narrower box.
+    pub fn width_holds(&self) -> crate::Holds {
+        let Some(width) = self.wrap_width() else {
+            return crate::Holds::ANY;
+        };
+        let width = Px::from_f32(width);
+        let soft_wrapped = self.layout.lines().any(|line| {
+            matches!(
+                line.break_reason(),
+                parley::layout::BreakReason::Regular | parley::layout::BreakReason::Emergency
+            )
+        });
+        let upper = if soft_wrapped { width } else { Px::MAX };
+        crate::Holds::from(Px::ceil_from_f32(self.layout.width()).min(width)..=upper)
+    }
+
     pub fn size(&self) -> Vec2 {
         Vec2::new(self.layout.width(), self.layout.height())
     }
@@ -139,8 +199,62 @@ impl TextBuffer {
             max_width: width,
         };
         if self.layout_key.as_ref() == Some(&layout_key) {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::TextShapeHits);
             return;
         }
+        // A greedy break at one width is the same break at every width down
+        // to the longest line it produced: each line still fits, and none can
+        // take a word that would not fit in the wider box. So the layout in
+        // hand already answers, and re-breaking would only be work.
+        //
+        // At the longest line exactly, with no margin below it. A narrower
+        // width really does break differently, so answering one from the
+        // break in hand is how a warm tree keeps lines a cold tree would
+        // never produce. The margin was here because a text reports the
+        // width it used and a parent hands that back; the report is the step
+        // at or above its longest line now, so what comes back fits.
+        if let Some(key) = &self.layout_key
+            && key.attrs == *attrs
+            && let (Some(broke_at), Some(want)) = (key.max_width, width)
+            && want <= broke_at
+            && want >= self.layout.width()
+        {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::TextShapeHits);
+            return;
+        }
+        let same_shaping = self
+            .layout_key
+            .as_ref()
+            .is_some_and(|key| key.attrs == *attrs);
+        let old_key = self.layout_key.replace(layout_key);
+        // The glyphs it holds are of the width it held, which the layout may
+        // well come back to.
+        if let Some(key) = old_key
+            && let Some(glyphs) = self.placed.take()
+        {
+            data.keep_placed(Placed {
+                text: self.text.clone(),
+                key,
+                glyphs,
+            });
+        }
+        // Only the line breaking depends on the width: the shaped runs under
+        // it are a function of the text and the attrs, and parley re-breaks
+        // them in place. So a new width is a break, not a shaping.
+        if same_shaping {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::TextBreaks);
+            #[cfg(feature = "layout-diagnostics")]
+            let _break = diag::timer(TimerKind::TextBreak);
+            self.break_lines(width);
+            return;
+        }
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::TextShapes);
+        #[cfg(feature = "layout-diagnostics")]
+        let _shape = diag::timer(TimerKind::TextShape);
         let mut builder = data
             .layout_ctx
             .ranged_builder(&mut data.font_ctx, &self.text, 1.0, true);
@@ -151,15 +265,18 @@ impl TextBuffer {
         )));
         builder.push_default(StyleProperty::Brush(attrs.color));
         builder.build_into(&mut self.layout, &self.text);
+        self.break_lines(width);
+    }
+
+    fn break_lines(&mut self, width: Option<f32>) {
         self.layout.break_all_lines(width);
         self.layout
             .align(Alignment::Start, AlignmentOptions::default());
-        self.layout_key = Some(layout_key);
     }
 }
 
 impl TextData {
-    pub fn place(&mut self, buffer: &TextBuffer, textures: &mut Textures) -> Vec<PlacedGlyph> {
+    pub fn place(&mut self, buffer: &TextBuffer) -> Vec<PlacedGlyph> {
         let mut placed = Vec::new();
         for line in buffer.layout.lines() {
             for item in line.items() {
@@ -185,24 +302,21 @@ impl TextData {
                         subpixel,
                         coords: coords_hash,
                     };
-                    let Some(entry) = self.glyph_entry(
+                    let Some(entry) = self.glyph_entry(GlyphRaster {
-                        GlyphRaster {
                         key,
                         font: font_ref,
                         font_size,
                         coords,
                         subpixel,
                         glyph_id: glyph.id,
-                        },
+                    }) else {
-                        textures,
-                    ) else {
                         continue;
                     };
                     placed.push(PlacedGlyph {
                         entry,
-                        offset: Vec2::new(
+                        offset: PxVec2::new(
-                            glyph.x.floor() + entry.left as f32,
+                            Px::from_int(glyph.x.floor() as i32 + entry.left),
-                            glyph.y.floor() - entry.top as f32,
+                            Px::from_int(glyph.y.floor() as i32 - entry.top),
                         ),
                     });
                 }
@@ -211,11 +325,7 @@ impl TextData {
         placed
     }
 
-    fn glyph_entry(
+    fn glyph_entry(&mut self, glyph: GlyphRaster<'_>) -> Option<GlyphEntry> {
-        &mut self,
-        glyph: GlyphRaster<'_>,
-        textures: &mut Textures,
-    ) -> Option<GlyphEntry> {
         if let Some(entry) = self.atlas.get(&glyph.key) {
             return entry;
         }
@@ -237,7 +347,7 @@ impl TextData {
         .render(&mut scaler, glyph.glyph_id as u16);
 
         if let Some(image) = image {
-            self.atlas.insert(glyph.key, &image, textures)
+            self.atlas.insert(glyph.key, &image)
         } else {
             self.atlas.insert_empty(glyph.key);
             None
@@ -273,19 +383,54 @@ pub struct RenderedText {
 }
 
 impl TextData {
-    pub fn render(
+    /// The glyphs of this text at this width, taken out of what is kept.
+    fn take_placed(&mut self, text: &str, key: &LayoutKey) -> Option<RenderedText> {
+        // From the newest, since a re-ask is usually of something recent.
+        let at = self
+            .spare
+            .iter()
+            .rposition(|spare| spare.key == *key && spare.text == text)?;
+        self.spare.remove(at).map(|spare| spare.glyphs)
+    }
+
+    fn keep_placed(&mut self, placed: Placed) {
+        if self.spare.len() >= SPARE_PLACED {
+            self.spare.pop_front();
+        }
+        self.spare.push_back(placed);
+    }
+
+    pub fn render<'b>(
         &mut self,
-        buffer: &mut TextBuffer,
+        buffer: &'b mut TextBuffer,
         attrs: &TextAttrs,
         width: Option<f32>,
-        textures: &mut Textures,
+    ) -> &'b RenderedText {
-    ) -> RenderedText {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::TextRenders);
+        #[cfg(feature = "layout-diagnostics")]
+        let _render = diag::timer(TimerKind::TextRender);
         buffer.shape(self, attrs, width);
-        let glyphs = self.place(buffer, textures);
+        // Only asked for when the buffer no longer holds them: taking one out
+        // of the store to then drop it would throw an answer away.
+        let placed = buffer.placed.take().or_else(|| {
+            let key = buffer.layout_key.as_ref()?;
+            self.take_placed(&buffer.text, key)
+        });
+        let placed = match placed {
+            Some(placed) => placed,
+            None => {
+                #[cfg(feature = "layout-diagnostics")]
+                diag::bump(Counter::GlyphPlacements);
+                #[cfg(feature = "layout-diagnostics")]
+                let _place = diag::timer(TimerKind::GlyphPlacement);
                 RenderedText {
-            glyphs,
+                    glyphs: self.place(buffer),
                     size: buffer.size(),
                     color: attrs.color,
                 }
             }
+        };
+        buffer.placed.insert(placed)
+    }
 }

core/src/primitive/texture.rs

@@ -1,7 +1,4 @@
-use crate::{
+use crate::util::{RefCounter, Vec2};
-    render::TexturePrimitive,
-    util::{RefCounter, Vec2},
-};
 use image::{DynamicImage, GenericImageView};
 use std::{
     ops::Index,
@@ -10,7 +7,7 @@ use std::{
 
 #[derive(Debug, Clone)]
 pub struct TextureHandle {
-    inner: TexturePrimitive,
+    slot: u32,
     size: Vec2,
     counter: RefCounter,
     send: Sender<u32>,
@@ -31,6 +28,8 @@ pub enum TextureUpdate<'a> {
     Set(u32, &'a DynamicImage),
     Patch(u32, PatchRect, &'a DynamicImage),
     Free(u32),
+    /// Added and freed before the renderer drained either update. It still has
+    /// to push a slot to stay lined up with `images`; `Free` then empties it.
     PushFree,
     SetFree,
 }
@@ -64,14 +63,8 @@ impl Textures {
     pub fn add(&mut self, image: impl Into<DynamicImage>) -> TextureHandle {
         let image = image.into();
         let size = image.dimensions().into();
-        let view_idx = self.push(image);
-        // 0 == default in renderer; TODO: actually create samplers here
-        let sampler_idx = 0;
         TextureHandle {
-            inner: TexturePrimitive {
+            slot: self.push(image),
-                view_idx,
-                sampler_idx,
-            },
             size,
             counter: RefCounter::new(),
             send: self.send.clone(),
@@ -92,15 +85,20 @@ impl Textures {
     }
 
     pub fn image_mut(&mut self, handle: &TextureHandle) -> &mut DynamicImage {
-        self.images[handle.inner.view_idx as usize]
+        self.images[handle.slot as usize]
             .as_mut()
             .expect("texture was freed while still held")
     }
 
     /// Queue an upload of just `rect`, after writing it with `image_mut`.
     pub fn patch(&mut self, handle: &TextureHandle, rect: PatchRect) {
-        self.updates
+        self.updates.push(Update::Patch(handle.slot, rect));
-            .push(Update::Patch(handle.inner.view_idx, rect));
+    }
+
+    /// How many textures are live, which is what a ui can ask; the renderer's
+    /// copies follow from the updates it drains.
+    pub fn count(&self) -> usize {
+        self.images.iter().flatten().count()
     }
 
     pub fn free(&mut self) {
@@ -131,8 +129,9 @@ impl Textures {
 }
 
 impl TextureHandle {
-    pub fn primitive(&self) -> TexturePrimitive {
+    /// Index into `Textures`, and into the renderer's parallel slots.
-        self.inner
+    pub fn slot(&self) -> u32 {
+        self.slot
     }
     pub fn size(&self) -> Vec2 {
         self.size
@@ -142,7 +141,7 @@ impl TextureHandle {
 impl Drop for TextureHandle {
     fn drop(&mut self) {
         if self.counter.drop() {
-            let _ = self.send.send(self.inner.view_idx);
+            let _ = self.send.send(self.slot);
         }
     }
 }
@@ -151,7 +150,7 @@ impl Index<&TextureHandle> for Textures {
     type Output = DynamicImage;
 
     fn index(&self, index: &TextureHandle) -> &Self::Output {
-        self.images[index.inner.view_idx as usize].as_ref().unwrap()
+        self.images[index.slot as usize].as_ref().unwrap()
     }
 }
 

core/src/render/atlas.rs

@@ -1,11 +1,12 @@
 use crate::{
-    PatchRect, TextureHandle, Textures,
+    PatchRect, PxVec2,
     util::{HashMap, Vec2},
 };
 use image::RgbaImage;
 use swash::scale::image::{Content, Image};
 
-const PAGE: u32 = 1024;
+/// Side of one page, and so of every layer of `render::page`'s array texture.
+pub(crate) const PAGE: u32 = 1024;
 
 /// Transparent margin kept around every glyph, so that sampling one cannot
 /// pick up its neighbour along a shared edge.
@@ -35,8 +36,8 @@ pub struct GlyphEntry {
     pub width: u32,
     pub height: u32,
     pub is_colored: bool,
-    pub view_idx: u32,
+    /// Which atlas array layer this glyph is on.
-    pub sampler_idx: u32,
+    pub layer: u32,
 }
 
 impl GlyphEntry {
@@ -48,18 +49,26 @@ impl GlyphEntry {
 }
 
 struct Page {
-    handle: TextureHandle,
+    image: RgbaImage,
     x: u32,
     y: u32,
     shelf_height: u32,
 }
 
+/// A rectangle of one page the renderer has not uploaded yet.
+#[derive(Clone, Copy)]
+pub struct PageUpload {
+    pub layer: u32,
+    pub rect: PatchRect,
+}
+
 #[derive(Default)]
 pub struct GlyphAtlas {
     pages: Vec<Page>,
     /// `None` for a glyph that rasterised to nothing -- a space, say. Cached
     /// too, so it is not re-rasterised on every layout.
     entries: HashMap<GlyphKey, Option<GlyphEntry>>,
+    uploads: Vec<PageUpload>,
 }
 
 impl GlyphAtlas {
@@ -67,12 +76,7 @@ impl GlyphAtlas {
         self.entries.get(key).copied()
     }
 
-    pub fn insert(
+    pub fn insert(&mut self, key: GlyphKey, image: &Image) -> Option<GlyphEntry> {
-        &mut self,
-        key: GlyphKey,
-        image: &Image,
-        textures: &mut Textures,
-    ) -> Option<GlyphEntry> {
         let w = image.placement.width;
         let h = image.placement.height;
         if w == 0 || h == 0 {
@@ -94,23 +98,11 @@ impl GlyphAtlas {
             return None;
         }
 
-        let (page_idx, x, y) = self.allocate(w, h, textures);
+        let upload = self.allocate(w, h);
-        let page = &self.pages[page_idx];
+        let PatchRect { x, y, .. } = upload.rect;
+        write_glyph(&mut self.pages[upload.layer as usize].image, image, x, y);
+        self.uploads.push(upload);
 
-        let img = textures.image_mut(&page.handle);
-        let rgba = img.as_mut_rgba8().expect("atlas page is rgba8");
-        write_glyph(rgba, image, x, y);
-
-        let handle = page.handle.clone();
-        let rect = PatchRect {
-            x,
-            y,
-            width: w,
-            height: h,
-        };
-        textures.patch(&handle, rect);
-
-        let page = &self.pages[page_idx];
         let scale = 1.0 / PAGE as f32;
         let entry = GlyphEntry {
             uv_min: Vec2::new(x as f32 * scale, y as f32 * scale),
@@ -120,43 +112,60 @@ impl GlyphAtlas {
             width: w,
             height: h,
             is_colored: matches!(image.content, Content::Color),
-            view_idx: page.handle.primitive().view_idx,
+            layer: upload.layer,
-            sampler_idx: page.handle.primitive().sampler_idx,
         };
         self.entries.insert(key, Some(entry));
         Some(entry)
     }
 
-    fn allocate(&mut self, w: u32, h: u32, textures: &mut Textures) -> (usize, u32, u32) {
+    /// Reserves room for a `w` by `h` glyph, adding a page if none has it.
+    fn allocate(&mut self, w: u32, h: u32) -> PageUpload {
+        let rect = |x, y| PatchRect {
+            x,
+            y,
+            width: w,
+            height: h,
+        };
         if let Some((i, (x, y))) = self
             .pages
             .iter_mut()
             .enumerate()
             .find_map(|(i, page)| page.allocate(w, h).map(|position| (i, position)))
         {
-            return (i, x, y);
+            return PageUpload {
+                layer: i as u32,
+                rect: rect(x, y),
+            };
         }
 
-        let handle = textures.add(RgbaImage::new(PAGE, PAGE));
         self.pages.push(Page {
-            handle,
+            image: RgbaImage::new(PAGE, PAGE),
             x: PAD + w + PAD,
             y: PAD,
             shelf_height: h + PAD,
         });
-        (self.pages.len() - 1, PAD, PAD)
+        PageUpload {
+            layer: self.pages.len() as u32 - 1,
+            rect: rect(PAD, PAD),
+        }
+    }
+
+    /// Drains what has been written since the last call, for the renderer to
+    /// upload. A new page needs nothing more: wgpu leaves the rest of a fresh
+    /// layer transparent, which is what an atlas wants.
+    pub fn uploads(&mut self) -> impl Iterator<Item = (PageUpload, &RgbaImage)> {
+        let pages = &self.pages;
+        self.uploads
+            .drain(..)
+            .map(|upload| (upload, &pages[upload.layer as usize].image))
     }
 
     pub fn insert_empty(&mut self, key: GlyphKey) {
         self.entries.insert(key, None);
     }
 
-    pub fn page_count(&self) -> usize {
+    pub fn page_count(&self) -> u32 {
-        self.pages.len()
+        self.pages.len() as u32
-    }
-
-    pub fn glyph_count(&self) -> usize {
-        self.entries.len()
     }
 }
 
@@ -228,5 +237,7 @@ fn write_glyph(page: &mut RgbaImage, image: &Image, x: u32, y: u32) {
 #[derive(Clone, Copy)]
 pub struct PlacedGlyph {
     pub entry: GlyphEntry,
-    pub offset: Vec2,
+    /// Whole pixels from the origin of the text to this glyph's top-left,
+    /// on the grid once here rather than on every frame that draws it.
+    pub offset: PxVec2,
 }

core/src/render/data.rs

@@ -1,31 +1,30 @@
-use crate::{UiRegion, util::Id};
+use crate::{UiRegion, util::Id, util::Vec2};
 use wgpu::*;
 
 #[repr(C)]
 #[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable, Default)]
 pub struct WindowUniform {
-    pub width: f32,
+    pub dim: Vec2,
-    pub height: f32,
 }
 
 #[repr(C)]
 #[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct PrimitiveInstance {
     pub region: UiRegion,
-    pub binding: u32,
-    pub idx: u32,
     pub mask_idx: MaskIdx,
+    pub move_idx: MoveIdx,
 }
 
 impl PrimitiveInstance {
-    const ATTRIBS: [VertexAttribute; 7] = vertex_attr_array![
+    // The region's four scalars, each a `Rel` beside a `Px`: whole counts
-        0 => Float32x2,
+    // that the shader decodes, rather than the numbers themselves.
-        1 => Float32x2,
+    const ATTRIBS: [VertexAttribute; 6] = vertex_attr_array![
-        2 => Float32x2,
+        0 => Sint32x2,
-        3 => Float32x2,
+        1 => Sint32x2,
+        2 => Sint32x2,
+        3 => Sint32x2,
         4 => Uint32,
         5 => Uint32,
-        6 => Uint32,
     ];
 
     pub fn desc() -> VertexBufferLayout<'static> {
@@ -47,4 +46,45 @@ impl MaskIdx {
 #[derive(Debug, Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct Mask {
     pub region: UiRegion,
+    pub move_idx: MoveIdx,
+}
+
+/// Its own type rather than another `Id<u32>`, because it sits beside
+/// `MaskIdx` in an instance and the two must not be swappable.
+#[repr(transparent)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, bytemuck::Pod, bytemuck::Zeroable)]
+pub struct MoveIdx(u32);
+
+impl MoveIdx {
+    pub const NONE: Self = Self(u32::MAX);
+
+    pub(crate) fn slot(idx: usize) -> Self {
+        Self(idx as u32)
+    }
+
+    pub(crate) fn idx(self) -> usize {
+        self.0 as usize
+    }
+}
+
+/// One link of the chain a primitive's position is resolved through: the box
+/// its contents are placed within, given in the coordinates of the slot it
+/// names. Moving or resizing a subtree writes its own slot and nothing else.
+///
+/// The identity is `UiRegion::FULL`, not zero: a zeroed entry is a box of no
+/// extent, which collapses everything under it to a point.
+#[repr(C)]
+#[derive(Debug, Copy, Clone)]
+pub struct MoveOffset {
+    pub region: UiRegion,
+    pub parent: MoveIdx,
+}
+
+unsafe impl bytemuck::Pod for MoveOffset {}
+unsafe impl bytemuck::Zeroable for MoveOffset {}
+
+impl MoveOffset {
+    pub fn new(parent: MoveIdx, region: UiRegion) -> Self {
+        Self { region, parent }
+    }
 }

core/src/render/mod.rs

@@ -1,8 +1,6 @@
-use std::num::NonZero;
-
 use crate::{
     UiData, UiRenderState,
-    render::{data::PrimitiveInstance, texture::GpuTextures, util::ArrBuf},
+    render::{data::PrimitiveInstance, util::ArrBuf},
     util::{HashMap, Vec2},
 };
 use data::WindowUniform;
@@ -13,50 +11,93 @@ use wgpu::{
 
 mod atlas;
 mod data;
+mod page;
 mod primitive;
 mod texture;
 mod util;
 
 pub use atlas::*;
-pub use data::{Mask, MaskIdx};
+pub use data::{Mask, MaskIdx, MoveIdx, MoveOffset};
 pub use primitive::*;
 
-const SHAPE_SHADER: &str = include_str!("./shader.wgsl");
+const PRELUDE: &str = include_str!("./shader/prelude.wgsl");
+
+fn module_source(wgsl: &str) -> String {
+    // Every number both sides count in, written once here rather than a
+    // second time in the shader: a grid the two disagree about puts every
+    // coordinate somewhere else, and a sentinel they disagree about makes one
+    // of them walk a chain from a slot the other says is not there.
+    format!(
+        "const PX_STEP: f32 = 1.0 / {}.0;\n\
+         const REL_STEP: f32 = 1.0 / {}.0;\n\
+         const MASK_NONE: u32 = {}u;\n\
+         const MOVE_NONE: u32 = {}u;\n\
+         const CHAIN_LIMIT: u32 = {}u;\n\
+         {PRELUDE}\n{wgsl}",
+        1u32 << crate::PX_SHIFT,
+        1u32 << crate::REL_SHIFT,
+        MaskIdx::NONE.idx(),
+        MoveIdx::NONE.idx(),
+        crate::CHAIN_LIMIT,
+    )
+}
 
 pub struct UiRenderNode {
-    uniform_group: BindGroup,
+    shared_layout: BindGroupLayout,
-    primitive_layout: BindGroupLayout,
+    shared_group: BindGroup,
-    rsc_layout: BindGroupLayout,
+    format: TextureFormat,
-    rsc_group: BindGroup,
 
-    pipeline: RenderPipeline,
+    /// One per registered primitive, in id order.
+    primitives: Vec<PrimitivePipeline>,
 
     layers: HashMap<usize, RenderLayer>,
     active: Vec<usize>,
     window_buffer: Buffer,
-    textures: GpuTextures,
     masks: ArrBuf<Mask>,
+    moves: ArrBuf<MoveOffset>,
 }
 
 struct RenderLayer {
+    /// One per registered primitive, `None` where this layer draws none.
+    primitives: Vec<Option<ListBuffers>>,
+}
+
+/// What draws one registered primitive.
+struct PrimitivePipeline {
+    data_layout: BindGroupLayout,
+    pipeline: RenderPipeline,
+    render: Box<dyn PrimitiveRender>,
+}
+
+/// One list's vertex buffer and the data its shader reads.
+struct ListBuffers {
     instance: ArrBuf<PrimitiveInstance>,
-    primitives: PrimitiveBuffers,
+    data: ArrBuf<u8>,
-    primitive_group: BindGroup,
+    group: Option<BindGroup>,
+    /// What the primitive asked to keep per instance, if anything.
+    bindings: Vec<u32>,
 }
 
 impl UiRenderNode {
     pub fn draw<'a>(&'a self, pass: &mut RenderPass<'a>) {
-        pass.set_pipeline(&self.pipeline);
+        pass.set_bind_group(0, &self.shared_group, &[]);
-        pass.set_bind_group(0, &self.uniform_group, &[]);
-        pass.set_bind_group(2, &self.rsc_group, &[]);
         for i in &self.active {
             let layer = &self.layers[i];
-            if layer.instance.len() == 0 {
+            for (id, list) in layer.primitives.iter().enumerate() {
-                continue;
+                let Some(list) = list else { continue };
+                let Some(group) = &list.group else { continue };
+                let primitive = &self.primitives[id];
+                pass.set_pipeline(&primitive.pipeline);
+                pass.set_bind_group(1, group, &[]);
+                pass.set_vertex_buffer(0, list.instance.buffer.slice(..));
+                primitive.render.draw(
+                    pass,
+                    ListDraw {
+                        instances: list.instance.len() as u32,
+                        bindings: &list.bindings,
+                    },
+                );
             }
-            pass.set_bind_group(1, &layer.primitive_group, &[]);
-            pass.set_vertex_buffer(0, layer.instance.buffer.slice(..));
-            pass.draw(0..4, 0..layer.instance.len() as u32);
         }
     }
 
@@ -67,81 +108,76 @@ impl UiRenderNode {
         ui: &mut UiData,
         ui_render: &mut UiRenderState,
     ) {
+        // Before the layers: each list is given its pipeline's data layout.
+        self.build_pipelines(device, queue, &ui.primitives);
         self.active.clear();
-        for (i, primitives) in ui_render.layers.iter_mut() {
+        for (i, draws) in ui_render.layers.iter_mut() {
             self.active.push(i);
-            for change in primitives.apply_free() {
+            for change in draws.apply_free() {
                 if let Some(inst) = ui_render.active.get_mut(&change.id) {
-                    for h in &mut inst.primitives {
+                    for primitive in &mut inst.primitives {
-                        if h.layer == i && h.inst_idx == change.old {
+                        let h = &mut primitive.handle;
+                        if h.layer == i && h.kind == change.kind && h.inst_idx == change.old {
                             h.inst_idx = change.new;
                             break;
                         }
                     }
                 }
             }
-            let rlayer = self.layers.entry(i).or_insert_with(|| {
+            let rlayer = self.layers.entry(i).or_insert_with(RenderLayer::new);
-                let primitives = PrimitiveBuffers::new(device);
+            if draws.updated {
-                let primitive_group =
+                let lists = draws.primitives();
-                    Self::primitive_group(device, &self.primitive_layout, primitives.buffers());
+                // The zip would otherwise skip a list with no pipeline.
-                RenderLayer {
+                assert!(lists.len() <= self.primitives.len());
-                    instance: ArrBuf::new(
+                rlayer.primitives.resize_with(lists.len(), || None);
-                        device,
+                for ((buffers, list), primitive) in rlayer
-                        BufferUsages::VERTEX | BufferUsages::COPY_DST,
+                    .primitives
-                        "instance",
+                    .iter_mut()
-                    ),
+                    .zip(lists)
-                    primitives,
+                    .zip(&self.primitives)
-                    primitive_group,
+                {
+                    let Some(list) = list else {
+                        continue;
+                    };
+                    buffers
+                        .get_or_insert_with(|| ListBuffers::new(device))
+                        .update(device, queue, primitive, list);
                 }
-            });
+                draws.updated = false;
-            if primitives.updated {
-                rlayer
-                    .instance
-                    .update(device, queue, primitives.instances());
-                rlayer.primitives.update(device, queue, primitives.data());
-                rlayer.primitive_group = Self::primitive_group(
-                    device,
-                    &self.primitive_layout,
-                    rlayer.primitives.buffers(),
-                );
-                primitives.updated = false;
             }
         }
-        let mut changed = false;
+        for primitive in &mut self.primitives {
-        changed |= self.textures.update(&mut ui.textures);
+            primitive.render.update(ui);
+        }
+        let mut regroup = false;
         if ui.masks.changed {
             ui.masks.changed = false;
-            self.masks.update(device, queue, &ui.masks[..]);
+            regroup |= self.masks.update(device, queue, &ui.masks[..]);
-            changed = true;
         }
-        if changed {
+        if ui_render.moves.changed {
-            self.rsc_group = Self::rsc_group(device, &self.rsc_layout, &self.textures, &self.masks);
+            ui_render.moves.changed = false;
+            regroup |= self.moves.update(device, queue, ui_render.moves.entries());
+        }
+        if regroup {
+            self.shared_group = Self::shared_group(
+                device,
+                &self.shared_layout,
+                &self.window_buffer,
+                &self.masks,
+                &self.moves,
+            );
         }
     }
 
     pub fn resize(&mut self, size: impl Into<Vec2>, queue: &Queue) {
         let size = size.into();
-        let slice = &[WindowUniform {
+        let slice = &[WindowUniform { dim: size }];
-            width: size.x,
-            height: size.y,
-        }];
         queue.write_buffer(&self.window_buffer, 0, bytemuck::cast_slice(slice));
     }
 
-    pub fn new(
+    pub fn new(device: &Device, config: &SurfaceConfiguration) -> Self {
-        device: &Device,
-        queue: &Queue,
-        config: &SurfaceConfiguration,
-        limits: UiLimits,
-    ) -> Self {
-        let shader = device.create_shader_module(ShaderModuleDescriptor {
-            label: Some("UI Shape Shader"),
-            source: ShaderSource::Wgsl(SHAPE_SHADER.into()),
-        });
-
         let window_uniform = WindowUniform {
-            width: config.width as f32,
+            dim: Vec2::new(config.width as f32, config.height as f32),
-            height: config.height as f32,
         };
         let window_buffer = device.create_buffer_init(&BufferInitDescriptor {
             label: Some("window"),
@@ -149,67 +185,80 @@ impl UiRenderNode {
             usage: BufferUsages::UNIFORM | BufferUsages::COPY_DST,
         });
 
-        let uniform_layout = device.create_bind_group_layout(&BindGroupLayoutDescriptor {
+        let shared_layout = Self::shared_layout(device);
-            entries: &[BindGroupLayoutEntry {
-                binding: 0,
-                visibility: ShaderStages::VERTEX | ShaderStages::FRAGMENT,
-                ty: BindingType::Buffer {
-                    ty: BufferBindingType::Uniform,
-                    has_dynamic_offset: false,
-                    min_binding_size: None,
-                },
-                count: None,
-            }],
-            label: Some("window"),
-        });
-
-        let uniform_group = Self::bind_group_0(device, &uniform_layout, &window_buffer);
-
-        let primitive_layout = device.create_bind_group_layout(&BindGroupLayoutDescriptor {
-            entries: &core::array::from_fn::<_, { PrimitiveBuffers::LEN }, _>(|i| {
-                BindGroupLayoutEntry {
-                    binding: i as u32,
-                    visibility: ShaderStages::FRAGMENT,
-                    ty: BindingType::Buffer {
-                        ty: BufferBindingType::Storage { read_only: true },
-                        has_dynamic_offset: false,
-                        min_binding_size: None,
-                    },
-                    count: None,
-                }
-            }),
-            label: Some("primitive"),
-        });
-
-        let tex_manager = GpuTextures::new(device, queue);
         let masks = ArrBuf::new(
             device,
             BufferUsages::STORAGE | BufferUsages::COPY_DST,
             "ui masks",
         );
+        let moves = ArrBuf::new(
+            device,
+            BufferUsages::STORAGE | BufferUsages::COPY_DST,
+            "ui move offsets",
+        );
+        let shared_group =
+            Self::shared_group(device, &shared_layout, &window_buffer, &masks, &moves);
 
-        let rsc_layout = Self::rsc_layout(device, &limits);
+        Self {
-        let rsc_group = Self::rsc_group(device, &rsc_layout, &tex_manager, &masks);
+            shared_layout,
+            shared_group,
+            format: config.format,
+            primitives: Vec::new(),
+            window_buffer,
+            layers: HashMap::default(),
+            active: Vec::new(),
+            masks,
+            moves,
+        }
+    }
 
-        let pipeline_layout = device.create_pipeline_layout(&PipelineLayoutDescriptor {
+    /// Compiles a pipeline for every primitive registered since the last call.
-            label: Some("UI Shape Pipeline Layout"),
+    /// Sources only ever arrive at the end, so an id keeps its pipeline.
-            bind_group_layouts: &[&uniform_layout, &primitive_layout, &rsc_layout],
+    fn build_pipelines(&mut self, device: &Device, queue: &Queue, registry: &PrimitiveRegistry) {
+        for source in &registry.sources()[self.primitives.len()..] {
+            let render = (source.render)(device, queue);
+            let data_layout = Self::data_layout(device, source.stride);
+            let mut groups = vec![Some(&self.shared_layout), Some(&data_layout)];
+            groups.extend(render.layout().map(Some));
+            let layout = device.create_pipeline_layout(&PipelineLayoutDescriptor {
+                label: Some(source.label),
+                bind_group_layouts: &groups,
                 immediate_size: 0,
             });
-        let pipeline = device.create_render_pipeline(&RenderPipelineDescriptor {
+            let pipeline = Self::pipeline(device, &layout, self.format, source.wgsl, source.label);
-            label: Some("UI Shape Pipeline"),
+            self.primitives.push(PrimitivePipeline {
-            layout: Some(&pipeline_layout),
+                data_layout,
+                pipeline,
+                render,
+            });
+        }
+    }
+
+    fn pipeline(
+        device: &Device,
+        layout: &PipelineLayout,
+        format: TextureFormat,
+        wgsl: &str,
+        label: &str,
+    ) -> RenderPipeline {
+        let module = device.create_shader_module(ShaderModuleDescriptor {
+            label: Some(label),
+            source: ShaderSource::Wgsl(module_source(wgsl).into()),
+        });
+        device.create_render_pipeline(&RenderPipelineDescriptor {
+            label: Some(label),
+            layout: Some(layout),
             vertex: VertexState {
-                module: &shader,
+                module: &module,
                 entry_point: Some("vs_main"),
-                buffers: &[PrimitiveInstance::desc()],
+                buffers: &[Some(PrimitiveInstance::desc())],
                 compilation_options: Default::default(),
             },
             fragment: Some(FragmentState {
-                module: &shader,
+                module: &module,
                 entry_point: Some("fs_main"),
                 targets: &[Some(ColorTargetState {
-                    format: config.format,
+                    format,
                     blend: Some(BlendState::ALPHA_BLENDING),
                     write_mask: ColorWrites::ALL,
                 })],
@@ -232,136 +281,176 @@ impl UiRenderNode {
             },
             multiview_mask: None,
             cache: None,
-        });
-
-        Self {
-            uniform_group,
-            primitive_layout,
-            rsc_layout,
-            rsc_group,
-            pipeline,
-            window_buffer,
-            layers: HashMap::default(),
-            active: Vec::new(),
-            textures: tex_manager,
-            masks,
-        }
-    }
-
-    fn bind_group_0(
-        device: &Device,
-        layout: &BindGroupLayout,
-        window_buffer: &Buffer,
-    ) -> BindGroup {
-        device.create_bind_group(&BindGroupDescriptor {
-            layout,
-            entries: &[BindGroupEntry {
-                binding: 0,
-                resource: window_buffer.as_entire_binding(),
-            }],
-            label: Some("ui window"),
         })
     }
 
-    fn primitive_group(
+    /// What every draw in the ui is given: the window, the masks and the
-        device: &Device,
+    /// move chain every position is resolved through.
-        layout: &BindGroupLayout,
+    fn shared_layout(device: &Device) -> BindGroupLayout {
-        buffers: [(u32, &Buffer); PrimitiveBuffers::LEN],
-    ) -> BindGroup {
-        device.create_bind_group(&BindGroupDescriptor {
-            layout,
-            entries: &buffers.map(|(binding, buf)| BindGroupEntry {
-                binding,
-                resource: buf.as_entire_binding(),
-            }),
-            label: Some("ui primitives"),
-        })
-    }
-
-    fn rsc_layout(device: &Device, limits: &UiLimits) -> BindGroupLayout {
         device.create_bind_group_layout(&BindGroupLayoutDescriptor {
             entries: &[
                 BindGroupLayoutEntry {
                     binding: 0,
-                    visibility: ShaderStages::FRAGMENT,
+                    visibility: ShaderStages::VERTEX | ShaderStages::FRAGMENT,
-                    ty: BindingType::Texture {
+                    ty: BindingType::Buffer {
-                        sample_type: TextureSampleType::Float { filterable: false },
+                        ty: BufferBindingType::Uniform,
-                        view_dimension: TextureViewDimension::D2,
+                        has_dynamic_offset: false,
-                        multisampled: false,
+                        min_binding_size: BufferSize::new(size_of::<WindowUniform>() as u64),
                     },
-                    count: Some(NonZero::new(limits.max_textures).unwrap()),
+                    count: None,
                 },
                 BindGroupLayoutEntry {
                     binding: 1,
                     visibility: ShaderStages::FRAGMENT,
-                    ty: BindingType::Sampler(SamplerBindingType::NonFiltering),
-                    count: Some(NonZero::new(limits.max_samplers).unwrap()),
-                },
-                BindGroupLayoutEntry {
-                    binding: 2,
-                    visibility: ShaderStages::FRAGMENT,
                     ty: BindingType::Buffer {
                         ty: BufferBindingType::Storage { read_only: true },
                         has_dynamic_offset: false,
-                        min_binding_size: None,
+                        min_binding_size: BufferSize::new(size_of::<Mask>() as u64),
+                    },
+                    count: None,
+                },
+                BindGroupLayoutEntry {
+                    binding: 2,
+                    visibility: ShaderStages::VERTEX | ShaderStages::FRAGMENT,
+                    ty: BindingType::Buffer {
+                        ty: BufferBindingType::Storage { read_only: true },
+                        has_dynamic_offset: false,
+                        min_binding_size: BufferSize::new(size_of::<MoveOffset>() as u64),
                     },
                     count: None,
                 },
             ],
-            label: Some("ui rsc"),
+            label: Some("ui shared"),
         })
     }
 
-    fn rsc_group(
+    fn shared_group(
         device: &Device,
         layout: &BindGroupLayout,
-        tex_manager: &GpuTextures,
+        window: &Buffer,
         masks: &ArrBuf<Mask>,
+        moves: &ArrBuf<MoveOffset>,
     ) -> BindGroup {
         device.create_bind_group(&BindGroupDescriptor {
             layout,
             entries: &[
                 BindGroupEntry {
                     binding: 0,
-                    resource: BindingResource::TextureViewArray(&tex_manager.views()),
+                    resource: window.as_entire_binding(),
                 },
                 BindGroupEntry {
                     binding: 1,
-                    resource: BindingResource::SamplerArray(&tex_manager.samplers()),
+                    resource: masks.buffer.as_entire_binding(),
                 },
                 BindGroupEntry {
                     binding: 2,
-                    resource: masks.buffer.as_entire_binding(),
+                    resource: moves.buffer.as_entire_binding(),
                 },
             ],
-            label: Some("ui rsc"),
+            label: Some("ui shared"),
         })
     }
 
-    pub fn view_count(&self) -> usize {
+    /// Layout for a list of one primitive's data. Every size in the ui is
-        self.textures.view_count()
+    /// stated, so "is the buffer big enough for one entry?" is answered when
+    /// the bind group is made; a `None` size is wgpu's to check on every draw.
+    fn data_layout(device: &Device, stride: u64) -> BindGroupLayout {
+        device.create_bind_group_layout(&BindGroupLayoutDescriptor {
+            entries: &[BindGroupLayoutEntry {
+                binding: 0,
+                visibility: ShaderStages::FRAGMENT,
+                ty: BindingType::Buffer {
+                    ty: BufferBindingType::Storage { read_only: true },
+                    has_dynamic_offset: false,
+                    min_binding_size: BufferSize::new(stride),
+                },
+                count: None,
+            }],
+            label: Some("ui primitive data"),
+        })
     }
 }
 
-pub struct UiLimits {
+impl RenderLayer {
-    max_textures: u32,
+    fn new() -> Self {
-    max_samplers: u32,
-}
-
-impl Default for UiLimits {
-    fn default() -> Self {
         Self {
-            max_textures: 100000,
+            primitives: Vec::new(),
-            max_samplers: 1000,
         }
     }
 }
 
-impl UiLimits {
+impl ListBuffers {
-    pub fn max_binding_array_elements_per_shader_stage(&self) -> u32 {
+    fn new(device: &Device) -> Self {
-        self.max_textures + self.max_samplers
+        Self {
+            instance: ArrBuf::new(
+                device,
+                BufferUsages::VERTEX | BufferUsages::COPY_DST,
+                "instance",
+            ),
+            data: ArrBuf::new(
+                device,
+                BufferUsages::STORAGE | BufferUsages::COPY_DST,
+                "primitive data",
+            ),
+            group: None,
+            bindings: Vec::new(),
         }
-    pub fn max_binding_array_sampler_elements_per_shader_stage(&self) -> u32 {
+    }
-        self.max_samplers
+
+    fn update(
+        &mut self,
+        device: &Device,
+        queue: &Queue,
+        primitive: &PrimitivePipeline,
+        list: &InstanceList,
+    ) {
+        self.bindings.clear();
+        primitive.render.instance_bindings(list, &mut self.bindings);
+        self.instance.update(device, queue, list.instances());
+        let resized = self.data.update(device, queue, list.data());
+        if list.instances().is_empty() {
+            self.group = None;
+        } else if resized || self.group.is_none() {
+            self.group = Some(device.create_bind_group(&BindGroupDescriptor {
+                layout: &primitive.data_layout,
+                entries: &[BindGroupEntry {
+                    binding: 0,
+                    resource: self.data.buffer.as_entire_binding(),
+                }],
+                label: Some("ui primitive data"),
+            }));
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::module_source;
+    use wgpu::naga::{
+        front::wgsl,
+        valid::{Capabilities, ValidationFlags, Validator},
+    };
+
+    /// Every shader file, composed as the renderer composes it, parses and
+    /// validates with no device -- so an edit that breaks one fails here and
+    /// not in the first window opened.
+    #[test]
+    fn every_shader_validates() {
+        let dir = concat!(env!("CARGO_MANIFEST_DIR"), "/src/render/shader");
+        let mut checked = 0;
+        for entry in std::fs::read_dir(dir).unwrap() {
+            let path = entry.unwrap().path();
+            if path.extension().is_none_or(|e| e != "wgsl") || path.ends_with("prelude.wgsl") {
+                continue;
+            }
+            let source = module_source(&std::fs::read_to_string(&path).unwrap());
+            let module = wgsl::parse_str(&source)
+                .unwrap_or_else(|e| panic!("{}: {}", path.display(), e.emit_to_string(&source)));
+            Validator::new(ValidationFlags::all(), Capabilities::all())
+                .validate(&module)
+                .unwrap_or_else(|e| panic!("{}: {e:?}", path.display()));
+            checked += 1;
+        }
+        assert!(checked > 0, "no shaders found in {dir}");
     }
 }

core/src/render/page.rs

@@ -0,0 +1,156 @@
+use wgpu::*;
+
+use crate::{GlyphAtlas, UiData};
+
+use super::{
+    atlas::PAGE,
+    primitive::{ListDraw, PrimitiveRender},
+    texture::{default_sampler, sampled_group, sampled_layout, write_region},
+};
+
+/// Draws glyphs from the atlas, which it owns: one array texture bound once
+/// for a whole list, since every glyph in it reads the same pages.
+pub struct GlyphRender {
+    pages: GpuPages,
+    layout: BindGroupLayout,
+    sampler: Sampler,
+}
+
+impl GlyphRender {
+    pub fn new(device: &Device, queue: &Queue) -> Self {
+        let layout = sampled_layout(device, TextureViewDimension::D2Array, "ui atlas");
+        let sampler = default_sampler(device);
+        Self {
+            pages: GpuPages::new(device, queue, &layout, &sampler),
+            layout,
+            sampler,
+        }
+    }
+}
+
+impl PrimitiveRender for GlyphRender {
+    fn layout(&self) -> Option<&BindGroupLayout> {
+        Some(&self.layout)
+    }
+
+    fn update(&mut self, ui: &mut UiData) {
+        self.pages
+            .update(&mut ui.text.atlas, &self.layout, &self.sampler);
+    }
+
+    fn draw<'a>(&'a self, pass: &mut RenderPass<'a>, list: ListDraw<'a>) {
+        pass.set_bind_group(2, self.pages.group(), &[]);
+        pass.draw(0..4, 0..list.instances);
+    }
+}
+
+/// The glyph atlas on the GPU: one array texture whose layers are the pages
+/// `GlyphAtlas` packs.
+///
+/// One array rather than a texture per page because a layer index is ordinary
+/// Vulkan 1.0 / GLES sampling, where a `binding_array` would need
+/// `VK_EXT_descriptor_indexing`, which a real share of Android GPUs lack.
+pub struct GpuPages {
+    device: Device,
+    queue: Queue,
+    texture: Texture,
+    group: BindGroup,
+}
+
+impl GpuPages {
+    pub fn new(
+        device: &Device,
+        queue: &Queue,
+        layout: &BindGroupLayout,
+        sampler: &Sampler,
+    ) -> Self {
+        let texture = create_array(device, 1);
+        Self {
+            device: device.clone(),
+            queue: queue.clone(),
+            group: atlas_group(device, layout, &texture, sampler),
+            texture,
+        }
+    }
+
+    pub fn update(&mut self, atlas: &mut GlyphAtlas, layout: &BindGroupLayout, sampler: &Sampler) {
+        if atlas.page_count() > self.texture.depth_or_array_layers() {
+            self.grow(atlas.page_count(), layout, sampler);
+        }
+        for (upload, page) in atlas.uploads() {
+            let dst = TexelCopyTextureInfo {
+                texture: &self.texture,
+                mip_level: 0,
+                origin: Origin3d {
+                    x: upload.rect.x,
+                    y: upload.rect.y,
+                    z: upload.layer,
+                },
+                aspect: TextureAspect::All,
+            };
+            write_region(&self.queue, dst, page, upload.rect);
+        }
+    }
+
+    pub fn group(&self) -> &BindGroup {
+        &self.group
+    }
+
+    /// Doubles until `needed` fits and copies the old layers across GPU side.
+    /// The new texture stales the group, so that is rebuilt here.
+    fn grow(&mut self, needed: u32, layout: &BindGroupLayout, sampler: &Sampler) {
+        let old = self.texture.depth_or_array_layers();
+        let mut layers = old;
+        while layers < needed {
+            layers *= 2;
+        }
+        let texture = create_array(&self.device, layers);
+        let mut encoder = self
+            .device
+            .create_command_encoder(&CommandEncoderDescriptor {
+                label: Some("atlas grow"),
+            });
+        encoder.copy_texture_to_texture(
+            self.texture.as_image_copy(),
+            texture.as_image_copy(),
+            Extent3d {
+                width: PAGE,
+                height: PAGE,
+                depth_or_array_layers: old,
+            },
+        );
+        self.queue.submit(std::iter::once(encoder.finish()));
+        self.group = atlas_group(&self.device, layout, &texture, sampler);
+        self.texture = texture;
+    }
+}
+
+fn atlas_group(
+    device: &Device,
+    layout: &BindGroupLayout,
+    texture: &Texture,
+    sampler: &Sampler,
+) -> BindGroup {
+    let view = texture.create_view(&TextureViewDescriptor {
+        dimension: Some(TextureViewDimension::D2Array),
+        ..Default::default()
+    });
+    sampled_group(device, layout, &view, sampler, "ui atlas")
+}
+
+fn create_array(device: &Device, layers: u32) -> Texture {
+    device.create_texture(&TextureDescriptor {
+        label: Some("glyph atlas"),
+        size: Extent3d {
+            width: PAGE,
+            height: PAGE,
+            depth_or_array_layers: layers,
+        },
+        mip_level_count: 1,
+        sample_count: 1,
+        dimension: TextureDimension::D2,
+        format: TextureFormat::Rgba8Unorm,
+        usage: TextureUsages::TEXTURE_BINDING | TextureUsages::COPY_DST | TextureUsages::COPY_SRC,
+        view_formats: &[],
+    })
+}

core/src/render/primitive.rs

@@ -1,181 +1,321 @@
-use std::ops::{Deref, DerefMut};
+use std::{any::TypeId, marker::PhantomData};
 
 use crate::{
-    Color, UiRegion, WidgetId,
+    Color, TextureHandle, UiData, UiRegion, WidgetId,
     render::{
-        ArrBuf,
+        data::{MaskIdx, MoveIdx, PrimitiveInstance},
-        data::{MaskIdx, PrimitiveInstance},
+        page::GlyphRender,
+        texture::ImageRender,
     },
-    util::Vec2,
+    util::{HashMap, Vec2},
 };
 use bytemuck::Pod;
-use wgpu::*;
+use wgpu::{BindGroupLayout, Device, Queue, RenderPass};
 
-pub struct Primitives {
+/// One instance of a primitive, laid out as the struct its shader reads.
-    instances: Vec<PrimitiveInstance>,
+///
-    assoc: Vec<WidgetId>,
+/// The type carries its own shader, so drawing one is all the wiring it needs:
-    data: PrimitiveData,
+/// its list, free list, buffers and pipeline follow from being registered.
-    free: Vec<usize>,
+pub trait Primitive: Pod + 'static {
-    pub updated: bool,
+    /// Compiled after `prelude.wgsl`, which states what it declares and what
+    /// it is given.
+    const WGSL: &'static str;
+
+    /// Made once, the first time the renderer sees this primitive. It owns
+    /// whatever the shader samples and records the primitive's own draws; the
+    /// default owns nothing and draws every instance in one call.
+    fn render(device: &Device, queue: &Queue) -> Box<dyn PrimitiveRender>
+    where
+        Self: Sized,
+    {
+        let _ = (device, queue);
+        Box::new(Instanced)
+    }
 }
 
-impl Default for Primitives {
+/// The renderer's half of a primitive: what it samples, what it uploads, and
-    fn default() -> Self {
+/// what draws it records.
+///
+/// Everything a draw shares -- the pipeline, the window and masks, the list's
+/// own data and instance buffer -- is set before this is called. What is left
+/// is what only this primitive knows: its group 2, and how many draws its
+/// instances are.
+pub trait PrimitiveRender {
+    /// The layout its shader reads at group 2. `None` for a primitive whose
+    /// shader samples nothing, whose pipeline then has no group 2 at all.
+    fn layout(&self) -> Option<&BindGroupLayout> {
+        None
+    }
+
+    /// Uploads whatever this primitive owns, once a frame, before any draw.
+    fn update(&mut self, ui: &mut UiData) {
+        let _ = ui;
+    }
+
+    /// Keeps what the primitive needs per instance at draw time, read from
+    /// the list's own data. A primitive that binds nothing per instance --
+    /// most of them -- leaves this empty and draws in one call.
+    fn instance_bindings(&self, list: &InstanceList, out: &mut Vec<u32>) {
+        let _ = (list, out);
+    }
+
+    fn draw<'a>(&'a self, pass: &mut RenderPass<'a>, list: ListDraw<'a>);
+}
+
+/// What a `PrimitiveRender` draws: this list's instances, and whatever
+/// `instance_bindings` kept for them.
+pub struct ListDraw<'a> {
+    pub instances: u32,
+    pub bindings: &'a [u32],
+}
+
+/// The default: nothing sampled, every instance in one call.
+pub struct Instanced;
+
+impl PrimitiveRender for Instanced {
+    fn draw<'a>(&'a self, pass: &mut RenderPass<'a>, list: ListDraw<'a>) {
+        pass.draw(0..4, 0..list.instances);
+    }
+}
+
+/// Which registered primitive an instance is.
+pub struct PrimitiveKind<P> {
+    id: u32,
+    _p: PhantomData<fn(P)>,
+}
+
+impl<P> PrimitiveKind<P> {
+    fn new(id: u32) -> Self {
         Self {
-            instances: Default::default(),
-            assoc: Default::default(),
-            data: Default::default(),
-            free: Vec::new(),
-            updated: true,
-        }
-    }
-}
-
-pub trait Primitive: Pod {
-    const BINDING: u32;
-    fn vec(data: &mut PrimitiveData) -> &mut PrimitiveVec<Self>;
-}
-
-macro_rules! primitives {
-    ($($name:ident: $ty:ty => $binding:expr,)*) => {
-        #[derive(Default)]
-        pub struct PrimitiveData {
-            $(pub(crate) $name: PrimitiveVec<$ty>,)*
-        }
-
-        pub struct PrimitiveBuffers {
-            $($name: ArrBuf<$ty>,)*
-        }
-
-        impl PrimitiveBuffers {
-            pub fn update(&mut self, device: &Device, queue: &Queue, data: &PrimitiveData) {
-                $(self.$name.update(device, queue, &data.$name);)*
-            }
-        }
-
-        impl PrimitiveBuffers {
-            pub const LEN: usize = primitives!(@count $($name)*);
-            pub fn buffers(&self) -> [(u32, &Buffer); Self::LEN] {
-                [
-                    $((<$ty>::BINDING, &self.$name.buffer),)*
-                ]
-            }
-            pub fn new(device: &Device) -> Self {
-                Self {
-                    $($name: ArrBuf::new(
-                        device,
-                        BufferUsages::STORAGE | BufferUsages::COPY_DST,
-                        stringify!($name),
-                    ),)*
-                }
-            }
-        }
-
-        impl PrimitiveData {
-            pub fn clear(&mut self) {
-                $(self.$name.clear();)*
-            }
-            pub fn free(&mut self, binding: u32, idx: usize) {
-                match binding {
-                    $(<$ty>::BINDING => self.$name.free(idx),)*
-                    _ => unreachable!()
-                }
-            }
-        }
-
-        $(
-            unsafe impl bytemuck::Pod for $ty {}
-            unsafe impl bytemuck::Zeroable for $ty {}
-            impl Primitive for $ty {
-                const BINDING: u32 = $binding;
-                fn vec(data: &mut PrimitiveData) -> &mut PrimitiveVec<Self> {
-                    &mut data.$name
-                }
-            }
-        )*
-    };
-    (@count $t1:tt $($t:tt)+) => { 1 + primitives!(@count $($t)+) };
-    (@count $t:tt) => { 1 };
-}
-
-pub struct PrimitiveInst<P> {
-    pub id: WidgetId,
-    pub primitive: P,
-    pub region: UiRegion,
-    pub mask_idx: MaskIdx,
-}
-
-impl Primitives {
-    pub fn write<P: Primitive>(
-        &mut self,
-        layer: usize,
-        PrimitiveInst {
             id,
-            primitive,
+            _p: PhantomData,
-            region,
+        }
-            mask_idx,
+    }
-        }: PrimitiveInst<P>,
+}
-    ) -> PrimitiveHandle {
+
-        self.updated = true;
+impl<P> Clone for PrimitiveKind<P> {
-        let vec = P::vec(&mut self.data);
+    fn clone(&self) -> Self {
-        let i = vec.add(primitive);
+        *self
-        let inst = PrimitiveInstance {
+    }
-            region,
+}
-            idx: i as u32,
+
-            mask_idx,
+impl<P> Copy for PrimitiveKind<P> {}
-            binding: P::BINDING,
+
-        };
+/// Every primitive a ui can draw, in the order they were first drawn.
-        let inst_i = if let Some(i) = self.free.pop() {
+#[derive(Default)]
+pub struct PrimitiveRegistry {
+    kinds: Vec<PrimitiveSource>,
+    ids: HashMap<TypeId, u32>,
+}
+
+pub struct PrimitiveSource {
+    pub wgsl: &'static str,
+    pub label: &'static str,
+    /// Size of one instance's entry, stated as the data binding's minimum.
+    pub stride: u64,
+    pub render: fn(&Device, &Queue) -> Box<dyn PrimitiveRender>,
+}
+
+impl PrimitiveRegistry {
+    /// Registers `P` if this is the first time it has been drawn.
+    pub fn kind<P: Primitive>(&mut self) -> PrimitiveKind<P> {
+        let Self { kinds, ids } = self;
+        let id = *ids.entry(TypeId::of::<P>()).or_insert_with(|| {
+            kinds.push(PrimitiveSource {
+                wgsl: P::WGSL,
+                label: std::any::type_name::<P>(),
+                stride: size_of::<P>() as u64,
+                render: P::render,
+            });
+            kinds.len() as u32 - 1
+        });
+        PrimitiveKind::new(id)
+    }
+
+    pub fn sources(&self) -> &[PrimitiveSource] {
+        &self.kinds
+    }
+}
+
+/// One registered primitive's instances in one layer. Everything per-instance
+/// rides here, so it stays in step through a `swap_remove`.
+pub struct InstanceList {
+    instances: Vec<PrimitiveInstance>,
+    /// The widget each instance belongs to, for renumbering its handles.
+    assoc: Vec<WidgetId>,
+    free: Vec<usize>,
+    /// `stride` bytes of the primitive's own data per instance.
+    data: Vec<u8>,
+    /// From the type the list was made for, so a write is never checked.
+    stride: usize,
+}
+
+impl InstanceList {
+    fn new<P: Primitive>() -> Self {
+        Self {
+            instances: Vec::new(),
+            assoc: Vec::new(),
+            free: Vec::new(),
+            data: Vec::new(),
+            stride: size_of::<P>(),
+        }
+    }
+
+    pub fn instances(&self) -> &[PrimitiveInstance] {
+        &self.instances
+    }
+
+    pub fn data(&self) -> &[u8] {
+        &self.data
+    }
+
+    pub fn stride(&self) -> usize {
+        self.stride
+    }
+
+    fn push(&mut self, id: WidgetId, inst: PrimitiveInstance, data: &[u8]) -> usize {
+        if let Some(i) = self.free.pop() {
             self.instances[i] = inst;
             self.assoc[i] = id;
+            self.data[i * self.stride..][..self.stride].copy_from_slice(data);
             i
         } else {
             let i = self.instances.len();
             self.instances.push(inst);
             self.assoc.push(id);
+            self.data.extend_from_slice(data);
             i
-        };
+        }
-        PrimitiveHandle::new::<P>(layer, inst_i, i)
     }
 
-    /// returns (old index, new index)
+    fn free(&mut self, i: usize) -> MaskIdx {
-    pub fn apply_free(&mut self) -> impl Iterator<Item = PrimitiveChange> {
+        self.free.push(i);
+        self.instances[i].mask_idx
+    }
+
+    fn apply_free(&mut self, kind: u32) -> impl Iterator<Item = PrimitiveChange> {
         self.free.sort_by(|a, b| b.cmp(a));
-        self.free.drain(..).filter_map(|i| {
+        let instances = &mut self.instances;
-            self.instances.swap_remove(i);
+        let assoc = &mut self.assoc;
-            self.assoc.swap_remove(i);
+        let data = &mut self.data;
-            if i == self.instances.len() {
+        let stride = self.stride;
+        self.free.drain(..).filter_map(move |i| {
+            instances.swap_remove(i);
+            assoc.swap_remove(i);
+            let last = instances.len();
+            data.copy_within(last * stride..(last + 1) * stride, i * stride);
+            data.truncate(last * stride);
+            if i == last {
                 return None;
             }
-            let id = self.assoc[i];
+            let id = assoc[i];
-            let old = self.instances.len();
+            Some(PrimitiveChange {
-            Some(PrimitiveChange { id, old, new: i })
+                id,
+                kind,
+                old: last,
+                new: i,
             })
+        })
+    }
+}
+
+/// Everything one layer draws, one list per registered primitive.
+pub struct LayerDraws {
+    /// `None` until this layer draws that primitive, because only the write
+    /// knows the type the list is for.
+    primitives: Vec<Option<InstanceList>>,
+    pub updated: bool,
+}
+
+impl Default for LayerDraws {
+    fn default() -> Self {
+        Self {
+            primitives: Vec::new(),
+            updated: true,
+        }
+    }
+}
+
+impl LayerDraws {
+    pub fn write<P: Primitive>(
+        &mut self,
+        layer: usize,
+        PrimitiveInst {
+            kind,
+            id,
+            primitive,
+            region,
+            mask_idx,
+            move_idx,
+        }: PrimitiveInst<P>,
+    ) -> PrimitiveHandle {
+        self.updated = true;
+        // Grown on first use rather than sized from the registry, which a
+        // layer cannot see.
+        if self.primitives.len() <= kind.id as usize {
+            self.primitives.resize_with(kind.id as usize + 1, || None);
+        }
+        let inst_idx = self.primitives[kind.id as usize]
+            .get_or_insert_with(InstanceList::new::<P>)
+            .push(
+                id,
+                PrimitiveInstance {
+                    region,
+                    mask_idx,
+                    move_idx,
+                },
+                bytemuck::bytes_of(&primitive),
+            );
+        PrimitiveHandle {
+            layer,
+            kind: kind.id,
+            inst_idx,
+        }
+    }
+
+    pub fn primitives(&self) -> &[Option<InstanceList>] {
+        &self.primitives
+    }
+
+    pub fn apply_free(&mut self) -> impl Iterator<Item = PrimitiveChange> {
+        self.primitives
+            .iter_mut()
+            .enumerate()
+            .filter_map(|(kind, list)| Some((kind as u32, list.as_mut()?)))
+            .flat_map(|(kind, list)| list.apply_free(kind))
     }
 
     pub fn free(&mut self, h: &PrimitiveHandle) -> MaskIdx {
         self.updated = true;
-        self.data.free(h.binding, h.data_idx);
+        self.list(h).free(h.inst_idx)
-        self.free.push(h.inst_idx);
-        self.instances[h.inst_idx].mask_idx
-    }
-
-    pub fn data(&self) -> &PrimitiveData {
-        &self.data
-    }
-
-    pub fn instances(&self) -> &Vec<PrimitiveInstance> {
-        &self.instances
     }
 
     pub fn region_mut(&mut self, h: &PrimitiveHandle) -> &mut UiRegion {
         self.updated = true;
-        &mut self.instances[h.inst_idx].region
+        &mut self.list(h).instances[h.inst_idx].region
     }
+
+    /// A handle is only ever made by `write`, which is what created the list.
+    fn list(&mut self, h: &PrimitiveHandle) -> &mut InstanceList {
+        self.primitives[h.kind as usize]
+            .as_mut()
+            .expect("handle names a primitive this layer never drew")
+    }
+}
+
+pub struct PrimitiveInst<P> {
+    pub kind: PrimitiveKind<P>,
+    pub id: WidgetId,
+    pub primitive: P,
+    pub region: UiRegion,
+    pub mask_idx: MaskIdx,
+    pub move_idx: MoveIdx,
 }
 
 pub struct PrimitiveChange {
     pub id: WidgetId,
+    /// Which registered primitive's list moved, since they index separately.
+    pub kind: u32,
     pub old: usize,
     pub new: usize,
 }
@@ -183,30 +323,12 @@ pub struct PrimitiveChange {
 #[derive(Debug)]
 pub struct PrimitiveHandle {
     pub layer: usize,
+    pub kind: u32,
     pub inst_idx: usize,
-    pub data_idx: usize,
-    pub binding: u32,
 }
 
-impl PrimitiveHandle {
-    fn new<P: Primitive>(layer: usize, inst_idx: usize, data_idx: usize) -> Self {
-        Self {
-            layer,
-            inst_idx,
-            data_idx,
-            binding: P::BINDING,
-        }
-    }
-}
-
-primitives!(
-    rects: RectPrimitive => 0,
-    textures: TexturePrimitive => 1,
-    glyphs: GlyphPrimitive => 2,
-);
-
 #[repr(C)]
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct RectPrimitive {
     pub color: Color<u8>,
     pub radius: f32,
@@ -214,6 +336,10 @@ pub struct RectPrimitive {
     pub inner_radius: f32,
 }
 
+impl Primitive for RectPrimitive {
+    const WGSL: &'static str = include_str!("shader/rect.wgsl");
+}
+
 impl RectPrimitive {
     pub fn color(color: Color<u8>) -> Self {
         Self {
@@ -225,71 +351,51 @@ impl RectPrimitive {
     }
 }
 
-#[repr(C)]
+/// `color` is multiplied by the atlas alpha for a mask glyph; a colour glyph
-#[derive(Debug, Copy, Clone)]
+/// takes the texel unchanged, which `GlyphEntry::IS_COLORED` selects.
-pub struct TexturePrimitive {
-    pub view_idx: u32,
-    pub sampler_idx: u32,
-}
-
 #[repr(C)]
 #[derive(Debug, Copy, Clone)]
 pub struct GlyphPrimitive {
     pub uv_min: Vec2,
     pub uv_max: Vec2,
-    pub view_idx: u32,
+    /// Which atlas array layer this glyph is on.
-    pub sampler_idx: u32,
+    pub layer: u32,
     pub color: Color<u8>,
     pub flags: u32,
 }
 
-pub struct PrimitiveVec<T> {
+// Manual rather than derived: `Vec2`'s alignment leaves four bytes of padding
-    vec: Vec<T>,
+// here, which is how WGSL lays the struct out.
-    free: Vec<usize>,
+unsafe impl bytemuck::Pod for GlyphPrimitive {}
+unsafe impl bytemuck::Zeroable for GlyphPrimitive {}
+impl Primitive for GlyphPrimitive {
+    const WGSL: &'static str = include_str!("shader/glyph.wgsl");
+
+    fn render(device: &Device, queue: &Queue) -> Box<dyn PrimitiveRender> {
+        Box::new(GlyphRender::new(device, queue))
+    }
 }
 
-impl<T> PrimitiveVec<T> {
+/// One drawn image. Its shader reads nothing per instance; the slot names the
-    pub fn new() -> Self {
+/// texture to bind for it.
+#[repr(C)]
+#[derive(Debug, Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
+pub struct TexturePrimitive {
+    pub slot: u32,
+}
+
+impl Primitive for TexturePrimitive {
+    const WGSL: &'static str = include_str!("shader/texture.wgsl");
+
+    fn render(device: &Device, queue: &Queue) -> Box<dyn PrimitiveRender> {
+        Box::new(ImageRender::new(device, queue))
+    }
+}
+
+impl From<&TextureHandle> for TexturePrimitive {
+    fn from(handle: &TextureHandle) -> Self {
         Self {
-            vec: Vec::new(),
+            slot: handle.slot(),
-            free: Vec::new(),
         }
     }
-    pub fn add(&mut self, t: T) -> usize {
-        if let Some(i) = self.free.pop() {
-            self.vec[i] = t;
-            i
-        } else {
-            let i = self.vec.len();
-            self.vec.push(t);
-            i
-        }
-    }
-    pub fn free(&mut self, i: usize) {
-        self.free.push(i);
-    }
-    pub fn clear(&mut self) {
-        self.free.clear();
-        self.vec.clear();
-    }
-}
-
-impl<T> Default for PrimitiveVec<T> {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
-impl<T> Deref for PrimitiveVec<T> {
-    type Target = Vec<T>;
-
-    fn deref(&self) -> &Self::Target {
-        &self.vec
-    }
-}
-
-impl<T> DerefMut for PrimitiveVec<T> {
-    fn deref_mut(&mut self) -> &mut Self::Target {
-        &mut self.vec
-    }
 }

core/src/render/shader.wgsl

@@ -1,204 +0,0 @@
-const RECT: u32 = 0u;
-const TEXTURE: u32 = 1u;
-const GLYPH: u32 = 2u;
-
-@group(0) @binding(0)
-var<uniform> window: WindowUniform;
-@group(1) @binding(RECT)
-var<storage> rects: array<Rect>;
-@group(1) @binding(TEXTURE)
-var<storage> textures: array<TextureInfo>;
-@group(1) @binding(GLYPH)
-var<storage> glyphs: array<GlyphInfo>;
-
-struct Rect {
-    color: u32,
-    radius: f32,
-    thickness: f32,
-    inner_radius: f32,
-}
-
-struct TextureInfo {
-    view_idx: u32,
-    sampler_idx: u32,
-}
-
-struct GlyphInfo {
-    uv_min: vec2<f32>,
-    uv_max: vec2<f32>,
-    view_idx: u32,
-    sampler_idx: u32,
-    color: u32,
-    flags: u32,
-}
-
-struct Mask {
-    x: UiSpan,
-    y: UiSpan,
-}
-
-struct UiSpan {
-    start: UiScalar,
-    end: UiScalar,
-}
-
-struct UiScalar {
-    rel: f32,
-    abs: f32,
-}
-
-struct UiVec2 {
-    rel: vec2<f32>,
-    abs: vec2<f32>,
-}
-
-@group(2) @binding(0)
-var views: binding_array<texture_2d<f32>>;
-@group(2) @binding(1)
-var samplers: binding_array<sampler>;
-@group(2) @binding(2)
-var<storage> masks: array<Mask>;
-
-struct WindowUniform {
-    dim: vec2<f32>,
-};
-
-struct InstanceInput {
-    @location(0) x_start: vec2<f32>,
-    @location(1) x_end: vec2<f32>,
-    @location(2) y_start: vec2<f32>,
-    @location(3) y_end: vec2<f32>,
-    @location(4) binding: u32,
-    @location(5) idx: u32,
-    @location(6) mask_idx: u32,
-}
-
-struct VertexOutput {
-    @location(0) top_left: vec2<f32>,
-    @location(1) bot_right: vec2<f32>,
-    @location(2) uv: vec2<f32>,
-    @location(3) binding: u32,
-    @location(4) idx: u32,
-    @location(5) mask_idx: u32,
-    @builtin(position) clip_position: vec4<f32>,
-};
-
-struct Region {
-    pos: vec2<f32>,
-    uv: vec2<f32>,
-    top_left: vec2<f32>,
-    bot_right: vec2<f32>,
-}
-
-@vertex
-fn vs_main(
-    @builtin(vertex_index) vi: u32,
-    in: InstanceInput,
-) -> VertexOutput {
-    var out: VertexOutput;
-
-    let top_left_rel = vec2(in.x_start.x, in.y_start.x);
-    let top_left_abs = vec2(in.x_start.y, in.y_start.y);
-    let bot_right_rel = vec2(in.x_end.x, in.y_end.x);
-    let bot_right_abs = vec2(in.x_end.y, in.y_end.y);
-
-    let top_left = floor(top_left_rel * window.dim) + floor(top_left_abs);
-    let bot_right = floor(bot_right_rel * window.dim) + floor(bot_right_abs);
-    let size = bot_right - top_left;
-
-    let uv = vec2<f32>(
-        f32(vi % 2u),
-        f32(vi / 2u)
-    );
-    let pos = (top_left + uv * size) / window.dim * 2.0 - 1.0;
-    out.clip_position = vec4<f32>(pos.x, -pos.y, 0.0, 1.0);
-    out.uv = uv;
-    out.binding = in.binding;
-    out.idx = in.idx;
-    out.top_left = top_left;
-    out.bot_right = bot_right;
-    out.mask_idx = in.mask_idx;
-
-    return out;
-}
-
-@fragment
-fn fs_main(
-    in: VertexOutput
-) -> @location(0) vec4<f32> {
-    let pos = in.clip_position.xy;
-    let region = Region(pos, in.uv, in.top_left, in.bot_right);
-    let i = in.idx;
-    var color: vec4<f32>;
-    switch in.binding {
-        case RECT: {
-            color = draw_rounded_rect(region, rects[i]);
-        }
-        case TEXTURE: {
-            color = draw_texture(region, textures[i]);
-        }
-        case GLYPH: {
-            color = draw_glyph(region, glyphs[i]);
-        }
-        default: {
-            color = vec4(1.0, 0.0, 1.0, 1.0);
-        }
-    }
-    if in.mask_idx != 4294967295u {
-        let mask = masks[in.mask_idx];
-        let tl = UiVec2(vec2(mask.x.start.rel, mask.y.start.rel), vec2(mask.x.start.abs, mask.y.start.abs));
-        let br = UiVec2(vec2(mask.x.end.rel, mask.y.end.rel), vec2(mask.x.end.abs, mask.y.end.abs));
-
-        let top_left = floor(tl.rel * window.dim) + floor(tl.abs);
-        let bot_right = floor(br.rel * window.dim) + floor(br.abs);
-        if pos.x < top_left.x || pos.x > bot_right.x || pos.y < top_left.y || pos.y > bot_right.y {
-            color *= 0.0;
-        }
-    }
-    return color;
-}
-
-// TODO: this seems really inefficient (per frag indexing)?
-fn draw_texture(region: Region, info: TextureInfo) -> vec4<f32> {
-    return textureSample(views[info.view_idx], samplers[info.sampler_idx], region.uv);
-}
-
-fn draw_glyph(region: Region, g: GlyphInfo) -> vec4<f32> {
-    let uv = mix(g.uv_min, g.uv_max, region.uv);
-    let texel = textureSample(views[g.view_idx], samplers[g.sampler_idx], uv);
-    if (g.flags & 1u) != 0u {
-        return texel;
-    }
-    var color = unpack4x8unorm(g.color);
-    color.a *= texel.a;
-    return color;
-}
-
-fn draw_rounded_rect(region: Region, rect: Rect) -> vec4<f32> {
-    var color = unpack4x8unorm(rect.color);
-
-    let edge = 0.5;
-
-    let size = region.bot_right - region.top_left;
-    let corner = size / 2.0;
-    let center = region.top_left + corner;
-
-    let dist = distance_from_rect(region.pos, center, corner, rect.radius);
-    color.a *= 1.0 - smoothstep(-min(edge, rect.radius), edge, dist);
-
-    if rect.thickness > 0.0 {
-        let dist2 = distance_from_rect(region.pos, center, corner - rect.thickness, rect.inner_radius);
-        color.a *= smoothstep(-min(edge, rect.inner_radius), edge, dist2);
-    }
-
-    return color;
-}
-
-fn distance_from_rect(pixel_pos: vec2<f32>, rect_center: vec2<f32>, rect_corner: vec2<f32>, radius: f32) -> f32 {
-    // vec from center to pixel
-    let p = pixel_pos - rect_center;
-    // vec from inner rect corner to pixel
-    let q = abs(p) - (rect_corner - radius);
-    return length(max(q, vec2(0.0))) - radius;
-}
-

core/src/render/shader/glyph.wgsl

@@ -0,0 +1,33 @@
+// Matches `GlyphEntry::IS_COLORED`.
+const COLORED: u32 = 1u;
+
+// The glyph atlas, whose array layers are its pages.
+@group(2) @binding(0)
+var atlas: texture_2d_array<f32>;
+@group(2) @binding(1)
+var samp: sampler;
+
+struct GlyphInfo {
+    uv_min: vec2<f32>,
+    uv_max: vec2<f32>,
+    // Which layer of the atlas array this glyph's page is.
+    layer: u32,
+    color: u32,
+    flags: u32,
+}
+
+@group(1) @binding(0)
+var<storage> glyphs: array<GlyphInfo>;
+
+@fragment
+fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
+    let g = glyphs[in.idx];
+    let uv = mix(g.uv_min, g.uv_max, in.uv);
+    let texel = textureSample(atlas, samp, uv, i32(g.layer));
+    if (g.flags & COLORED) != 0u {
+        return masked(in, texel);
+    }
+    var color = unpack4x8unorm(g.color);
+    color.a *= texel.a;
+    return masked(in, color);
+}

core/src/render/shader/prelude.wgsl

@@ -0,0 +1,189 @@
+// Prepended to every primitive's shader, which declares its own instance data
+// as `var<storage> <name>: array<T>` at group 1 binding 0, and an `fs_main`
+// shading one instance of it. What it samples, if anything, is bound at group
+// 2: the texture at binding 0 and the sampler at binding 1.
+
+@group(0) @binding(0)
+var<uniform> window: WindowUniform;
+@group(0) @binding(1)
+var<storage> masks: array<Mask>;
+@group(0) @binding(2)
+var<storage> move_offsets: array<MoveOffset>;
+
+struct WindowUniform {
+    dim: vec2<f32>,
+};
+
+struct Mask {
+    x: RawSpan,
+    y: RawSpan,
+    move_idx: u32,
+}
+
+struct MoveOffset {
+    x: RawSpan,
+    y: RawSpan,
+    parent: u32,
+}
+
+// `PX_STEP`, `REL_STEP`, `MASK_NONE`, `MOVE_NONE` and `CHAIN_LIMIT` are
+// prepended from `iris_core`'s own constants, so none of them is written
+// twice. What the CPU stores is a whole count of each step, and both steps
+// are powers of two, so decoding is exact and the number here is the number
+// the CPU decided.
+
+// Every coordinate the CPU decided is a whole count of `PX_STEP`, so one that
+// composes to within half a step of a pixel boundary is on that boundary and
+// belongs to the pixel above it. Flooring the product instead drops a pixel
+// wherever a fraction divides a window exactly: a fifth of 1920 comes out of
+// `REL_STEP` as 383.99998, and five tabs each lose their last column.
+//
+// Taken over the whole coordinate, fraction and pixels summed, since a floor
+// does not distribute over a sum: floored apart, a half of one and a half of
+// the other lose the pixel the two together make.
+fn snap_floor(v: vec2<f32>) -> vec2<f32> {
+    return floor(v + PX_STEP * 0.5);
+}
+
+struct RawScalar {
+    rel: i32,
+    px: i32,
+}
+
+struct RawSpan {
+    start: RawScalar,
+    end: RawScalar,
+}
+
+fn scalar_of(raw: RawScalar) -> Len {
+    return Len(f32(raw.rel) * REL_STEP, f32(raw.px) * PX_STEP);
+}
+
+fn span_of(raw: RawSpan) -> UiSpan {
+    return UiSpan(scalar_of(raw.start), scalar_of(raw.end));
+}
+
+fn scalar_of_pair(raw: vec2<i32>) -> Len {
+    return Len(f32(raw.x) * REL_STEP, f32(raw.y) * PX_STEP);
+}
+
+struct Region {
+    x: UiSpan,
+    y: UiSpan,
+}
+
+// The same expression `Len::within` uses, in floats rather than on the
+// CPU's grid: a move is resolved here so that scrolling a subtree writes one
+// entry instead of walking it. What has to hold is that this agrees with
+// itself frame to frame, not that it matches the CPU to the last bit.
+fn scalar_within(s: Len, p: UiSpan) -> Len {
+    return Len(
+        p.start.rel + (p.end.rel - p.start.rel) * s.rel,
+        s.px + (p.start.px + (p.end.px - p.start.px) * s.rel),
+    );
+}
+
+fn span_within(s: UiSpan, p: UiSpan) -> UiSpan {
+    return UiSpan(scalar_within(s.start, p), scalar_within(s.end, p));
+}
+
+fn resolve_move(idx: u32, local: Region) -> Region {
+    var r = local;
+    var at = idx;
+    for (var step = 0u; step < CHAIN_LIMIT; step++) {
+        if at == MOVE_NONE {
+            break;
+        }
+        let entry = move_offsets[at];
+        r = Region(span_within(r.x, span_of(entry.x)), span_within(r.y, span_of(entry.y)));
+        at = entry.parent;
+    }
+    return r;
+}
+
+struct UiSpan {
+    start: Len,
+    end: Len,
+}
+
+struct Len {
+    rel: f32,
+    px: f32,
+}
+
+struct InstanceInput {
+    @location(0) x_start: vec2<i32>,
+    @location(1) x_end: vec2<i32>,
+    @location(2) y_start: vec2<i32>,
+    @location(3) y_end: vec2<i32>,
+    @location(4) mask_idx: u32,
+    @location(5) move_idx: u32,
+}
+
+struct VertexOutput {
+    @location(0) top_left: vec2<f32>,
+    @location(1) bot_right: vec2<f32>,
+    @location(2) uv: vec2<f32>,
+    @location(3) @interpolate(flat) mask_idx: u32,
+    @location(4) @interpolate(flat) idx: u32,
+    @builtin(position) clip_position: vec4<f32>,
+};
+
+@vertex
+fn vs_main(
+    @builtin(vertex_index) vi: u32,
+    @builtin(instance_index) ii: u32,
+    in: InstanceInput,
+) -> VertexOutput {
+    var out: VertexOutput;
+
+    let local = Region(
+        UiSpan(scalar_of_pair(in.x_start), scalar_of_pair(in.x_end)),
+        UiSpan(scalar_of_pair(in.y_start), scalar_of_pair(in.y_end)),
+    );
+    let r = resolve_move(in.move_idx, local);
+    let top_left_rel = vec2(r.x.start.rel, r.y.start.rel);
+    let top_left_px = vec2(r.x.start.px, r.y.start.px);
+    let bot_right_rel = vec2(r.x.end.rel, r.y.end.rel);
+    let bot_right_px = vec2(r.x.end.px, r.y.end.px);
+
+    let top_left = snap_floor(top_left_rel * window.dim + top_left_px);
+    let bot_right = snap_floor(bot_right_rel * window.dim + bot_right_px);
+    let size = bot_right - top_left;
+
+    let uv = vec2<f32>(
+        f32(vi % 2u),
+        f32(vi / 2u)
+    );
+    let pos = (top_left + uv * size) / window.dim * 2.0 - 1.0;
+    out.clip_position = vec4<f32>(pos.x, -pos.y, 0.0, 1.0);
+    out.uv = uv;
+    out.top_left = top_left;
+    out.bot_right = bot_right;
+    out.mask_idx = in.mask_idx;
+    out.idx = ii;
+
+    return out;
+}
+
+fn masked(in: VertexOutput, color: vec4<f32>) -> vec4<f32> {
+    if in.mask_idx == MASK_NONE {
+        return color;
+    }
+    let mask = masks[in.mask_idx];
+    // Its own chain, not the drawn primitive's, so a stationary viewport
+    // clips content that moves inside it.
+    let m = resolve_move(mask.move_idx, Region(span_of(mask.x), span_of(mask.y)));
+    let tl = vec2(m.x.start.rel, m.y.start.rel);
+    let tl_px = vec2(m.x.start.px, m.y.start.px);
+    let br = vec2(m.x.end.rel, m.y.end.rel);
+    let br_px = vec2(m.x.end.px, m.y.end.px);
+
+    let top_left = snap_floor(tl * window.dim + tl_px);
+    let bot_right = snap_floor(br * window.dim + br_px);
+    let pos = in.clip_position.xy;
+    if pos.x < top_left.x || pos.x > bot_right.x || pos.y < top_left.y || pos.y > bot_right.y {
+        return color * 0.0;
+    }
+    return color;
+}

core/src/render/shader/rect.wgsl

@@ -0,0 +1,39 @@
+struct Rect {
+    color: u32,
+    radius: f32,
+    thickness: f32,
+    inner_radius: f32,
+}
+
+@group(1) @binding(0)
+var<storage> rects: array<Rect>;
+
+@fragment
+fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
+    let rect = rects[in.idx];
+    var color = unpack4x8unorm(rect.color);
+
+    let edge = 0.5;
+    let size = in.bot_right - in.top_left;
+    let corner = size / 2.0;
+    let center = in.top_left + corner;
+    let pos = in.clip_position.xy;
+
+    let dist = distance_from_rect(pos, center, corner, rect.radius);
+    color.a *= 1.0 - smoothstep(-min(edge, rect.radius), edge, dist);
+
+    if rect.thickness > 0.0 {
+        let dist2 = distance_from_rect(pos, center, corner - rect.thickness, rect.inner_radius);
+        color.a *= smoothstep(-min(edge, rect.inner_radius), edge, dist2);
+    }
+
+    return masked(in, color);
+}
+
+fn distance_from_rect(pixel_pos: vec2<f32>, rect_center: vec2<f32>, rect_corner: vec2<f32>, radius: f32) -> f32 {
+    // vec from center to pixel
+    let p = pixel_pos - rect_center;
+    // vec from inner rect corner to pixel
+    let q = abs(p) - (rect_corner - radius);
+    return length(max(q, vec2(0.0))) - radius;
+}

core/src/render/shader/texture.wgsl

@@ -0,0 +1,10 @@
+// The image this instance draws, bound for it alone.
+@group(2) @binding(0)
+var image: texture_2d<f32>;
+@group(2) @binding(1)
+var samp: sampler;
+
+@fragment
+fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
+    return masked(in, textureSample(image, samp, in.uv));
+}

core/src/render/texture.rs

@@ -1,119 +1,119 @@
-use image::{DynamicImage, EncodableLayout, GenericImageView};
+use image::{DynamicImage, EncodableLayout, GenericImageView, RgbaImage};
 use wgpu::{util::DeviceExt, *};
 
-use crate::{PatchRect, TextureUpdate, Textures};
+use crate::{
+    PatchRect, TextureUpdate, Textures, UiData,
+    render::{
+        TexturePrimitive,
+        primitive::{ListDraw, PrimitiveRender},
+    },
+};
 
+/// Draws standalone images, which it owns. Each is its own texture, so each
+/// instance binds its own and is a draw of its own.
+pub struct ImageRender {
+    textures: GpuTextures,
+    layout: BindGroupLayout,
+    sampler: Sampler,
+}
+
+impl ImageRender {
+    pub fn new(device: &Device, queue: &Queue) -> Self {
+        Self {
+            textures: GpuTextures::new(device, queue),
+            layout: sampled_layout(device, TextureViewDimension::D2, "ui image"),
+            sampler: default_sampler(device),
+        }
+    }
+}
+
+impl PrimitiveRender for ImageRender {
+    fn layout(&self) -> Option<&BindGroupLayout> {
+        Some(&self.layout)
+    }
+
+    fn update(&mut self, ui: &mut UiData) {
+        self.textures
+            .update(&mut ui.textures, &self.layout, &self.sampler);
+    }
+
+    fn instance_bindings(&self, list: &super::InstanceList, out: &mut Vec<u32>) {
+        let slots = list
+            .data()
+            .chunks_exact(list.stride())
+            .map(|data| bytemuck::pod_read_unaligned::<TexturePrimitive>(data).slot);
+        out.extend(slots);
+    }
+
+    fn draw<'a>(&'a self, pass: &mut RenderPass<'a>, list: ListDraw<'a>) {
+        for (i, &slot) in list.bindings.iter().enumerate() {
+            let Some(image) = self.textures.group(slot) else {
+                continue;
+            };
+            pass.set_bind_group(2, image, &[]);
+            pass.draw(0..4, i as u32..i as u32 + 1);
+        }
+    }
+}
+
+/// The standalone images a ui draws, each its own texture and bind group --
+/// unlike the glyph atlas in `super::page`, which is one array they share.
 pub struct GpuTextures {
     device: Device,
     queue: Queue,
-    /// Parallel to `views`; patches require textures rather than views.
+    slots: Vec<Option<ImageGpu>>,
-    textures: Vec<Option<Texture>>,
+}
-    views: Vec<TextureView>,
+
-    view_count: usize,
+struct ImageGpu {
-    samplers: Vec<Sampler>,
+    /// Kept for `patch`, which needs the texture rather than the view.
-    null_view: TextureView,
+    texture: Texture,
-    no_views: Vec<TextureView>,
+    group: BindGroup,
 }
 
 impl GpuTextures {
-    pub fn update(&mut self, textures: &mut Textures) -> bool {
+    pub fn new(device: &Device, queue: &Queue) -> Self {
-        let mut bindings_changed = false;
+        Self {
+            device: device.clone(),
+            queue: queue.clone(),
+            slots: Vec::new(),
+        }
+    }
+
+    pub fn update(&mut self, textures: &mut Textures, layout: &BindGroupLayout, sampler: &Sampler) {
         for update in textures.updates() {
-            bindings_changed |= match update {
+            match update {
                 TextureUpdate::Push(image) => {
-                    self.push(image);
+                    let image = self.create(image, layout, sampler);
-                    true
+                    self.slots.push(Some(image));
                 }
                 TextureUpdate::Set(i, image) => {
-                    self.set(i, image);
+                    let image = self.create(image, layout, sampler);
-                    true
+                    self.slots[i as usize] = Some(image);
                 }
-                TextureUpdate::Patch(i, rect, image) => {
+                TextureUpdate::Patch(i, rect, image) => self.patch(i, rect, image),
-                    self.patch(i, rect, image);
+                TextureUpdate::PushFree => self.slots.push(None),
-                    false
+                TextureUpdate::SetFree => {}
+                TextureUpdate::Free(i) => self.slots[i as usize] = None,
             }
-                TextureUpdate::SetFree => {
-                    self.view_count += 1;
-                    true
         }
-                TextureUpdate::Free(i) => {
-                    self.free(i);
-                    true
-                }
-                TextureUpdate::PushFree => {
-                    self.push_free();
-                    true
-                }
-            };
-        }
-        bindings_changed
-    }
-    fn set(&mut self, i: u32, image: &DynamicImage) {
-        self.view_count += 1;
-        let (texture, view) = self.create(image);
-        self.textures[i as usize] = Some(texture);
-        self.views[i as usize] = view;
-    }
-    fn free(&mut self, i: u32) {
-        self.view_count -= 1;
-        self.textures[i as usize] = None;
-        self.views[i as usize] = self.null_view.clone();
-    }
-    fn push(&mut self, image: &DynamicImage) {
-        self.view_count += 1;
-        let (texture, view) = self.create(image);
-        self.textures.push(Some(texture));
-        self.views.push(view);
-    }
-    fn push_free(&mut self) {
-        self.view_count += 1;
-        self.textures.push(None);
-        self.views.push(self.null_view.clone());
     }
 
-    fn patch(&mut self, i: u32, rect: PatchRect, image: &DynamicImage) {
+    pub fn group(&self, slot: u32) -> Option<&BindGroup> {
-        let Some(texture) = &self.textures[i as usize] else {
+        self.slots.get(slot as usize)?.as_ref().map(|i| &i.group)
-            return;
-        };
-        if rect.width == 0 || rect.height == 0 {
-            return;
-        }
-        // `write_texture` requires tightly packed rows, unlike the atlas image.
-        let sub = image
-            .view(rect.x, rect.y, rect.width, rect.height)
-            .to_image();
-        self.queue.write_texture(
-            TexelCopyTextureInfo {
-                texture,
-                mip_level: 0,
-                origin: Origin3d {
-                    x: rect.x,
-                    y: rect.y,
-                    z: 0,
-                },
-                aspect: TextureAspect::All,
-            },
-            sub.as_bytes(),
-            TexelCopyBufferLayout {
-                offset: 0,
-                bytes_per_row: Some(rect.width * 4),
-                rows_per_image: Some(rect.height),
-            },
-            Extent3d {
-                width: rect.width,
-                height: rect.height,
-                depth_or_array_layers: 1,
-            },
-        );
     }
 
-    fn create(&self, image: &DynamicImage) -> (Texture, TextureView) {
+    fn create(
-        let image = image.to_rgba8();
+        &self,
-        let (width, height) = image.dimensions();
+        image: &DynamicImage,
+        layout: &BindGroupLayout,
+        sampler: &Sampler,
+    ) -> ImageGpu {
+        let rgba = image.to_rgba8();
+        let (width, height) = rgba.dimensions();
         let texture = self.device.create_texture_with_data(
             &self.queue,
             &TextureDescriptor {
-                label: None,
+                label: Some("image"),
                 size: Extent3d {
                     width,
                     height,
@@ -127,63 +127,115 @@ impl GpuTextures {
                 view_formats: &[],
             },
             wgt::TextureDataOrder::MipMajor,
-            image.as_bytes(),
+            rgba.as_bytes(),
         );
         let view = texture.create_view(&TextureViewDescriptor::default());
-        (texture, view)
+        let group = sampled_group(&self.device, layout, &view, sampler, "ui image");
+        ImageGpu { texture, group }
     }
 
-    pub fn new(device: &Device, queue: &Queue) -> Self {
+    fn patch(&mut self, i: u32, rect: PatchRect, image: &DynamicImage) {
-        let null_view = null_texture_view(device);
+        let Some(Some(slot)) = self.slots.get(i as usize) else {
-        Self {
+            return;
-            device: device.clone(),
+        };
-            queue: queue.clone(),
+        let dst = TexelCopyTextureInfo {
-            textures: Vec::new(),
+            texture: &slot.texture,
-            views: Vec::new(),
+            mip_level: 0,
-            samplers: vec![default_sampler(device)],
+            origin: Origin3d {
-            no_views: vec![null_view.clone()],
+                x: rect.x,
-            null_view,
+                y: rect.y,
-            view_count: 0,
+                z: 0,
+            },
+            aspect: TextureAspect::All,
+        };
+        match image.as_rgba8() {
+            Some(rgba) => write_region(&self.queue, dst, rgba, rect),
+            // The texture is rgba8, so any other layout has to be converted --
+            // and converting the rectangle is cheaper than the whole image.
+            None => {
+                let sub = image
+                    .view(rect.x, rect.y, rect.width, rect.height)
+                    .to_image();
+                write_region(&self.queue, dst, &sub, PatchRect { x: 0, y: 0, ..rect });
+            }
+        }
     }
 }
 
-    pub fn views(&self) -> Vec<&TextureView> {
+pub fn write_region(queue: &Queue, dst: TexelCopyTextureInfo, src: &RgbaImage, rect: PatchRect) {
-        if self.views.is_empty() {
+    if rect.width == 0 || rect.height == 0 {
-            &self.no_views
+        return;
-        } else {
-            &self.views
     }
-        .iter()
+    let stride = src.width() * 4;
-        .by_ref()
+    queue.write_texture(
-        .collect()
+        dst,
-    }
+        src.as_bytes(),
-
+        TexelCopyBufferLayout {
-    pub fn samplers(&self) -> Vec<&Sampler> {
+            offset: (rect.y * stride + rect.x * 4) as u64,
-        self.samplers.iter().by_ref().collect()
+            bytes_per_row: Some(stride),
-    }
+            rows_per_image: Some(rect.height),
-
+        },
-    pub fn view_count(&self) -> usize {
+        Extent3d {
-        self.view_count
+            width: rect.width,
-    }
+            height: rect.height,
-}
-
-pub fn null_texture_view(device: &Device) -> TextureView {
-    device
-        .create_texture(&TextureDescriptor {
-            label: Some("null"),
-            size: Extent3d {
-                width: 1,
-                height: 1,
             depth_or_array_layers: 1,
         },
-            mip_level_count: 1,
+    );
-            sample_count: 1,
+}
-            dimension: TextureDimension::D2,
+
-            format: TextureFormat::Rgba8Unorm,
+/// What a primitive that samples binds: a texture, and the sampler that reads
-            usage: TextureUsages::TEXTURE_BINDING,
+/// it.
-            view_formats: &[],
+pub fn sampled_group(
+    device: &Device,
+    layout: &BindGroupLayout,
+    view: &TextureView,
+    sampler: &Sampler,
+    label: &'static str,
+) -> BindGroup {
+    device.create_bind_group(&BindGroupDescriptor {
+        layout,
+        entries: &[
+            BindGroupEntry {
+                binding: 0,
+                resource: BindingResource::TextureView(view),
+            },
+            BindGroupEntry {
+                binding: 1,
+                resource: BindingResource::Sampler(sampler),
+            },
+        ],
+        label: Some(label),
+    })
+}
+
+/// The layout for one of those. The dimension differs -- the atlas is an
+/// array of pages and an image is not -- and nothing else does.
+pub fn sampled_layout(
+    device: &Device,
+    dimension: TextureViewDimension,
+    label: &'static str,
+) -> BindGroupLayout {
+    device.create_bind_group_layout(&BindGroupLayoutDescriptor {
+        entries: &[
+            BindGroupLayoutEntry {
+                binding: 0,
+                visibility: ShaderStages::FRAGMENT,
+                ty: BindingType::Texture {
+                    sample_type: TextureSampleType::Float { filterable: false },
+                    view_dimension: dimension,
+                    multisampled: false,
+                },
+                count: None,
+            },
+            BindGroupLayoutEntry {
+                binding: 1,
+                visibility: ShaderStages::FRAGMENT,
+                ty: BindingType::Sampler(SamplerBindingType::NonFiltering),
+                count: None,
+            },
+        ],
+        label: Some(label),
     })
-        .create_view(&TextureViewDescriptor::default())
 }
 
 pub fn default_sampler(device: &Device) -> Sampler {

core/src/render/util/mod.rs

@@ -21,17 +21,25 @@ impl<T: Pod> ArrBuf<T> {
             _pd: PhantomData,
         }
     }
-    pub fn update(&mut self, device: &Device, queue: &Queue, data: &[T]) {
+    /// Returns whether the `Buffer` was recreated, which stales any cached
-        if self.len != data.len() {
+    /// `BindGroup` holding it.
+    pub fn update(&mut self, device: &Device, queue: &Queue, data: &[T]) -> bool {
+        let resized = self.len != data.len();
+        if resized {
             self.len = data.len();
             self.buffer =
                 Self::init_buf(device, std::mem::size_of_val(data), self.usage, self.label);
         }
         queue.write_buffer(&self.buffer, 0, bytemuck::cast_slice(data));
+        resized
+    }
+    pub fn len(&self) -> usize {
+        self.len
     }
     fn init_buf(device: &Device, size: usize, usage: BufferUsages, label: &'static str) -> Buffer {
         let mut size = size as u64;
         if usage.contains(BufferUsages::STORAGE) {
+            // A binding cannot be empty or under the layout's minimum.
             size = size.max(std::mem::size_of::<T>() as u64);
         }
         device.create_buffer(&BufferDescriptor {
@@ -41,8 +49,4 @@ impl<T: Pod> ArrBuf<T> {
             usage,
         })
     }
-    #[allow(clippy::len_without_is_empty)]
-    pub fn len(&self) -> usize {
-        self.len
-    }
 }

core/src/ui/active.rs

@@ -1,14 +1,99 @@
-use crate::{LayerId, MaskIdx, PrimitiveHandle, TextureHandle, UiRegion, WidgetId};
+use crate::{
+    Declared, LayerId, LayoutHolds, MaskIdx, MoveIdx, PlaceDesc, RegionAlign, RetainedPrimitive,
+    Size, TextureHandle, UiRegion, UiVec2, WidgetId,
+};
 
-/// important non rendering data for retained drawing
+/// What is kept of a widget its parent has asked about. `drawn` says whether
+/// it currently draws; one that does not is kept so that a change to it, or
+/// under it, still reaches whoever asked.
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
+    /// Where its drawing goes, in its region node's coordinates.
+    pub placement: UiRegion,
+    /// What a fraction declared or reported under this widget is a fraction
+    /// of, as a length of the window.
+    pub rel_base: UiVec2,
+    /// Where its drawing was put, and where it was asked. The two differ
+    /// where a container asks in one place and puts the answer in another --
+    /// a row measures from its cursor and puts the child in its slot. Each
+    /// carries the rel base that ask stated, so asking again from either is
+    /// the same question it was.
+    pub placed: PlaceDesc,
+    pub asked: PlaceDesc,
+    /// The box it was asked in, in the parent's region-node coordinates: the
+    /// box its drawing was made in and the one its contract is about. Its
+    /// drawing is placed elsewhere by re-expression, never by asking again.
     pub region: UiRegion,
+    /// The measured answer and its dependencies. A hint-only dependency or
+    /// a widget first encountered during placement has no measurement yet.
+    pub answer: Option<Answer>,
+    /// Asked more than once in its parent's last draw -- measured in one box
+    /// and then asked in the one the parent decided. The parent's layout
+    /// rests on the first answer and its drawing on the last, so only the
+    /// parent can ask either again.
+    pub re_asked: bool,
+    /// What the widget reported, in window-unit lengths.
+    pub size: Size,
+    /// The window and region reads that this drawing holds for, and the
+    /// rel base and region it pinned.
+    pub holds: LayoutHolds,
+    pub drawn: bool,
     pub parent: Option<WidgetId>,
+    /// How far down the tree it was drawn, the root being 1. Carried down a
+    /// draw rather than worked out by walking up, so it is right for every
+    /// widget a frame visits and cannot drift while one is being drawn.
+    pub depth: usize,
     pub textures: Vec<TextureHandle>,
-    pub primitives: Vec<PrimitiveHandle>,
+    /// Its primitives, each keeping the box it was written in -- in this
+    /// widget's placement coordinates, which is what a move recomposes from.
+    pub primitives: Vec<RetainedPrimitive>,
+    /// An owned mask holds one reference independently of its primitives.
+    pub mask_region: Option<UiRegion>,
     pub children: Vec<WidgetId>,
+    /// The movable region its primitives are positioned through: its own when
+    /// opted in, otherwise the nearest ancestor's.
+    pub move_idx: MoveIdx,
+    /// The declared lengths whoever drew this widget resolved into its rel base.
+    /// A change to one moves a box this widget cannot fix by drawing again,
+    /// and comparing them is what says so.
+    pub declared: Declared,
+    /// Its alignment when it was last drawn, which a change to the property
+    /// is found against.
+    pub own_align: RegionAlign,
+    /// The movable region whose coordinates its placement is in when this
+    /// widget does not own a region node.
+    pub parent_move: MoveIdx,
+    /// The mask its drawing is clipped to: one it set itself, or the one it
+    /// inherited from whoever drew it.
     pub mask: MaskIdx,
+    /// That inherited one. The two differ exactly where the widget set a
+    /// mask of its own, which is the one it owns and the one a move rewrites
+    /// -- and the one a redraw of it must not be handed back, since setting
+    /// a mask asserts there is none.
+    pub parent_mask: MaskIdx,
     pub layer: LayerId,
 }
+
+impl ActiveData {
+    /// What it answered when its parent asked, where it has been asked at
+    /// all. Not `size`, which is what its last drawing reported: a drawing
+    /// re-expressed in the box that answer chose is not a second answer.
+    pub fn measured(&self) -> Option<Size> {
+        self.answer.map(|answer| answer.size)
+    }
+
+    /// Whether it owns a region node rather than sharing the one it was drawn
+    /// under, which is what its two move indices being different says.
+    pub fn is_region_node(&self) -> bool {
+        self.move_idx != self.parent_move
+    }
+}
+
+/// What a widget answered when it was asked: the size it reported, and the
+/// boxes and windows that answer holds for.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Answer {
+    pub size: Size,
+    pub holds: LayoutHolds,
+}

core/src/ui/cache.rs

@@ -1,18 +0,0 @@
-use crate::{BothAxis, Len, UiVec2, WidgetId, util::HashMap};
-
-#[derive(Default)]
-pub struct Cache {
-    pub size: BothAxis<HashMap<WidgetId, (UiVec2, Len)>>,
-}
-
-impl Cache {
-    pub fn remove(&mut self, id: WidgetId) {
-        self.size.x.remove(&id);
-        self.size.y.remove(&id);
-    }
-
-    pub fn clear(&mut self) {
-        self.size.x.clear();
-        self.size.y.clear();
-    }
-}

core/src/ui/holds.rs

@@ -0,0 +1,204 @@
+use crate::{Len, Px, REL_SHIFT, fixed::div_toward, fixed::narrow};
+use std::ops::RangeInclusive;
+
+/// The lengths of a box, in pixels, that one drawing of a widget holds for:
+/// give the widget any box in this range and it draws the same thing and
+/// reports the same size. A widget that never reads its box in pixels holds
+/// for every length; one that does holds for the one it read unless it says
+/// otherwise, and a parent holds for whatever keeps every child it asked
+/// about or drew inside its own range.
+///
+/// The ends are lengths on the grid rather than floats with a tolerance
+/// around them: a box offered back at the length a widget reported comes back
+/// as the same number, so a range means what it says. The one place a range
+/// is wider than the length it came from is [`Self::through`], and what it is
+/// wider by is the floor that inverting a fraction undoes.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Holds {
+    pub lo: Px,
+    pub hi: Px,
+}
+
+impl Len {
+    /// Whether this is longer than `than` in a window this wide, and the
+    /// windows that answer holds for.
+    ///
+    /// Which is longer is a question in pixels -- `rel(0.5)` is longer than
+    /// 300 px at a box of 600 and shorter at 400 -- and it is asked of the
+    /// difference and answered back through that same difference, so the
+    /// boundary is the comparison's own rather than a second way of finding
+    /// it.
+    pub fn longer_than(&self, than: Len, window: Px) -> (bool, Holds) {
+        let over = *self - than;
+        let longer = over.to_px(window) > Px::ZERO;
+        let side = match longer {
+            true => Px::STEP..=Px::MAX,
+            false => Px::MIN..=Px::ZERO,
+        };
+        (longer, Holds::from(side).through(over))
+    }
+}
+
+impl Holds {
+    pub const ANY: Self = Self {
+        lo: Px::MIN,
+        hi: Px::MAX,
+    };
+
+    pub const fn at(len: Px) -> Self {
+        Self { lo: len, hi: len }
+    }
+
+    pub const fn contains(&self, len: Px) -> bool {
+        len.raw() >= self.lo.raw() && len.raw() <= self.hi.raw()
+    }
+
+    /// Every length `other` holds for is one this holds for, so a drawing
+    /// made under this range is still good wherever `other` is.
+    pub const fn covers(&self, other: Self) -> bool {
+        self.lo.raw() <= other.lo.raw() && self.hi.raw() >= other.hi.raw()
+    }
+
+    pub const fn and(&self, other: Self) -> Self {
+        Self {
+            lo: self.lo.max(other.lo),
+            hi: self.hi.min(other.hi),
+        }
+    }
+
+    /// What a box has to be for a part of it, `len` of the box long, to stay
+    /// in this range: the exact preimage of `px + floor(rel * box)`, which is
+    /// the one way a box in pixels is reached. A part with no relative extent
+    /// is a fixed length -- it was drawn at that length and any box keeps it
+    /// there.
+    ///
+    /// The answer is an interval even where this range is a single length,
+    /// because the multiply on the way in drops to the step below and many
+    /// boxes therefore give one length. That is a floor rather than an
+    /// allowance: inverting it is two divisions and nothing else, and the
+    /// whole of a box maps back to itself.
+    pub const fn through(&self, len: Len) -> Self {
+        if self.lo.raw() == Px::MIN.raw() && self.hi.raw() == Px::MAX.raw() {
+            return Self::ANY;
+        }
+        let rel = len.rel.raw() as i64;
+        if rel == 0 {
+            return Self::ANY;
+        }
+        let px = len.px.raw() as i64;
+        // `floor(rel * box) >= lo - px` is `rel * box >= (lo - px) << REL`, and
+        // `floor(rel * box) <= hi - px` is `rel * box < (hi - px + 1) << REL`.
+        let lo = (self.lo.raw() as i64 - px) << REL_SHIFT;
+        let hi = (((self.hi.raw() as i64 - px) + 1) << REL_SHIFT) - 1;
+        // Dividing by a negative fraction turns the ends around, so which
+        // bound each comes from is decided before dividing rather than by
+        // taking the min and max of four divisions.
+        match rel > 0 {
+            true => Self::raws(div_toward(lo, rel, true), div_toward(hi, rel, false)),
+            false => Self::raws(div_toward(hi, rel, true), div_toward(lo, rel, false)),
+        }
+    }
+
+    const fn raws(lo: i64, hi: i64) -> Self {
+        Self {
+            lo: Px::from_raw(narrow(lo)),
+            hi: Px::from_raw(narrow(hi)),
+        }
+    }
+}
+
+impl From<RangeInclusive<Px>> for Holds {
+    fn from(range: RangeInclusive<Px>) -> Self {
+        Self {
+            lo: *range.start(),
+            hi: *range.end(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::Rel;
+
+    #[test]
+    fn an_unrestricted_range_stays_unrestricted_through_any_length() {
+        for rel in [-2.0, -0.5, 0.0, 0.5, 1.0, 2.0] {
+            for px in [-8, 0, 8] {
+                let len = Len::from_parts(Rel::from_f32(rel), Px::from_int(px));
+                assert_eq!(Holds::ANY.through(len), Holds::ANY);
+            }
+        }
+    }
+
+    #[test]
+    fn through_reverses_a_range_for_a_negative_fraction() {
+        // `10 - box / 2` is between 20 and 40 for boxes from -60 to -20.
+        let part = Len::from_parts(Rel::from_f32(-0.5), Px::from_int(10));
+        let holds = Holds::from(Px::from_int(20)..=Px::from_int(40)).through(part);
+        assert!(holds.contains(Px::from_int(-60)) && holds.contains(Px::from_int(-20)));
+        assert!(!holds.contains(Px::from_int(-61)) && !holds.contains(Px::from_int(-19)));
+    }
+
+    /// The case the widening is for: a part that holds only for the length it
+    /// was drawn at has to hold for the box it was drawn in, and a third of a
+    /// box is not a whole number of steps.
+    #[test]
+    fn a_part_maps_back_onto_the_box_it_was_measured_in() {
+        let part = Len::from_parts(Rel::from_f32(1.0 / 3.0), Px::from_int(-146));
+        for box_len in (440..460).map(Px::from_int) {
+            let holds = Holds::at(part.to_px(box_len)).through(part);
+            assert!(holds.contains(box_len), "{box_len:?} left out by {holds:?}");
+        }
+    }
+
+    /// A widget handed the whole of its parent's box, with or without pixels
+    /// taken off it, has no fraction to invert: multiplying by one is exact
+    /// and taking the pixels off again is too, so the box maps back to
+    /// itself. Allowing for anything here compounded a step a level down a
+    /// chain of widgets each taking the whole of its parent.
+    #[test]
+    fn the_whole_of_a_box_maps_back_to_itself() {
+        let at = Px::from_int(956);
+        assert_eq!(Holds::at(at).through(Len::FULL), Holds::at(at));
+        let less_eight = Len::from_parts(Rel::ONE, Px::from_int(-8));
+        assert_eq!(
+            Holds::at(at).through(less_eight),
+            Holds::at(at + Px::from_int(8))
+        );
+    }
+
+    /// The range is the exact preimage at both ends, so a box one step
+    /// outside it really does give a length outside this range. What a wider
+    /// range costs is a drawing reused where it does not hold.
+    #[test]
+    fn a_box_one_step_outside_the_range_is_outside_it() {
+        let part = Len::from_parts(Rel::from_f32(1.0 / 3.0), Px::from_int(-146));
+        let at = Px::from_int(300);
+        let holds = Holds::at(at).through(part);
+        for inside in [holds.lo, holds.hi] {
+            assert_eq!(part.to_px(inside), at, "{inside:?} left out of {holds:?}");
+        }
+        for outside in [holds.lo.next_down(), holds.hi.next_up()] {
+            assert_ne!(part.to_px(outside), at, "{outside:?} admitted by {holds:?}");
+        }
+    }
+
+    /// A truncating multiply only ever drops, so the step it needs allowing
+    /// for on the way in belongs at the top of the range and not the bottom.
+    #[test]
+    fn a_fraction_widens_further_up_than_down() {
+        let half = Len::from_parts(Rel::from_f32(0.5), Px::ZERO);
+        let holds = Holds::at(Px::from_int(100)).through(half);
+        let box_len = Px::from_int(200);
+        assert!(holds.hi - box_len > box_len - holds.lo, "{holds:?}");
+    }
+
+    #[test]
+    fn a_boundary_the_next_step_along_does_not_admit_it() {
+        let boundary = Px::from_int(10);
+        let above = Holds::from(boundary.next_up()..=Px::MAX);
+        assert!(!above.contains(boundary));
+        assert!(above.contains(boundary.next_up()));
+    }
+}

core/src/ui/layout_holds.rs

@@ -0,0 +1,108 @@
+use crate::util::impl_axis_index;
+use crate::{Axis, Holds, Len, Px, PxVec2, UiRegion, UiVec2};
+
+/// What one evaluation of a widget depends on along one axis: the window
+/// lengths its reads hold for, the pixel lengths of its own box, and the
+/// symbolic lengths of that box and of its rel base where either one is what
+/// it was expressed in.
+///
+/// The symbolic lengths are pins rather than ranges: a container places its
+/// children as lengths of its rel base measured from where its own box starts,
+/// so what it draws turns on that box's length and on nothing about where it
+/// is. A box pin reaches the parent only where the box it pinned is the
+/// parent's own; anywhere else the parent chose that length itself, and a
+/// widget pinned this way is checked when it is re-placed.
+///
+/// A rel base pin says the answer or the drawing is a fraction of the rel base,
+/// which is a different length wherever the rel base is a different one -- at
+/// the same window size, so no range of window pixels can say it. A length
+/// of the rel base that is only pixels is not one: it is that many pixels
+/// whatever the rel base turns out to be.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct AxisHolds {
+    pub window: Holds,
+    pub rel_base: Option<Len>,
+    pub region: Holds,
+    pub region_len: Option<Len>,
+}
+
+impl AxisHolds {
+    pub const ANY: Self = Self {
+        window: Holds::ANY,
+        rel_base: None,
+        region: Holds::ANY,
+        region_len: None,
+    };
+
+    pub fn and(&self, other: Self) -> Self {
+        // Two pins of the same length disagreeing would mean one drawing was
+        // a fraction of two different lengths at once.
+        debug_assert!(
+            self.region_len.is_none()
+                || other.region_len.is_none()
+                || self.region_len == other.region_len
+        );
+        debug_assert!(
+            self.rel_base.is_none() || other.rel_base.is_none() || self.rel_base == other.rel_base
+        );
+        Self {
+            window: self.window.and(other.window),
+            rel_base: self.rel_base.or(other.rel_base),
+            region: self.region.and(other.region),
+            region_len: self.region_len.or(other.region_len),
+        }
+    }
+
+    pub fn covers(&self, other: Self) -> bool {
+        self.window.covers(other.window)
+            && self.region.covers(other.region)
+            && self
+                .region_len
+                .is_none_or(|len| other.region_len == Some(len))
+            && self.rel_base.is_none_or(|len| other.rel_base == Some(len))
+    }
+
+    /// Whether a widget in a box `len` long, with that rel base, in that
+    /// window, is one this drawing holds for.
+    pub fn contains(&self, window: Px, rel_base: Len, len: Len) -> bool {
+        self.window.contains(window)
+            && self.rel_base.is_none_or(|pinned| pinned == rel_base)
+            && self.region.contains(len.to_px(window))
+            && self.region_len.is_none_or(|pinned| pinned == len)
+    }
+}
+
+/// [`AxisHolds`] on both axes. Every question asked of it is asked of one
+/// axis at a time, since a widget that read one length holds for any length
+/// of the other.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct LayoutHolds {
+    pub x: AxisHolds,
+    pub y: AxisHolds,
+}
+
+impl LayoutHolds {
+    pub const ANY: Self = Self {
+        x: AxisHolds::ANY,
+        y: AxisHolds::ANY,
+    };
+
+    pub fn and(&self, other: Self) -> Self {
+        Self {
+            x: self.x.and(other.x),
+            y: self.y.and(other.y),
+        }
+    }
+
+    pub fn covers(&self, other: Self) -> bool {
+        self.x.covers(other.x) && self.y.covers(other.y)
+    }
+
+    pub fn contains(&self, window: PxVec2, rel_base: UiVec2, region: UiRegion) -> bool {
+        Axis::BOTH
+            .into_iter()
+            .all(|axis| self[axis].contains(window[axis], rel_base[axis], region[axis].len()))
+    }
+}
+
+impl_axis_index!(LayoutHolds => AxisHolds);

core/src/ui/mod.rs

@@ -1,24 +1,126 @@
-use crate::{Mask, TextData, Textures, WeakWidget, WidgetId, Widgets, util::TrackedArena};
+use crate::{
+    Mask, MoveIdx, MoveOffset, PrimitiveRegistry, TextData, Textures, UiRegion, WeakWidget,
+    WidgetId, Widgets,
+    util::{Arena, Id, TrackedArena},
+};
+
+/// How far the shader will walk a move chain. It bounds a malformed cycle
+/// rather than any real tree; `Moves::resolve` uses the same number so the
+/// two agree on what a deep tree resolves to.
+pub const CHAIN_LIMIT: u32 = 64;
 
 mod active;
-mod cache;
+mod holds;
+mod layout_holds;
 mod painter;
+mod place;
 mod render_state;
-mod size;
 
 pub use active::*;
-pub use painter::Painter;
+pub use holds::*;
+pub use layout_holds::*;
+pub use painter::{Painter, PrimitiveLike};
+pub use place::{PlaceDesc, PlaceDescAxis, RetainedPrimitive};
 pub use render_state::*;
-pub use size::*;
 
 #[derive(Default)]
 pub struct UiData {
     pub widgets: Widgets,
+    /// Every primitive this ui can draw.
+    pub primitives: PrimitiveRegistry,
     pub textures: Textures,
     pub text: TextData,
     pub masks: TrackedArena<Mask, u32>,
 }
 
+/// Where each widget's drawing sits relative to its parent's slot, so moving
+/// a subtree writes one entry rather than every descendant's primitives.
+#[derive(Default)]
+pub struct Moves {
+    arena: Arena<MoveOffset, u32>,
+    pub changed: bool,
+}
+
+impl Moves {
+    pub fn push(&mut self, parent: MoveIdx, region: UiRegion) -> MoveIdx {
+        self.changed = true;
+        MoveIdx::slot(self.arena.push(MoveOffset::new(parent, region)).idx())
+    }
+
+    /// Re-points a slot at a different parent, for a widget drawn somewhere
+    /// else in the tree than it was.
+    pub fn set_parent(&mut self, idx: MoveIdx, parent: MoveIdx) {
+        let entry = self.arena.get_mut(Id::preset(idx.idx() as u32));
+        if entry.parent != parent {
+            entry.parent = parent;
+            self.changed = true;
+        }
+    }
+
+    pub fn remove(&mut self, idx: MoveIdx) {
+        self.changed = true;
+        self.arena.remove(Id::preset(idx.idx() as u32));
+    }
+
+    /// Sets the box a slot's contents are placed within, itself given in the
+    /// coordinates of its parent slot.
+    pub fn set(&mut self, idx: MoveIdx, region: UiRegion) {
+        let entry = self.arena.get_mut(Id::preset(idx.idx() as u32));
+        if entry.region != region {
+            entry.region = region;
+            self.changed = true;
+        }
+    }
+
+    /// The same walk the vertex shader does, in the same `Len` the shader is
+    /// handed, for asking where a drawing will actually land -- hit testing,
+    /// and nothing layout decides on. Layout threads its lengths down the
+    /// draw instead, so no box it compares is composed back up this chain.
+    pub fn resolve(&self, idx: MoveIdx, local: UiRegion) -> UiRegion {
+        let mut region = local;
+        self.walk(idx, |entry| region = region.within(entry));
+        region
+    }
+
+    fn walk(&self, idx: MoveIdx, mut step: impl FnMut(&UiRegion)) {
+        let mut at = idx;
+        for _ in 0..CHAIN_LIMIT {
+            if at == MoveIdx::NONE {
+                return;
+            }
+            let entry = &self.arena[at.idx()];
+            step(&entry.region);
+            at = entry.parent;
+        }
+        debug_assert!(
+            at == MoveIdx::NONE,
+            "a move chain longer than {CHAIN_LIMIT} resolves to the wrong place, \
+             and the shader stops at the same depth"
+        );
+    }
+
+    /// How many slots a region in `idx` is composed through, which is what
+    /// the shader's walk costs per primitive.
+    pub fn depth(&self, idx: MoveIdx) -> usize {
+        let mut depth = 0;
+        let mut at = idx;
+        while at != MoveIdx::NONE && depth < CHAIN_LIMIT as usize {
+            at = self.arena[at.idx()].parent;
+            depth += 1;
+        }
+        depth
+    }
+
+    pub fn entries(&self) -> &[MoveOffset] {
+        &self.arena
+    }
+
+    pub fn clear(&mut self) {
+        self.changed = true;
+        self.arena = Arena::default();
+    }
+}
+
 pub trait UiRsc {
     fn ui(&self) -> &UiData;
     fn ui_mut(&mut self) -> &mut UiData;

core/src/ui/painter.rs

@@ -1,8 +1,17 @@
+#[cfg(feature = "layout-diagnostics")]
+use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Len, RenderedText, Size, SizeCtx, StrongWidget, TextAttrs, TextBuffer, TextData,
+    Axis, Declared, Holds, LayoutHolds, LayoutLen, Len, PlaceDesc, Px, PxVec2, RegionAlign, Rel,
-    TextureHandle, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, Widget, WidgetId,
+    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextureHandle,
-    render::{GlyphPrimitive, Mask, MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst},
+    UiRegion, UiRenderState, UiRsc, UiVec2, Weight, WidgetId, Widgets,
-    util::Vec2,
+    render::{
+        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
+        TexturePrimitive,
+    },
+    ui::{
+        place::{PlaceSpan, RelBase},
+        render_state::{DrawInfo, Placing},
+    },
 };
 
 /// makes your surfaces look pretty
@@ -10,149 +19,511 @@ pub struct Painter<'a> {
     pub(super) state: &'a mut UiRenderState,
     pub(super) rsc: &'a mut dyn UiRsc,
 
+    /// This widget's rel base, per axis: a length of the window, and what a
+    /// fraction it or anything under it declares or reports is a fraction
+    /// of. A length rather than a box, so padding can take from both the
+    /// rel base and the box without either becoming the other.
+    pub(super) rel_base: UiVec2,
+    /// The box this widget was asked in, in its region node's coordinates:
+    /// what it draws in, and what its children's places are parts of.
     pub(super) region: UiRegion,
+    /// The window in pixels. Frames and boxes become pixels against this one
+    /// unit, regardless of region-node boundaries.
+    pub(super) window: PxVec2,
     pub(super) mask: MaskIdx,
     pub(super) textures: Vec<TextureHandle>,
-    pub(super) primitives: Vec<PrimitiveHandle>,
+    pub(super) primitives: Vec<RetainedPrimitive>,
+    pub(super) mask_region: Option<UiRegion>,
+    /// The previous drawing's owned mask, available for this draw to reclaim.
+    pub(super) mask_slot: Option<MaskIdx>,
+    /// Only children whose answers were read constrain this widget's answer.
+    pub(super) answer_under: LayoutHolds,
     pub(super) children: Vec<WidgetId>,
+    /// The children whose size this widget read while drawing.
+    pub(super) size_deps: Vec<WidgetId>,
+    /// What this draw itself reads, as against what its children's drawings
+    /// hold for: every window and every length of its own region until it
+    /// reads one, then that one unless it says otherwise, and the rel base or
+    /// region length it read symbolically, each of which makes the drawing
+    /// hold for that length alone.
+    pub(super) own: LayoutHolds,
+    /// What each child's drawing depends on. Asking a child again replaces
+    /// its drawing, so it replaces this too rather than narrowing it.
+    pub(super) under: Vec<(WidgetId, LayoutHolds)>,
+    /// The movable region this widget's primitives are positioned through:
+    /// its own when opted in, otherwise the nearest ancestor's.
+    pub(super) move_idx: MoveIdx,
     pub layer: usize,
+    /// The layer this widget was entered on, which its children's layers are
+    /// counted from however far `layer` has walked.
+    pub(super) own_layer: usize,
+    pub(super) depth: usize,
     pub(super) id: WidgetId,
 }
 
 impl<'a> Painter<'a> {
     fn primitive_at<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
+        let kind = self.rsc.ui_mut().primitives.kind::<P>();
+        self.write(kind, primitive, region);
+    }
+
+    /// Takes the kind, for a caller writing many of one primitive.
+    fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: UiRegion) {
+        self.write_resolved(kind, primitive, region, self.resolve(region));
+    }
+
+    /// A box in this widget's region, composed into its region node's
+    /// coordinates.
+    fn resolve(&self, region: UiRegion) -> UiRegion {
+        region.within(&self.region)
+    }
+
+    fn write_resolved<P: Primitive>(
+        &mut self,
+        kind: PrimitiveKind<P>,
+        primitive: P,
+        region: UiRegion,
+        resolved: UiRegion,
+    ) {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::PrimitiveWrites);
         let h = self.state.layers.write(
             self.layer,
             PrimitiveInst {
+                kind,
                 id: self.id,
                 primitive,
-                region,
+                region: resolved,
                 mask_idx: self.mask,
+                move_idx: self.move_idx,
             },
         );
+        self.push_primitive(RetainedPrimitive { handle: h, region });
+    }
+
+    fn push_primitive(&mut self, h: RetainedPrimitive) {
         if self.mask != MaskIdx::NONE {
-            // TODO: I have no clue if this works at all :joy:
             self.rsc.ui_mut().masks.push_ref(self.mask);
         }
         self.primitives.push(h);
     }
 
-    /// Writes a primitive to be rendered
+    /// Writes a primitive over the whole of this widget's own box.
-    pub fn primitive<P: Primitive>(&mut self, primitive: P) {
+    pub fn primitive(&mut self, primitive: impl PrimitiveLike) {
-        self.primitive_at(primitive, self.region)
+        let primitive = primitive.into_primitive(self);
+        self.primitive_at(primitive, UiRegion::FULL)
     }
 
-    pub fn primitive_within<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
+    /// Writes a primitive in a part of this widget's own box, in that box's
-        self.primitive_at(primitive, region.within(&self.region));
+    /// coordinates.
+    pub fn primitive_within(&mut self, primitive: impl PrimitiveLike, region: UiRegion) {
+        let primitive = primitive.into_primitive(self);
+        self.primitive_at(primitive, region);
     }
 
+    /// Sets a mask, in this widget's own box's coordinates.
     pub fn set_mask(&mut self, region: UiRegion) {
+        self.mask_region = Some(region);
         assert!(self.mask == MaskIdx::NONE);
-        self.mask = self.rsc.ui_mut().masks.push(Mask { region });
+        let resolved = self.resolve(region);
+        let move_idx = self.move_idx;
+        let mask = Mask {
+            region: resolved,
+            move_idx,
+        };
+        let masks = &mut self.rsc.ui_mut().masks;
+        self.mask = match self.mask_slot.take() {
+            Some(idx) => {
+                *masks.get_mut(idx) = mask;
+                idx
+            }
+            None => {
+                let idx = masks.push(mask);
+                // The owner keeps the slot alive even with no primitives.
+                masks.push_ref(idx);
+                idx
+            }
+        };
     }
 
-    /// Draws a widget within this widget's region.
+    /// Draws a widget in the whole of this widget's own box, with the rel
-    pub fn widget<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
+    /// base forwarded unchanged: what a container that is only a wrapper
-        self.widget_at(id, self.region);
+    /// around one child wants.
+    pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
+        self.widget_at(id, UiRegion::FULL)
     }
 
-    /// Draws a widget somewhere within this one.
+    /// Resolves what the place says about the child's rel base into a length,
-    /// Useful for drawing child widgets in select areas.
+    /// where that is this widget's own narrowed the way the region is. An
-    pub fn widget_within<W: ?Sized>(&mut self, id: &StrongWidget<W>, region: UiRegion) {
+    /// axis the region leaves whole is not read at all, so a wrapper that
-        self.widget_at(id, region.within(&self.region));
+    /// only moves its child does not pin its drawing to a rel base.
+    fn resolve_rel_base(&mut self, mut place: PlaceDesc) -> PlaceDesc {
+        for axis in Axis::BOTH {
+            let at = &mut place[axis];
+            let (RelBase::WithRegion, PlaceSpan::Within(span)) = (at.rel_base, at.span) else {
+                continue;
+            };
+            let len = span.len();
+            at.rel_base = match len == Len::FULL {
+                true => RelBase::Inherit,
+                false => RelBase::Len(len.within_len(self.rel_base(axis))),
+            };
+        }
+        place
     }
 
-    fn widget_at<W: ?Sized>(&mut self, id: &StrongWidget<W>, region: UiRegion) {
+    /// Asks a child, saying what its fractions are of and where it is asked.
-        self.children.push(id.id());
+    ///
-        self.state.draw_inner(
+    /// `place` says where the child goes and what its fractions are of:
-            self.layer,
+    /// see [`PlaceDesc`]. A `UiRegion` converts into the common case, which
-            id.id(),
+    /// is a box of this widget's own with the answer placed inside it.
+    ///
+    /// The child draws once, in the region that comes of it, and its answer
+    /// is placed inside that region by re-expressing the drawing. Nothing is
+    /// drawn again in a box an answer chose; a container that puts the
+    /// answer somewhere else says so with [`Self::place_at`].
+    pub fn widget_at<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
+        place: impl Into<PlaceDesc>,
+    ) -> DrawResult<'s, 'a, W> {
+        let offer = self.resolve_rel_base(place.into());
+        let Ask {
+            rel_base,
             region,
-            Some(self.id),
+            place,
-            self.mask,
+            declared,
+            holds: ask_holds,
+        } = self
+            .placing()
+            .ask(self.rsc.widgets(), self.window, id.id(), offer);
+        let region_node = self.rsc.widgets().is_region_node(id.id());
+        #[cfg(feature = "layout-diagnostics")]
+        if region_node {
+            diag::bump(Counter::RegionNodeDraws);
+            diag::region_node(id.id(), self.id, region);
+        }
+        // A child listed twice would be moved twice.
+        let re_asked = self.children.contains(&id.id());
+        if !re_asked {
+            self.children.push(id.id());
+        }
+        let drawn = self.state.draw_inner(
+            id.id(),
+            DrawInfo {
+                layer: self.layer,
+                parent: Some(self.id),
+                depth: self.depth + 1,
+                parent_move: self.move_idx,
+                region_node,
+                mask: self.mask,
+                rel_base,
+                region,
+                placed: place,
+                asked: offer,
+                ask_holds,
+                re_asked,
+            },
             None,
             self.rsc,
         );
+        let holds = self.in_parent(drawn.drawing_holds, region, place, declared);
+        let answer_holds = self.in_parent(drawn.answer.holds, region, place, declared);
+        match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
+            Some((_, kept)) => *kept = holds,
+            None => self.under.push((id.id(), holds)),
+        }
+        DrawResult {
+            child: id,
+            painter: self,
+            size: drawn.answer.size,
+            answer_holds,
+        }
     }
 
-    pub fn texture_within(&mut self, handle: &TextureHandle, region: UiRegion) {
+    /// Takes back a child that was drawn only to find out how long it is.
-        self.textures.push(handle.clone());
+    /// Its drawing is dropped and it is not one of this widget's children
-        self.primitive_at(handle.primitive(), region.within(&self.region));
+    /// this frame; what it answered is still something this widget asked.
+    pub fn undraw<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
+        self.children.retain(|child| *child != id.id());
+        self.under.retain(|(child, _)| *child != id.id());
+        self.state.undraw_rec(id.id(), self.rsc);
     }
 
-    pub fn texture(&mut self, handle: &TextureHandle) {
+    /// Puts a child in `place` of this widget's box, where that box is the
-        self.textures.push(handle.clone());
+    /// answer the child already gave: the drawing is re-expressed there
-        self.primitive(handle.primitive());
+    /// rather than made again -- what a row does once it knows every slot,
+    /// having measured each child from its cursor.
+    ///
+    /// A child this draw has not asked about, and one whose rel base this
+    /// narrows, is asked here instead: there is no answer to re-express, or
+    /// the question has changed. So a container that places every child the
+    /// same way says it once, and which of the two happens is this widget's
+    /// business rather than the caller's.
+    pub fn place_at<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
+        place: impl Into<PlaceDesc>,
+    ) -> DrawResult<'s, 'a, W> {
+        let place = self.resolve_rel_base(place.into());
+        let states_rel_base = Axis::BOTH
+            .iter()
+            .any(|&axis| matches!(place[axis].rel_base, RelBase::Len(_)));
+        if states_rel_base || !self.children.contains(&id.id()) {
+            return self.widget_at(id, place);
+        }
+        let at = self.placing();
+        self.state.place_in(id.id(), &at, place, self.rsc);
+        let active = &self.state.active[&id.id()];
+        let size = active.measured().unwrap_or(active.size);
+        DrawResult {
+            child: id,
+            painter: self,
+            size,
+            // Read where it was asked; moving it is not a second answer.
+            answer_holds: LayoutHolds::ANY,
+        }
     }
 
-    pub fn texture_at(&mut self, handle: &TextureHandle, region: UiRegion) {
+    /// This widget as the thing its children are placed within.
-        self.textures.push(handle.clone());
+    fn placing(&self) -> Placing {
-        self.primitive_at(handle.primitive(), region);
+        Placing {
+            id: Some(self.id),
+            region: self.region,
+            rel_base: self.rel_base,
+            depth: self.depth,
+            move_idx: self.move_idx,
+            mask: self.mask,
+        }
     }
 
-    pub fn render_text(
+    /// What a child says its length is without being drawn, if it can say,
+    /// as the length its draw would report: a fraction in it is resolved
+    /// against this widget's rel base, which is the rel base a child asked with
+    /// nothing narrowed gets. Asking counts as reading its size.
+    pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
+        let hint = self.rsc.widgets().exact_len(id.id(), axis);
+        let rel_base = self.rel_base[axis];
+        let resolved = hint.map(|hint| hint.within_len(rel_base));
+        #[cfg(feature = "layout-diagnostics")]
+        {
+            diag::hint_read(id.id(), self.id, axis, resolved);
+            diag::bump(match resolved {
+                Some(_) => Counter::HintHits,
+                None => Counter::HintMisses,
+            });
+        }
+        if let Some(hint) = hint {
+            self.depend_on(id);
+            // Resolving a fraction against this rel base makes this draw a
+            // function of the rel base's length. The fraction to ask about is
+            // the child's own: resolved against a rel base of pixels, none is
+            // left to see it by.
+            if hint.rel != Rel::ZERO {
+                self.own[axis].rel_base = Some(rel_base);
+            }
+        }
+        resolved
+    }
+
+    fn depend_on<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
+        if !self.size_deps.contains(&child.id()) {
+            self.size_deps.push(child.id());
+        }
+    }
+
+    pub fn render_text<'b>(
         &mut self,
-        buffer: &mut TextBuffer,
+        buffer: &'b mut TextBuffer,
         attrs: &TextAttrs,
         width: Option<f32>,
-    ) -> RenderedText {
+    ) -> &'b RenderedText {
+        #[cfg(feature = "layout-diagnostics")]
+        diag::render_text(self.id, self.rsc.widgets().label(self.id), width);
         let ui = self.rsc.ui_mut();
-        ui.text.render(buffer, attrs, width, &mut ui.textures)
+        ui.text.render(buffer, attrs, width)
     }
 
+    /// Writes glyphs in the selected rel base or region coordinates.
+    // TODO: merge the text methods into the primitive ones.
     pub fn glyphs(&mut self, text: &RenderedText, origin: UiRegion) {
+        // Glyph offsets and sizes are pixels, which compose additively.
+        // Only the shared origin needs composing through the region.
+        let resolved = self.resolve(origin);
+        let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {
-            let mut region = origin;
+            let place = |mut region: UiRegion| {
                 region.x.end = region.x.start;
                 region.y.end = region.y.start;
-            let mut region = region.offset(UiVec2::abs(glyph.offset));
+                let mut region = region.offset(UiVec2::from_px(glyph.offset));
-            region.x.end = region.x.start + UiScalar::abs(glyph.entry.width as f32);
+                let size = PxVec2::new(
-            region.y.end = region.y.start + UiScalar::abs(glyph.entry.height as f32);
+                    Px::from_int(glyph.entry.width as i32),
-            self.primitive_at(
+                    Px::from_int(glyph.entry.height as i32),
+                );
+                region.x.end = region.x.start.offset(size.x);
+                region.y.end = region.y.start.offset(size.y);
+                region
+            };
+            self.write_resolved(
+                kind,
                 GlyphPrimitive {
                     uv_min: glyph.entry.uv_min,
                     uv_max: glyph.entry.uv_max,
-                    view_idx: glyph.entry.view_idx,
+                    layer: glyph.entry.layer,
-                    sampler_idx: glyph.entry.sampler_idx,
                     color: text.color,
                     flags: glyph.entry.flags(),
                 },
-                region,
+                place(origin),
+                place(resolved),
             );
         }
     }
 
-    pub fn region(&self) -> UiRegion {
+    /// The symbolic length of this widget's own box along one axis, in the
-        self.region
+    /// lengths of its rel base that it places its children in. Reading it pins
+    /// the drawing to that length -- and to nothing about where the box
+    /// starts, which is what lets a container move without being drawn
+    /// again. One axis at a time, because a container that divides one axis
+    /// holds for any length of the other.
+    pub fn region_len(&mut self, axis: Axis) -> Len {
+        let len = self.region[axis].len();
+        self.own[axis].region_len = Some(len);
+        len
     }
 
-    pub fn size<W: ?Sized + Widget>(&mut self, id: &StrongWidget<W>) -> Size {
+    /// This widget's rel base along one axis: what a fraction it or anything
-        self.size_ctx().size(id)
+    /// under it declares or reports is a fraction of. A container reads it
+    /// to hand a length of it down -- padding, which takes its pixels off.
+    /// Reading it pins the drawing to that rel base, the way
+    /// [`Self::region_len`] pins it to the box.
+    pub fn rel_base(&mut self, axis: Axis) -> Len {
+        let len = self.rel_base[axis];
+        self.own[axis].rel_base = Some(len);
+        len
     }
 
-    pub fn len_axis<W: ?Sized + Widget>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Len {
+    /// Where this widget sits in a box longer than the length it takes. A
-        match axis {
+    /// widget that positions its own content reads it to place that content
-            Axis::X => self.size_ctx().width(id),
+    /// the way the box around it would have placed the widget.
-            Axis::Y => self.size_ctx().height(id),
+    pub fn alignment(&self) -> RegionAlign {
-        }
+        self.rsc.widgets().alignment(self.id)
     }
 
-    pub fn output_size(&self) -> Vec2 {
+    /// Whether a rule beside this widget gives its length on `axis` outright,
-        self.state.output_size
+    /// which makes whatever it reports for that axis moot. A rule that only
+    /// bounds the length is not one of these: the answer is still the
+    /// widget's to give, and something still has to work it out.
+    ///
+    /// The widget under a rule does not otherwise learn of it -- this is for
+    /// a container deciding whether reading its children across an axis is
+    /// worth anything, since reading one is also what makes its own size
+    /// depend on it.
+    pub fn has_exact_size(&self, axis: Axis) -> bool {
+        self.rsc.widgets().size_rules(self.id)[axis]
+            .exact()
+            .is_some()
     }
 
-    pub fn px_size(&mut self) -> Vec2 {
+    /// This widget's own box in pixels. Reading it makes the drawing one
-        self.region.size().to_abs(self.state.output_size)
+    /// that holds for this box only, until `holds` says how far it goes.
+    pub fn px_size(&mut self) -> PxVec2 {
+        PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
     }
 
-    pub fn text_data(&mut self) -> &mut TextData {
+    /// One axis of this widget's own box in pixels. Prefer this to
-        &mut self.rsc.ui_mut().text
+    /// [`Self::px_size`] when the other axis cannot affect the drawing.
+    pub fn px_len(&mut self, axis: Axis) -> Px {
+        let len = self.region[axis].len();
+        let px = len.to_px(self.window[axis]);
+        let own = &mut self.own[axis].region;
+        if *own == Holds::ANY {
+            *own = Holds::at(px);
+        }
+        px
+    }
+
+    /// The lengths of this widget's own box on `axis` that what it is drawing
+    /// holds for -- the same primitives, in the same fractions and offsets
+    /// of the box, and the same reported size. A widget that read its length
+    /// in pixels holds for that one alone until it says otherwise.
+    pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+        let len = self.region[axis].len();
+        let holds = holds.into();
+        debug_assert!(
+            holds.contains(len.to_px(self.window[axis])),
+            "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
+            self.label(),
+            self.id
+        );
+        self.own[axis].region = holds;
+    }
+
+    /// A window length in pixels, which is what every length in layout is
+    /// measured in. Reading one pins the drawing to this window wherever the
+    /// length is a fraction of it; one that is only pixels is that many
+    /// pixels in any window and pins nothing.
+    pub fn to_px(&mut self, len: Len, axis: Axis) -> Px {
+        let window = self.window[axis];
+        if len.rel != Rel::ZERO {
+            let own = &mut self.own[axis].window;
+            if *own == Holds::ANY {
+                *own = Holds::at(window);
+            }
+        }
+        len.to_px(window)
+    }
+
+    /// [`Len::longer_than`], asked on this widget's behalf: the windows the
+    /// comparison comes out the same way on are windows its drawing holds
+    /// for, and nowhere else does it. What a container has left for the
+    /// shares it divides is the one thing that asks.
+    ///
+    /// Narrowed rather than stated, because whatever else this widget read
+    /// about the window is a reason its drawing holds where it does too.
+    pub fn longer_than(&mut self, len: Len, than: Len, axis: Axis) -> bool {
+        let window = self.window[axis];
+        let (longer, holds) = len.longer_than(than, window);
+        debug_assert!(
+            holds.contains(window),
+            "'{}' ({:?}) compared two lengths and kept a range without this window",
+            self.label(),
+            self.id
+        );
+        self.own[axis].window = self.own[axis].window.and(holds);
+        longer
+    }
+
+    /// The windows this drawing holds for, stated rather than taken: a
+    /// container that branched on a length in pixels says which side of the
+    /// boundary it was on, which is wider than the one window reading that
+    /// length pins, and replaces it.
+    pub fn window_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+        let holds = holds.into();
+        debug_assert!(
+            holds.contains(self.window[axis]),
+            "'{}' ({:?}) says its drawing holds for windows that leave out this one",
+            self.label(),
+            self.id
+        );
+        self.own[axis].window = holds;
     }
 
     pub fn child_layer(&mut self) {
         self.layer = self.state.layers.child(self.layer);
     }
 
+    /// The layer this widget's `n`th child draws on, addressed rather than
+    /// walked to. A container that measures one child by drawing it can ask
+    /// on the layer that child will end up on, and then the second ask is a
+    /// reuse rather than a second drawing on another layer.
+    pub fn child_layer_at(&mut self, n: usize) {
+        let mut at = self.state.layers.child(self.own_layer);
+        for _ in 0..n {
+            at = self.state.layers.next(at);
+        }
+        self.layer = at;
+    }
+
     pub fn next_layer(&mut self) {
         self.layer = self.state.layers.next(self.layer);
     }
@@ -164,8 +535,325 @@ impl<'a> Painter<'a> {
     pub fn id(&self) -> &WidgetId {
         &self.id
     }
+}
 
-    pub fn size_ctx(&mut self) -> SizeCtx<'_> {
+/// A child that has just been drawn. Reading its size records that this
-        self.state.size_ctx(self.id, self.region.size(), self.rsc)
+/// widget's own size depends on it; dropping it without reading draws the
+/// child and leaves the parent independent of what it came to.
+pub struct DrawResult<'p, 'a, W: ?Sized> {
+    painter: &'p mut Painter<'a>,
+    child: &'p StrongWidget<W>,
+    size: Size,
+    answer_holds: LayoutHolds,
+}
+
+impl<W: ?Sized> DrawResult<'_, '_, W> {
+    pub fn size(self) -> Size {
+        #[cfg(feature = "layout-diagnostics")]
+        {
+            diag::bump(Counter::SizeReads);
+            diag::size_read(self.child.id(), self.painter.id, self.size);
+        }
+        self.painter.depend_on(self.child);
+        self.painter.answer_under = self.painter.answer_under.and(self.answer_holds);
+        self.size
+    }
+
+    pub fn len(self, axis: Axis) -> LayoutLen {
+        self.size()[axis]
+    }
+}
+
+/// What `Painter::primitive` takes: a primitive, or something that yields one
+/// and does whatever else drawing it needs.
+pub trait PrimitiveLike {
+    type Primitive: Primitive;
+    fn into_primitive(self, painter: &mut Painter) -> Self::Primitive;
+}
+
+impl<P: Primitive> PrimitiveLike for P {
+    type Primitive = P;
+    fn into_primitive(self, _: &mut Painter) -> P {
+        self
+    }
+}
+
+impl PrimitiveLike for &TextureHandle {
+    type Primitive = TexturePrimitive;
+
+    /// Retains a share of the handle, so the slot the primitive names cannot
+    /// be freed and reused while it is still drawn.
+    fn into_primitive(self, painter: &mut Painter) -> TexturePrimitive {
+        painter.textures.push(self.clone());
+        self.into()
+    }
+}
+
+impl Painter<'_> {
+    /// Moves what a child depends on into this widget's own terms, taking
+    /// only what the child was asked with.
+    ///
+    /// Window ranges are already about the one unit and combine directly.
+    /// A rel base pin becomes this widget's own rel base wherever a length of it
+    /// is what reached the child; where only pixels did, no length of this
+    /// rel base can change the child's and the pin stops here.
+    ///
+    /// A child's validity maps back through the part of this widget's box,
+    /// where the box the child was asked in is that part; a declared length
+    /// places the box inside the part instead, and then only that length
+    /// reaches the child. A narrowed rel base is not one of these: it decides
+    /// what fractions under the child mean and leaves the box the part it
+    /// was given.
+    fn in_parent(
+        &self,
+        holds: LayoutHolds,
+        region: UiRegion,
+        place: PlaceDesc,
+        declared: Declared,
+    ) -> LayoutHolds {
+        let mut result = LayoutHolds::ANY;
+        for axis in Axis::BOTH {
+            let declared = declared[axis];
+            let holds = holds[axis];
+            let at = place[axis];
+            let result = &mut result[axis];
+            // Every read became pixels against the window, so a range on
+            // it is already in this widget's terms.
+            result.window = holds.window;
+            // A length this widget named -- a resolved share, a box a sibling
+            // decided, a box it sized outright, which is its own base -- is
+            // not a length of this widget's rel base, so a pin on it stops
+            // here. So does a declaration in pixels: no length of either base
+            // is in it to see.
+            let reaches = !matches!(at.rel_base, RelBase::Len(_))
+                && declared.is_none_or(|len| len.rel != Rel::ZERO);
+            result.rel_base = holds.rel_base.and(reaches.then(|| self.rel_base[axis]));
+            match (at.span, declared) {
+                // Its box is a part of this widget's own box, in that box's
+                // own lengths, so what it holds for maps back through that
+                // part into a range on this widget's box. A length it pinned
+                // is this widget's length less the part's pixels where the
+                // part is the whole of the box less pixels, which is the one
+                // shape that inverts exactly; any other part pins this
+                // widget's own length.
+                (PlaceSpan::Within(span), None) => {
+                    let part_len = span.len();
+                    result.region = holds.region.through(part_len);
+                    result.region_len = holds.region_len.map(|pinned| match part_len.rel {
+                        Rel::ONE => pinned - Len::from_parts(Rel::ZERO, part_len.px),
+                        _ => self.region[axis].len(),
+                    });
+                }
+                // Its box is a length this widget decided, from its own
+                // rel base or from a sibling's answer: no length of this
+                // widget's box reaches it, so what it holds for is a range
+                // on the window and none of it on that box.
+                _ => {
+                    result.window = result.window.and(holds.region.through(region[axis].len()));
+                }
+            }
+        }
+        result
+    }
+}
+
+impl Widgets {
+    /// What says a widget's length on one axis without drawing it, if anything
+    /// does. A rule is the answer where there is one: it wins over whatever the
+    /// widget would draw, so it has to win over what the widget says too -- a
+    /// share included, since a share is a length only to whoever divides one,
+    /// and that is the parent rather than this widget.
+    fn exact_len(&self, id: WidgetId, axis: Axis) -> Option<LayoutLen> {
+        self.size_rules(id)[axis].exact().or_else(|| {
+            // A hint still narrows the box where no rule does, which is how a
+            // widget with a natural pixel size -- an image, a gap -- gets that
+            // size rather than the whole offer. That is the offer's business
+            // rather than a declaration's, and this falls away once a widget
+            // occupies its reported size inside the box it was offered.
+            self.get_dyn(id)?.size_hint(axis)
+        })
+    }
+
+    /// What a widget's box is where a rule or its own hint gives one outright,
+    /// rather than a share for whoever draws it to divide.
+    pub(super) fn declared_lens(&self, id: WidgetId) -> Declared {
+        Declared::from_axes(|axis| self.exact_len(id, axis)?.declared())
+    }
+}
+
+/// One ask of a widget: the box it draws in, what its fractions are of, and
+/// what deciding those read.
+pub(super) struct Ask {
+    pub rel_base: UiVec2,
+    pub region: UiRegion,
+    /// The place the ask came to, which a rule of the widget's own can take
+    /// past the box its parent offered.
+    pub place: PlaceDesc,
+    pub declared: Declared,
+    /// What the ask itself holds for, kept on the widget asked about: a rule
+    /// compared against the offer in pixels holds only for the windows on its
+    /// side of the crossing, and that range reaches whoever asked through the
+    /// drawing it is part of. Kept on the widget asked about rather than on
+    /// the asker because the root has no asker.
+    pub holds: LayoutHolds,
+}
+
+impl Placing {
+    /// Asks about a widget at `place` of this box, with the widget's own
+    /// rules applied to what the place offers it. `place` is resolved: what
+    /// a rel base of the asker's is a fraction of, the asker worked out.
+    ///
+    /// Every ask is this one, the root's included -- there the box is the
+    /// window and nothing above narrowed it, which is what [`Self::WINDOW`]
+    /// says.
+    pub(super) fn ask(
+        &self,
+        widgets: &Widgets,
+        window: PxVec2,
+        id: WidgetId,
+        mut place: PlaceDesc,
+    ) -> Ask {
+        let align = widgets.alignment(id);
+        let mut holds = LayoutHolds::ANY;
+        // A share fills what the pixels and fraction beside it leave of the
+        // box and overflows where they are longer, which is the rule a span
+        // follows with one child. Only the overflow is a box of the child's
+        // own: a share that fits is the box it was given, which is what this
+        // place already says.
+        for axis in Axis::BOTH {
+            let (len, kept) =
+                self.share_past_the_offer(widgets, window[axis], id, place, align, axis);
+            holds[axis].window = holds[axis].window.and(kept);
+            if let Some(len) = len {
+                place[axis] = len.as_desc().fills();
+            }
+        }
+        let declared = widgets.declared_lens(id);
+        let (rel_base, region) =
+            place.rel_base_and_region(self.region, self.rel_base, declared, align);
+        Ask {
+            rel_base,
+            region,
+            place,
+            declared,
+            holds,
+        }
+    }
+
+    /// The box a widget's own share asks for where that is longer than the
+    /// box `place` gives it, and nothing where the share fits -- with the
+    /// windows that answer holds for, which is a range either way.
+    ///
+    /// A share is a length only to whoever divides one, and nothing divides a
+    /// box handed to one child: what is left of it after the pixels and the
+    /// fraction beside the share is what the share takes, so the length comes
+    /// to the whole box until those are longer than it and to them once they
+    /// are. Only that second case is a box its parent did not give, and the
+    /// crossing between them is a question in pixels.
+    fn share_past_the_offer(
+        &self,
+        widgets: &Widgets,
+        window: Px,
+        id: WidgetId,
+        place: PlaceDesc,
+        align: RegionAlign,
+        axis: Axis,
+    ) -> (Option<Len>, Holds) {
+        // A place that is the widget's placement outright is a box its parent
+        // decided, and a parent that divides one has already given the share
+        // whatever it was owed. Only an offer -- a box with the answer still
+        // to be placed inside it -- is a box a share reads.
+        if place[axis].fills {
+            return (None, Holds::ANY);
+        }
+        // A share with nothing beside it is the box whatever the box is, so
+        // there is no comparison to make and no range to keep for one.
+        let Some(stated) = widgets.exact_len(id, axis) else {
+            return (None, Holds::ANY);
+        };
+        if stated.leftover == Weight::ZERO || stated.is_only_leftover() {
+            return (None, Holds::ANY);
+        }
+        let fixed = stated
+            .without_leftover()
+            .within_len(place.base(axis, self.rel_base));
+        let offer = place.of(self.region, align)[axis].len();
+        let (longer, holds) = fixed.longer_than(offer, window);
+        (longer.then_some(fixed), holds)
+    }
+}
+
+impl LayoutLen {
+    /// Whether what a widget reported along an axis is the whole of the box
+    /// it is in rather than a part to be placed inside it. A share fills,
+    /// because a share is a length only to whoever divides one, and whoever
+    /// did is the one that handed down this box. A declared axis does too:
+    /// the rule already gave the region its length, and the rule's length is
+    /// what the widget reports there. And an axis the parent decided from
+    /// the answer is the answer already.
+    pub(super) fn fills(&self, declared: Option<Len>, decided: bool) -> bool {
+        self.leftover != Weight::ZERO || declared.is_some() || decided
+    }
+}
+
+impl PlaceDesc {
+    /// Where a widget's drawing goes inside the part its parent gave it: what
+    /// it reported, on the side of the part its alignment says, and the whole
+    /// part wherever the answer fills it.
+    ///
+    /// The length it reported is a length of its rel base, and the part is one
+    /// too, so this takes one from the other rather than composing it into the
+    /// part. That is what makes a fraction the same fraction wherever the part
+    /// it is placed in sits and however long it is -- the fraction is resolved
+    /// once, here, against the rel base it was reported of.
+    pub(super) fn placement(
+        self,
+        region: UiRegion,
+        size: Size,
+        declared: Declared,
+        align: RegionAlign,
+    ) -> UiRegion {
+        let mut placed = region;
+        for axis in Axis::BOTH {
+            let reported = size[axis];
+            if reported.fills(declared[axis], self[axis].fills) {
+                continue;
+            }
+            placed[axis] = placed[axis].place(reported.without_leftover(), align[axis]);
+        }
+        placed
+    }
+
+    /// The rel base length and the box a child is asked in, in the coordinates the
+    /// widget asking draws in.
+    ///
+    /// `own` is that widget's own box, and `place` what of it the child is
+    /// given, including any rel base it states -- a row's slot, or padding's rel
+    /// base less its pixels. That is a window length, like every other length
+    /// here, since a slot of a row is not a fraction of anything the row can
+    /// name. The child's declaration is a fraction of whichever reached it, and
+    /// is the only one that also places the box: a box the caller decided is
+    /// what `place` names.
+    pub(super) fn rel_base_and_region(
+        self,
+        own: UiRegion,
+        parent_rel_base: UiVec2,
+        declared: Declared,
+        align: RegionAlign,
+    ) -> (UiVec2, UiRegion) {
+        let given = self.of(own, align);
+        let mut rel_base = parent_rel_base;
+        let mut region = given;
+        for axis in Axis::BOTH {
+            let base = self.base(axis, parent_rel_base);
+            let len = declared[axis]
+                .map(|len| len.within_len(base))
+                .unwrap_or(base);
+            rel_base[axis] = len;
+            if declared[axis].is_some() {
+                region[axis] = given[axis].place(len, align[axis]);
+            }
+        }
+        (rel_base, region)
     }
 }

core/src/ui/place.rs

@@ -0,0 +1,214 @@
+use crate::util::impl_axis_index;
+use crate::{Axis, AxisAlign, Len, PrimitiveHandle, RegionAlign, UiRegion, UiSpan, UiVec2};
+
+/// How a child's region along one axis comes from the region of the widget
+/// asking, and what its fractions are of.
+///
+/// The three ways of saying a region are the three the geometry already has:
+/// a span composed into the caller's box, a span shifted to where that box
+/// starts, and a length placed in it by alignment. Which one is meant cannot
+/// be read off the numbers, since two of them take the same span and apply
+/// it differently, so it is said here.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct PlaceDescAxis {
+    pub span: PlaceSpan,
+    pub fills: bool,
+    pub rel_base: RelBase,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum PlaceSpan {
+    Within(UiSpan),
+    Shifted(UiSpan),
+    Sized(Len),
+}
+
+/// What a child's fractions are of. [`PlaceSpan::Sized`] is a length the
+/// caller named, which is always its own base, so nothing here constructs one
+/// beside anything but [`Self::Len`].
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum RelBase {
+    /// The caller's own, unchanged.
+    Inherit,
+    /// The caller's own, narrowed the way the region is.
+    WithRegion,
+    /// This length of the window.
+    Len(Len),
+}
+
+impl PlaceDescAxis {
+    /// The whole of the caller's box.
+    pub const WHOLE: Self = UiSpan::FULL.within_desc();
+
+    /// This region is the child's placement: its answer is not placed inside
+    /// it again. A container uses it where it hands back exactly what the
+    /// child asked for -- a row placing a child at the length it reported.
+    pub const fn fills(mut self) -> Self {
+        self.fills = true;
+        self
+    }
+
+    /// This along `axis`, and the whole of the caller's box across it: what
+    /// a container dividing one axis says, since nothing divides the other.
+    /// [`PlaceDesc::from_axis`] says the across one where it is not the
+    /// whole.
+    pub const fn on_axis(self, axis: Axis) -> PlaceDesc {
+        PlaceDesc::from_axis(axis, self, Self::WHOLE)
+    }
+
+    /// What the child's fractions are of, as a length of the window: a
+    /// resolved share, or a box a sibling's answer decided.
+    pub const fn rel_base(mut self, len: Len) -> Self {
+        self.rel_base = RelBase::Len(len);
+        self
+    }
+
+    /// Where it lands in the coordinates `own` is in.
+    pub fn of(self, own: UiSpan, align: AxisAlign) -> UiSpan {
+        match self.span {
+            PlaceSpan::Within(span) => span.within(&own),
+            PlaceSpan::Shifted(mut span) => {
+                span.shift(own.start);
+                span
+            }
+            PlaceSpan::Sized(len) => own.place(len, align),
+        }
+    }
+}
+
+/// Where a child is asked, on both axes. A [`UiRegion`] converts into the
+/// common case: that box of the caller's own, the answer placed inside it.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct PlaceDesc {
+    pub x: PlaceDescAxis,
+    pub y: PlaceDescAxis,
+}
+
+impl PlaceDesc {
+    /// The whole of the caller's box, on both axes.
+    pub const WHOLE: Self = Self::splat(PlaceDescAxis::WHOLE);
+
+    pub const fn new(x: PlaceDescAxis, y: PlaceDescAxis) -> Self {
+        Self { x, y }
+    }
+
+    /// The same on both axes.
+    pub const fn splat(place: PlaceDescAxis) -> Self {
+        Self { x: place, y: place }
+    }
+
+    /// A description per axis, where the two differ and neither is the
+    /// axis a container divides.
+    pub fn from_axes(f: impl Fn(Axis) -> PlaceDescAxis) -> Self {
+        Self::new(f(Axis::X), f(Axis::Y))
+    }
+
+    /// `aligned` on `axis` and `ortho` on the other, which is how a
+    /// container that divides one axis says what it is doing.
+    pub const fn from_axis(axis: Axis, aligned: PlaceDescAxis, ortho: PlaceDescAxis) -> Self {
+        match axis {
+            Axis::X => Self::new(aligned, ortho),
+            Axis::Y => Self::new(ortho, aligned),
+        }
+    }
+
+    /// Both regions are the child's placement. See [`PlaceDescAxis::fills`].
+    pub const fn fills(self) -> Self {
+        Self::new(self.x.fills(), self.y.fills())
+    }
+
+    /// The child's rel base on one axis. See [`PlaceDescAxis::rel_base`].
+    pub const fn rel_base(mut self, axis: Axis, len: Len) -> Self {
+        self[axis] = self[axis].rel_base(len);
+        self
+    }
+
+    /// What a child's fractions on one axis are of, as a length of the
+    /// window: a length this place names, or the rel base of the widget
+    /// giving it, which is `parent_rel_base`.
+    pub(super) fn base(&self, axis: Axis, parent_rel_base: UiVec2) -> Len {
+        match self[axis].rel_base {
+            RelBase::Len(len) => len,
+            RelBase::Inherit | RelBase::WithRegion => parent_rel_base[axis],
+        }
+    }
+
+    /// The box each axis names, in the coordinates `own` is in.
+    pub fn of(self, own: UiRegion, align: RegionAlign) -> UiRegion {
+        UiRegion::new(self.x.of(own.x, align.x), self.y.of(own.y, align.y))
+    }
+}
+
+impl UiSpan {
+    /// This span composed into the caller's own box, so it moves and scales
+    /// with it: [`UiSpan::within`], which is what a container that insets
+    /// one speaks. Taking eleven pixels off the end needs no length, where
+    /// saying the same thing in window lengths would make the container read
+    /// its own box -- and a box chosen from its own answer then feeds back
+    /// into the answer.
+    ///
+    /// The child's rel base is narrowed the same way, so padding takes its
+    /// pixels off both and `rel(1)` under it fills the caller rather than
+    /// overflowing it.
+    pub const fn within_desc(self) -> PlaceDescAxis {
+        PlaceDescAxis {
+            span: PlaceSpan::Within(self),
+            fills: false,
+            rel_base: RelBase::WithRegion,
+        }
+    }
+
+    /// This span shifted to where the caller's own box starts: window
+    /// lengths along a cursor, which is what a container dividing room among
+    /// its children speaks. A child's report is a window length, so the
+    /// cursor that sums those reports is one too, and a moved box re-places
+    /// every child by re-adding its start, exactly.
+    ///
+    /// The child's rel base passes through: how far along the cursor a child
+    /// sits says nothing about what a fraction under it is of. The same span
+    /// says [`Self::within_desc`] as a part of that box instead, and which is
+    /// meant cannot be read off the numbers.
+    pub const fn shifted_desc(self) -> PlaceDescAxis {
+        PlaceDescAxis {
+            span: PlaceSpan::Shifted(self),
+            fills: false,
+            rel_base: RelBase::Inherit,
+        }
+    }
+}
+
+impl Len {
+    /// A box this long, placed in the caller's own by the child's alignment:
+    /// the rule that places an answer, with the length given from above
+    /// rather than reported. What a stack's sizing child decides for the
+    /// rest. It is the child's rel base too.
+    pub const fn as_desc(self) -> PlaceDescAxis {
+        PlaceDescAxis {
+            span: PlaceSpan::Sized(self),
+            fills: false,
+            rel_base: RelBase::Len(self),
+        }
+    }
+}
+
+impl From<UiRegion> for PlaceDesc {
+    fn from(region: UiRegion) -> Self {
+        Self::new(region.x.within_desc(), region.y.within_desc())
+    }
+}
+
+impl From<PlaceDescAxis> for PlaceDesc {
+    fn from(place: PlaceDescAxis) -> Self {
+        Self::splat(place)
+    }
+}
+
+/// A primitive as it was written: its box in the widget's own box's
+/// coordinates, which is what a move of that box re-composes from.
+#[derive(Debug)]
+pub struct RetainedPrimitive {
+    pub handle: PrimitiveHandle,
+    pub region: UiRegion,
+}
+
+impl_axis_index!(PlaceDesc => PlaceDescAxis);

core/src/ui/size.rs

@@ -1,91 +0,0 @@
-use crate::{
-    Axis, AxisT, IdLike, Len, RenderedText, Size, TextAttrs, TextBuffer, TextData, Textures,
-    UiVec2, WidgetAxisFns, WidgetId, Widgets, XAxis, YAxis, ui::cache::Cache, util::Vec2,
-};
-
-pub struct SizeCtx<'a> {
-    pub text: &'a mut TextData,
-    pub textures: &'a mut Textures,
-    pub(super) source: WidgetId,
-    pub(super) widgets: &'a Widgets,
-    pub(super) cache: &'a mut Cache,
-    /// TODO: should this be pub? rn used for sized
-    pub outer: UiVec2,
-    pub(super) output_size: Vec2,
-    pub(super) id: WidgetId,
-}
-
-impl SizeCtx<'_> {
-    pub fn id(&self) -> &WidgetId {
-        &self.id
-    }
-
-    pub fn source(&self) -> &WidgetId {
-        &self.source
-    }
-
-    pub(super) fn len_inner<A: const AxisT>(&mut self, id: WidgetId) -> Len {
-        if let Some((_, len)) = self.cache.size.axis::<A>().get(&id) {
-            return *len;
-        }
-        let len = self
-            .widgets
-            .get_dyn_dynamic(id)
-            .desired_len::<A>(&mut SizeCtx {
-                text: self.text,
-                textures: self.textures,
-                source: self.source,
-                widgets: self.widgets,
-                cache: self.cache,
-                outer: self.outer,
-                output_size: self.output_size,
-                id,
-            });
-        self.cache.size.axis::<A>().insert(id, (self.outer, len));
-        len
-    }
-
-    pub fn width(&mut self, id: impl IdLike) -> Len {
-        self.len_inner::<XAxis>(id.id())
-    }
-
-    pub fn height(&mut self, id: impl IdLike) -> Len {
-        self.len_inner::<YAxis>(id.id())
-    }
-
-    pub fn len_axis(&mut self, id: impl IdLike, axis: Axis) -> Len {
-        match axis {
-            Axis::X => self.width(id),
-            Axis::Y => self.height(id),
-        }
-    }
-
-    pub fn size(&mut self, id: impl IdLike) -> Size {
-        let id = id.id();
-        Size {
-            x: self.width(id),
-            y: self.height(id),
-        }
-    }
-
-    pub fn px_size(&mut self) -> Vec2 {
-        self.outer.to_abs(self.output_size)
-    }
-
-    pub fn output_size(&mut self) -> Vec2 {
-        self.output_size
-    }
-
-    pub fn draw_text(
-        &mut self,
-        buffer: &mut TextBuffer,
-        attrs: &TextAttrs,
-        width: Option<f32>,
-    ) -> RenderedText {
-        self.text.render(buffer, attrs, width, self.textures)
-    }
-
-    pub fn label(&self, id: WidgetId) -> &String {
-        self.widgets.label(id)
-    }
-}

core/src/util/arena.rs

@@ -34,6 +34,10 @@ impl<T, I: IdNum> Arena<T, I> {
         self.tracker.free(id);
         self.data[i]
     }
+
+    pub fn get_mut(&mut self, id: Id<I>) -> &mut T {
+        &mut self.data[id.idx()]
+    }
 }
 
 impl<T, I: IdNum> Default for Arena<T, I> {
@@ -71,6 +75,11 @@ impl<T, I: IdNum> TrackedArena<T, I> {
         self.refs[i.idx()] += 1;
     }
 
+    pub fn get_mut(&mut self, id: Id<I>) -> &mut T {
+        self.changed = true;
+        self.inner.get_mut(id)
+    }
+
     pub fn remove(&mut self, id: Id<I>) -> T
     where
         T: Copy,

core/src/util/math.rs

@@ -1,34 +1,13 @@
-use std::ops::*;
-
 pub const trait LerpUtil {
     fn lerp(self, from: Self, to: Self) -> Self;
-    fn lerp_inv(self, from: Self, to: Self) -> Self;
 }
 
-pub const trait DivOr {
+const impl LerpUtil for f32 {
-    fn div_or(self, rhs: Self, other: Self) -> Self;
-}
-
-const impl DivOr for f32 {
-    fn div_or(self, rhs: Self, other: Self) -> Self {
-        let res = self / rhs;
-        if res.is_nan() { other } else { res }
-    }
-}
-
-const impl<
-    T: const Add<Output = T> + const Sub<Output = T> + const Mul<Output = T> + const DivOr + Copy,
-> LerpUtil for T
-{
     /// linear interpolation
     /// from * (1.0 - self) + to * self
     fn lerp(self, from: Self, to: Self) -> Self {
         from + (to - from) * self
     }
-    /// inverse of lerp
-    fn lerp_inv(self, from: Self, to: Self) -> Self {
-        (self - from).div_or(to - from, from)
-    }
 }
 
 macro_rules! impl_op {
@@ -77,6 +56,34 @@ macro_rules! impl_op {
             }
         }
     };
+    // Without the `f32` operations, for a type whose fields are not all the
+    // same kind of number: there is nothing a bare float means to a fraction
+    // and an offset at once.
+    (same $T:ident $op:ident $fn:ident $opa:ident $fna:ident; $($field:ident)*) => {
+        #[allow(non_snake_case)]
+        mod ${concat($T, _op_, $fn, _same_impl)} {
+            use super::*;
+            #[allow(unused_imports)]
+            use std::ops::*;
+            const impl $op for $T {
+                type Output = Self;
+
+                fn $fn(self, rhs: Self) -> Self::Output {
+                    Self {
+                        $($field: self.$field.$fn(rhs.$field),)*
+                    }
+                }
+            }
+            const impl $opa for $T {
+                fn $fna(&mut self, rhs: Self) {
+                    *self = self.$fn(rhs);
+                }
+            }
+        }
+    };
+    (same $T:ident $op:ident $fn:ident; $($field:ident)*) => {
+        impl_op!(same $T $op $fn ${concat($op,Assign)} ${concat($fn,_assign)}; $($field)*);
+    };
     ($T:ident $op:ident $fn:ident; $($field:ident)*) => {
         impl_op!($T $op $fn ${concat($op,Assign)} ${concat($fn,_assign)}; $($field)*);
     };
@@ -86,3 +93,31 @@ macro_rules! impl_op {
 }
 
 pub(crate) use impl_op;
+
+/// `Index<Axis>` for a pair, which is how every pair here is read by axis.
+/// The generics clause is given in braces where the type has one.
+macro_rules! impl_axis_index {
+    ($({$($gen:tt)*})? $T:ty => $Out:ty) => {
+        const impl $(<$($gen)*>)? std::ops::Index<crate::Axis> for $T {
+            type Output = $Out;
+
+            fn index(&self, axis: crate::Axis) -> &$Out {
+                match axis {
+                    crate::Axis::X => &self.x,
+                    crate::Axis::Y => &self.y,
+                }
+            }
+        }
+
+        const impl $(<$($gen)*>)? std::ops::IndexMut<crate::Axis> for $T {
+            fn index_mut(&mut self, axis: crate::Axis) -> &mut $Out {
+                match axis {
+                    crate::Axis::X => &mut self.x,
+                    crate::Axis::Y => &mut self.y,
+                }
+            }
+        }
+    };
+}
+
+pub(crate) use impl_axis_index;

core/src/util/slot.rs

@@ -1,4 +1,4 @@
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub struct SlotId {
     idx: u32,
     genr: u32,

core/src/util/trust.rs

@@ -1,8 +1,3 @@
-#[allow(clippy::missing_safety_doc)]
-pub(crate) unsafe fn forget_ref<'a, T>(x: &T) -> &'a T {
-    unsafe { std::mem::transmute::<&T, &T>(x) }
-}
-
 #[allow(clippy::missing_safety_doc)]
 pub(crate) unsafe fn forget_mut<'a, T>(x: &mut T) -> &'a mut T {
     unsafe { std::mem::transmute::<&mut T, &mut T>(x) }

core/src/util/vec2.rs

@@ -1,7 +1,11 @@
-use crate::util::{DivOr, impl_op};
+use crate::util::impl_op;
 use std::{hash::Hash, ops::*};
 
-#[repr(C)]
+/// `align(8)` because that is WGSL's alignment for a `vec2<f32>`, so any GPU
+/// struct holding one is laid out the way its shader reads it without having
+/// to say so itself. Those structs still need a manual `unsafe impl Pod`,
+/// since the trailing padding this introduces is what `derive(Pod)` refuses.
+#[repr(C, align(8))]
 #[derive(Clone, Copy, PartialEq, Default, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct Vec2 {
     pub x: f32,
@@ -67,15 +71,6 @@ impl_op!(Vec2 Sub sub; x y);
 impl_op!(Vec2 Mul mul; x y);
 impl_op!(Vec2 Div div; x y);
 
-const impl DivOr for Vec2 {
-    fn div_or(self, rhs: Self, other: Self) -> Self {
-        Self {
-            x: self.x.div_or(rhs.x, other.x),
-            y: self.y.div_or(rhs.y, other.y),
-        }
-    }
-}
-
 impl Neg for Vec2 {
     type Output = Self;
 

core/src/widget/data.rs

@@ -1,8 +1,11 @@
-use crate::Widget;
+use crate::{RegionAlign, SizeRules, Widget};
 
 pub struct WidgetData {
     pub widget: Box<dyn Widget>,
     pub label: String,
+    pub(super) region_node: bool,
+    pub(super) size: SizeRules,
+    pub(super) align: RegionAlign,
     /// dynamic borrow checking
     pub borrowed: bool,
 }
@@ -16,6 +19,9 @@ impl WidgetData {
         Self {
             widget: Box::new(widget),
             label,
+            region_node: false,
+            size: SizeRules::default(),
+            align: RegionAlign::default(),
             borrowed: false,
         }
     }

core/src/widget/mod.rs

@@ -1,9 +1,10 @@
-use crate::{Axis, AxisT, Len, Painter, SizeCtx};
+use crate::{Axis, LayoutLen, Painter, Size};
 use std::any::Any;
 
 mod data;
 mod handle;
 mod like;
+mod size_rule;
 mod tag;
 mod view;
 mod widgets;
@@ -11,36 +12,31 @@ mod widgets;
 pub use data::*;
 pub use handle::*;
 pub use like::*;
+pub use size_rule::*;
 pub use tag::*;
 pub use view::*;
 pub use widgets::*;
 
 pub trait Widget: Any {
-    fn draw(&mut self, painter: &mut Painter);
+    /// Draws the widget, and returns what it used of the box it was given.
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len;
+    fn draw(&mut self, painter: &mut Painter) -> Size;
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len;
-}
 
-pub trait WidgetAxisFns {
+    /// An exact length the widget can give without a painter or its children.
-    fn desired_len<A: AxisT>(&mut self, ctx: &mut SizeCtx) -> Len;
+    /// Optional, and saves a draw rather than changing one: a hint that
-}
+    /// disagrees with the eventual draw fails a debug assertion.
-
+    fn size_hint(&self, _axis: Axis) -> Option<LayoutLen> {
-impl<W: Widget + ?Sized> WidgetAxisFns for W {
+        None
-    fn desired_len<A: AxisT>(&mut self, ctx: &mut SizeCtx) -> Len {
-        match A::get() {
-            Axis::X => self.desired_width(ctx),
-            Axis::Y => self.desired_height(ctx),
-        }
     }
 }
 
 impl Widget for () {
-    fn draw(&mut self, _: &mut Painter) {}
+    /// A gap: nothing drawn, at the default length, so a span gives it a share.
-    fn desired_width(&mut self, _: &mut SizeCtx) -> Len {
+    fn draw(&mut self, _: &mut Painter) -> Size {
-        Len::ZERO
+        Size::default()
     }
-    fn desired_height(&mut self, _: &mut SizeCtx) -> Len {
+
-        Len::ZERO
+    fn size_hint(&self, _axis: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::default())
     }
 }
 

core/src/widget/size_rule.rs

@@ -0,0 +1,86 @@
+use crate::util::impl_axis_index;
+use crate::{Axis, LayoutLen, Len};
+
+/// What a widget's length on one axis is, as a rule its parent applies where
+/// it draws it rather than an answer the widget gives about itself.
+///
+/// A rule and a drawn size are not two opinions to reconcile: a rule wins on
+/// the axis it names, and the `Size` returned by `draw` answers only the axes
+/// with no rule. That is what lets a span divide its space around a length
+/// nobody has drawn yet, and it is why a rule lives beside the widget rather
+/// than inside it -- the widget under the rule never has to know about it.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub enum SizeRule {
+    /// Whatever the widget reports from drawing.
+    #[default]
+    Free,
+    /// This length, whatever the widget reports.
+    Exact(LayoutLen),
+}
+
+impl SizeRule {
+    /// The length this rule gives without the widget being drawn, if it can
+    /// give one.
+    pub fn declared(&self) -> Option<Len> {
+        self.exact().and_then(|len| len.declared())
+    }
+
+    /// The length this rule gives outright, whatever the widget reports --
+    /// which makes the widget's answer on that axis moot. A share counts: it
+    /// is a length the widget's parent still has to divide, so it is exact
+    /// here and resolved there, unlike `declared`, which is only the ones
+    /// that give a box directly.
+    pub fn exact(&self) -> Option<LayoutLen> {
+        match self {
+            Self::Free => None,
+            Self::Exact(len) => Some(*len),
+        }
+    }
+}
+
+impl From<LayoutLen> for SizeRule {
+    fn from(len: LayoutLen) -> Self {
+        Self::Exact(len)
+    }
+}
+
+impl From<Option<LayoutLen>> for SizeRule {
+    fn from(len: Option<LayoutLen>) -> Self {
+        len.map_or(Self::Free, Self::Exact)
+    }
+}
+
+/// One rule per axis, which is how a widget carries a length on one axis and
+/// leaves the other to whatever it draws.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct SizeRules {
+    pub x: SizeRule,
+    pub y: SizeRule,
+}
+
+impl_axis_index!(SizeRules => SizeRule);
+
+/// What a widget's box is on each axis where something says so outright,
+/// before it is drawn: a rule beside it, or a hint it gives about itself.
+/// Whoever draws the widget resolves these against its rel base.
+///
+/// A [`Len`] rather than a [`LayoutLen`], because a share can never be one
+/// -- see [`LayoutLen::declared`].
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Declared {
+    pub x: Option<Len>,
+    pub y: Option<Len>,
+}
+
+impl Declared {
+    pub const NONE: Self = Self { x: None, y: None };
+
+    pub fn from_axes(f: impl Fn(Axis) -> Option<Len>) -> Self {
+        Self {
+            x: f(Axis::X),
+            y: f(Axis::Y),
+        }
+    }
+}
+
+impl_axis_index!(Declared => Option<Len>);

core/src/widget/widgets.rs

@@ -1,7 +1,8 @@
 use std::sync::mpsc::{Receiver, Sender, channel};
 
 use crate::{
-    IdLike, StrongWidget, WeakWidget, Widget, WidgetData, WidgetId,
+    Axis, AxisAlign, IdLike, RegionAlign, SizeRule, SizeRules, StrongWidget, WeakWidget, Widget,
+    WidgetData, WidgetId,
     util::{DynBorrower, HashSet, SlotVec, forget_mut, to_mut},
 };
 
@@ -29,6 +30,14 @@ impl Widgets {
         !self.needs_redraw.is_empty()
     }
 
+    /// Marks this widget for the next frame to draw again, with nothing about
+    /// it changed. Taking a widget mutably marks it too, which is the ordinary
+    /// content-change signal; this is for a change the borrow cannot express,
+    /// and for asking for the same tree over again.
+    pub fn mark_for_redraw(&mut self, id: impl IdLike) {
+        self.needs_redraw.insert(id.id());
+    }
+
     pub fn get_dyn(&self, id: WidgetId) -> Option<&dyn Widget> {
         Some(self.vec.get(id)?.widget.as_ref())
     }
@@ -40,14 +49,14 @@ impl Widgets {
 
     /// get_dyn but dynamic borrow checking of widgets
     /// lets you do recursive (tree) operations, like the painter does
-    pub(crate) fn get_dyn_dynamic<'a>(&self, id: WidgetId) -> WidgetWrapper<'a> {
+    pub(crate) fn get_dyn_dynamic<'a>(&self, id: WidgetId) -> DynBorrower<'a, dyn Widget> {
         // SAFETY: must guarantee no other mutable references to this widget exist
         // done through the borrow variable
         let data = unsafe { forget_mut(to_mut(self.vec.get(id).unwrap())) };
         if data.borrowed {
             panic!("tried to mutably borrow the same widget twice");
         }
-        WidgetWrapper::new(data.widget.as_mut(), &mut data.borrowed)
+        DynBorrower::new(data.widget.as_mut(), &mut data.borrowed)
     }
 
     pub fn get<I: IdLike>(&self, id: &I) -> Option<&I::Widget>
@@ -100,6 +109,71 @@ impl Widgets {
         self.data_mut(id.id()).unwrap().label = label;
     }
 
+    /// Whether this widget owns a movable retained region.
+    pub fn is_region_node(&self, id: impl IdLike) -> bool {
+        self.data(id).unwrap().region_node
+    }
+
+    /// Chooses whether this widget's retained drawing has one movable region
+    /// of its own. Changing the boundary redraws the subtree once so every
+    /// primitive names the right coordinate space.
+    pub fn set_region_node(&mut self, id: impl IdLike, region_node: bool) {
+        let id = id.id();
+        let data = self.data_mut(id).unwrap();
+        if data.region_node == region_node {
+            return;
+        }
+        data.region_node = region_node;
+        self.needs_redraw.insert(id);
+    }
+
+    /// The length rules whoever draws this widget applies to its box.
+    pub fn size_rules(&self, id: impl IdLike) -> SizeRules {
+        self.data(id).unwrap().size
+    }
+
+    /// Sets one axis's rule. The widget is marked rather than its parent
+    /// because the parent is not known here; `redraw` escalates a changed
+    /// declared length to whoever resolves it.
+    pub fn set_size_rule(&mut self, id: impl IdLike, axis: Axis, rule: SizeRule) {
+        let id = id.id();
+        let data = self.data_mut(id).unwrap();
+        if data.size[axis] == rule {
+            return;
+        }
+        data.size[axis] = rule;
+        self.needs_redraw.insert(id);
+    }
+
+    /// Where this widget sits in a box longer than the length it takes.
+    pub fn alignment(&self, id: impl IdLike) -> RegionAlign {
+        self.data(id).unwrap().align
+    }
+
+    /// Sets one axis's alignment. Which box a widget ends up in is its
+    /// parent's to decide, so this is escalated the way a length rule is.
+    pub fn set_alignment(&mut self, id: impl IdLike, axis: Axis, align: AxisAlign) {
+        let id = id.id();
+        let data = self.data_mut(id).unwrap();
+        if data.align[axis] == align {
+            return;
+        }
+        data.align[axis] = align;
+        self.needs_redraw.insert(id);
+    }
+
+    /// Both axes at once.
+    pub fn set_size_rules(
+        &mut self,
+        id: impl IdLike,
+        x: impl Into<SizeRule>,
+        y: impl Into<SizeRule>,
+    ) {
+        let id = id.id();
+        self.set_size_rule(id, Axis::X, x.into());
+        self.set_size_rule(id, Axis::Y, y.into());
+    }
+
     pub fn data_mut(&mut self, id: impl IdLike) -> Option<&mut WidgetData> {
         self.vec.get_mut(id.id())
     }
@@ -122,8 +196,6 @@ impl Default for Widgets {
     }
 }
 
-pub type WidgetWrapper<'a> = DynBorrower<'a, dyn Widget>;
-
 impl<I: IdLike> std::ops::Index<I> for Widgets
 where
     I::Widget: Sized + Widget,

examples/minimal.rs

@@ -10,11 +10,7 @@ struct State {
 }
 
 impl DefaultAppState for State {
-    fn new(
+    fn new(mut ui_state: DefaultUiState, rsc: &mut DefaultRsc<Self>, _: Proxy<Self>) -> Self {
-        mut ui_state: DefaultUiState,
-        rsc: &mut DefaultRsc<Self>,
-        _: Proxy<Self::Event>,
-    ) -> Self {
         rect(Color::RED).set_root(rsc, &mut ui_state);
         Self { ui_state }
     }

examples/random.rs

@@ -0,0 +1,31 @@
+//! The seeded random tree `tests/generated.rs` checks, drawn so it can be
+//! looked at. `IRIS_SEED` and `IRIS_DEPTH` choose which one.
+
+use iris::prelude::*;
+use iris::random::Edits;
+
+fn env(name: &str, fallback: u64) -> u64 {
+    std::env::var(name)
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(fallback)
+}
+
+fn main() {
+    DefaultApp::<State>::run();
+}
+
+#[derive(DefaultUiState)]
+struct State {
+    ui_state: DefaultUiState,
+}
+
+impl DefaultAppState for State {
+    fn new(mut ui_state: DefaultUiState, rsc: &mut DefaultRsc<Self>, _: Proxy<Self>) -> Self {
+        let seed = env("IRIS_SEED", 1);
+        let depth = env("IRIS_DEPTH", 4) as usize;
+        let (root, _) = iris::random::grow(rsc, seed, depth, &Edits::default());
+        ui_state.set_root(root);
+        Self { ui_state }
+    }
+}

examples/tabs/main.rs

@@ -15,31 +15,31 @@ pub struct Client {
 }
 
 impl DefaultAppState for Client {
-    fn new(
+    fn new(mut ui_state: DefaultUiState, rsc: &mut DefaultRsc<Self>, _: Proxy<Self>) -> Self {
-        mut ui_state: DefaultUiState,
-        rsc: &mut DefaultRsc<Self>,
-        _: Proxy<Self::Event>,
-    ) -> Self {
         let rrect = rect(Color::WHITE).radius(20);
         let pad_test = (
             rrect.color(Color::BLUE),
             (
+                // The square is one widget and the two shares of the row it
+                // sits centred in are another: a length is a property of a
+                // widget, so `.width` here would overwrite the `.sized`.
                 rrect
                     .color(Color::RED)
                     .sized((100, 100))
                     .center()
-                    .width(rest(2)),
+                    .wrapper()
+                    .width(leftover(2)),
                 (
                     rrect.color(Color::ORANGE),
                     rrect.color(Color::LIME).pad(10.0),
                 )
                     .span(Dir::RIGHT)
-                    .width(rest(2)),
+                    .width(leftover(2)),
                 rrect.color(Color::YELLOW),
             )
                 .span(Dir::RIGHT)
                 .pad(10)
-                .width(rest(3)),
+                .width(leftover(3)),
         )
             .span(Dir::RIGHT)
             .add(rsc);
@@ -125,11 +125,11 @@ impl DefaultAppState for Client {
             .add(rsc);
 
         let text_edit_scroll = (
-            msg_area.height(rest(1)),
+            msg_area.height(leftover(1)),
             (
                 Rect::new(Color::WHITE.darker(0.9)),
                 (
-                    add_text.width(rest(1)),
+                    add_text.width(leftover(1)),
                     Rect::new(Color::GREEN)
                         .on(CursorSense::click(), move |ctx, rsc: &mut ClientRsc| {
                             rsc.run_event::<Submit>(add_text, (), ctx.state);
@@ -147,7 +147,7 @@ impl DefaultAppState for Client {
             .span(Dir::DOWN)
             .add(rsc);
 
-        let main = WidgetPtr::new().add(rsc);
+        let main = Wrapper::new().add(rsc);
 
         let vals = Rc::new(RefCell::new((0, Vec::new())));
         let mut switch_button = |color, to: WeakWidget, label| {
@@ -212,10 +212,10 @@ impl DefaultAppState for Client {
         render: &mut UiRenderState,
     ) {
         let new = format!(
-            "widgets: {}\nactive: {}\nviews: {}",
+            "widgets: {}\nactive: {}\ntextures: {}",
             rsc.widgets().len(),
             render.active_widgets(),
-            self.ui_state.renderer.ui.view_count(),
+            rsc.ui().textures.count(),
         );
         if new != *rsc.widgets()[self.info].content {
             *rsc.widgets_mut()[self.info].content = new;

examples/task.rs

@@ -11,11 +11,7 @@ struct State {
 }
 
 impl DefaultAppState for State {
-    fn new(
+    fn new(mut ui_state: DefaultUiState, rsc: &mut DefaultRsc<Self>, _: Proxy<Self>) -> Self {
-        mut ui_state: DefaultUiState,
-        rsc: &mut DefaultRsc<Self>,
-        _: Proxy<Self::Event>,
-    ) -> Self {
         let rect = rect(Color::RED).add(rsc);
         rect.task_on(CursorSense::click(), async move |mut ctx| {
             tokio::time::sleep(Duration::from_secs(1)).await;

examples/text.rs

@@ -0,0 +1,67 @@
+//! Text sizing: wrapped text reads the width it is offered, fixed text does
+//! not, and both report a height their container lays out around.
+
+use iris::prelude::*;
+
+fn main() {
+    DefaultApp::<State>::run();
+}
+
+#[derive(DefaultUiState)]
+struct State {
+    ui_state: DefaultUiState,
+}
+
+const SAMPLE: &str = "Wrapping shapes one source into as many lines as its container \
+    leaves room for, so the height of a paragraph is an answer rather than a setting, and \
+    the same words in a narrower box come back taller. Resize the window and watch the \
+    text below reflow into a different number of lines while nothing about it changes.";
+
+impl DefaultAppState for State {
+    fn new(mut ui_state: DefaultUiState, rsc: &mut DefaultRsc<Self>, _: Proxy<Self>) -> Self {
+        let panel = || rect(Color::WHITE.darker(0.85));
+
+        let wrapped = wtext(SAMPLE)
+            .size(28)
+            .wrap(true)
+            .text_align(Align::LEFT)
+            .pad(16)
+            .background(panel());
+
+        // Each one takes the whole width, because `text_align` puts the
+        // glyphs somewhere in the box the text is given and a text that
+        // reports the width of its own glyphs is given exactly that.
+        let label = |text: &str, align| wtext(text).size(24).text_align(align).width(rel(1.0));
+        let aligned = (
+            label("left", Align::LEFT),
+            label("centred", Align::H_CENTER),
+            label("right", Align::RIGHT),
+        )
+            .span(Dir::DOWN)
+            .gap(8)
+            .pad(16)
+            .background(panel());
+
+        // The same words in half the width, which is a different number of
+        // lines and so a different height. A declared width only holds along
+        // a span's own axis, hence the row.
+        let narrow = (
+            wtext(SAMPLE)
+                .size(20)
+                .wrap(true)
+                .pad(16)
+                .background(panel())
+                .align(Align::TOP)
+                .width(rel(0.5)),
+            rect(Color::WHITE.darker(0.95)),
+        )
+            .span(Dir::RIGHT);
+
+        (wrapped, aligned, narrow)
+            .span(Dir::DOWN)
+            .gap(12)
+            .pad(12)
+            .set_root(rsc, &mut ui_state);
+        Self { ui_state }
+    }
+}

examples/view.rs

@@ -36,11 +36,7 @@ impl Test {
 }
 
 impl DefaultAppState for State {
-    fn new(
+    fn new(mut ui_state: DefaultUiState, rsc: &mut DefaultRsc<Self>, _: Proxy<Self>) -> Self {
-        mut ui_state: DefaultUiState,
-        rsc: &mut DefaultRsc<Self>,
-        _: Proxy<Self::Event>,
-    ) -> Self {
         let test = Test::new(rsc);
 
         test.on(CursorSense::click(), move |_, rsc| {

rig-input/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "rig-input"
+version.workspace = true
+edition.workspace = true
+
+# Replays `.touch` recordings through Wayland's virtual-pointer protocol;
+# headless sway has no input devices for coordinate-driving tools to move.
+[[bin]]
+name = "replay-touch"
+path = "src/main.rs"
+
+[dependencies]
+# Share the harness parser so both ways of replaying read a file the same.
+iris = { path = ".." }
+wayland-client = { workspace = true }
+wayland-protocols-wlr = { workspace = true }

rig-input/src/main.rs

@@ -0,0 +1,141 @@
+use iris::harness::{TouchAction, TouchScript};
+use std::time::Duration;
+use wayland_client::protocol::wl_pointer::ButtonState;
+use wayland_client::protocol::{wl_registry, wl_seat};
+use wayland_client::{Connection, Dispatch, QueueHandle, delegate_noop};
+use wayland_protocols_wlr::virtual_pointer::v1::client::{
+    zwlr_virtual_pointer_manager_v1::ZwlrVirtualPointerManagerV1,
+    zwlr_virtual_pointer_v1::ZwlrVirtualPointerV1,
+};
+
+const BTN_LEFT: u32 = 0x110;
+
+const SETTLE: Duration = Duration::from_millis(200);
+
+#[derive(Default)]
+struct Globals {
+    seat: Option<wl_seat::WlSeat>,
+    manager: Option<ZwlrVirtualPointerManagerV1>,
+}
+
+impl Dispatch<wl_registry::WlRegistry, ()> for Globals {
+    fn event(
+        state: &mut Self,
+        registry: &wl_registry::WlRegistry,
+        event: wl_registry::Event,
+        _: &(),
+        _: &Connection,
+        qh: &QueueHandle<Self>,
+    ) {
+        let wl_registry::Event::Global {
+            name,
+            interface,
+            version,
+        } = event
+        else {
+            return;
+        };
+        match interface.as_str() {
+            "wl_seat" => {
+                state.seat = Some(registry.bind(name, version.min(7), qh, ()));
+            }
+            "zwlr_virtual_pointer_manager_v1" => {
+                state.manager = Some(registry.bind(name, version.min(2), qh, ()));
+            }
+            _ => {}
+        }
+    }
+}
+
+delegate_noop!(Globals: ignore wl_seat::WlSeat);
+delegate_noop!(Globals: ZwlrVirtualPointerManagerV1);
+delegate_noop!(Globals: ZwlrVirtualPointerV1);
+
+fn main() {
+    let args: Vec<String> = std::env::args().skip(1).collect();
+    let [width, height, path] = args.as_slice() else {
+        eprintln!("usage: replay-touch WIDTH HEIGHT FILE");
+        std::process::exit(2);
+    };
+    let (width, height) = (parse(width, "WIDTH"), parse(height, "HEIGHT"));
+    let text = std::fs::read_to_string(path)
+        .unwrap_or_else(|e| fail(&format!("could not read {path}: {e}")));
+    let script = TouchScript::parse(&text).unwrap_or_else(|e| fail(&e));
+
+    let conn = Connection::connect_to_env().unwrap_or_else(|e| {
+        fail(&format!(
+            "no wayland display ({e}); is WAYLAND_DISPLAY set?"
+        ))
+    });
+    let mut queue = conn.new_event_queue();
+    let qh = queue.handle();
+    let display = conn.display();
+    display.get_registry(&qh, ());
+    let mut globals = Globals::default();
+    queue
+        .roundtrip(&mut globals)
+        .unwrap_or_else(|e| fail(&format!("wayland roundtrip failed: {e}")));
+
+    let manager = globals.manager.as_ref().unwrap_or_else(|| {
+        fail(
+            "this compositor does not offer zwlr_virtual_pointer_manager_v1, so a pointer cannot \
+             be synthesised; sway and every wlroots compositor do",
+        )
+    });
+    let pointer = manager.create_virtual_pointer(globals.seat.as_ref(), &qh, ());
+
+    // Put the pointer where the gesture starts and let the compositor
+    // settle before anything is pressed. Without this the press is
+    // dropped: sway has just learned about this pointer, and a button
+    // sent in the same breath as the motion that first puts it over a
+    // window arrives before there is a focused surface to send it to --
+    // winit sees `CursorEntered`, the moves and the *release*, never the
+    // press, so the gesture reads as a hover and nothing scrolls. Found
+    // by printing winit's own events; the settle is what fixed it.
+    if let Some(first) = script.samples.first() {
+        pointer.motion_absolute(0, first.pos.x as u32, first.pos.y as u32, width, height);
+        pointer.frame();
+        conn.flush()
+            .unwrap_or_else(|e| fail(&format!("flush: {e}")));
+        std::thread::sleep(SETTLE);
+    }
+
+    let mut previous = 0;
+    for sample in &script.samples {
+        std::thread::sleep(Duration::from_millis(sample.t_ms - previous));
+        previous = sample.t_ms;
+        let t = sample.t_ms as u32;
+        pointer.motion_absolute(t, sample.pos.x as u32, sample.pos.y as u32, width, height);
+        pointer.frame();
+        // The button goes in a frame of its own, *after* the motion has
+        // been committed. Sent in the same frame as the motion that
+        // first puts the pointer over the window, sway drops it: the
+        // client sees `CursorEntered` and the moves but never a
+        // `MouseInput { state: Pressed }`, so the whole gesture reads as
+        // a hover and nothing scrolls. Found exactly that way, by
+        // printing winit's events.
+        let state = match sample.action {
+            TouchAction::Down => Some(ButtonState::Pressed),
+            TouchAction::Up => Some(ButtonState::Released),
+            TouchAction::Move => None,
+        };
+        if let Some(state) = state {
+            pointer.button(t, BTN_LEFT, state);
+            pointer.frame();
+        }
+        conn.flush()
+            .unwrap_or_else(|e| fail(&format!("flush: {e}")));
+    }
+    pointer.destroy();
+    conn.flush().ok();
+}
+
+fn parse(text: &str, what: &str) -> u32 {
+    text.parse()
+        .unwrap_or_else(|_| fail(&format!("{what} is not a whole number: {text:?}")))
+}
+
+fn fail(message: &str) -> ! {
+    eprintln!("replay-touch: {message}");
+    std::process::exit(1);
+}

scripts/headless.conf

@@ -0,0 +1,14 @@
+# The compositor `scripts/run-headless.sh` starts, so that an example has a
+# surface where there is no display. Nothing here is meant to be looked at
+# directly; `grim` is.
+#
+# No Xwayland: winit talks Wayland natively, so an X server is a second thing
+# to go wrong for no gain.
+xwayland disable
+
+# The default output, overridden per run by `--mode`. Larger than the window
+# an example opens, so nothing is scaled or clipped.
+output HEADLESS-1 mode 1920x1200@60Hz
+
+default_border none
+focus_follows_mouse no

scripts/run-headless.sh

@@ -0,0 +1,187 @@
+#!/bin/sh
+# Run an iris example on a machine with no display.
+#
+#   ./scripts/run-headless.sh tabs
+#   ./scripts/run-headless.sh tabs --shot /tmp/tabs.png --seconds 4
+#   ./scripts/run-headless.sh tabs --replay taps.touch --shot /tmp/tabs.png
+#   ./scripts/run-headless.sh app --dir ../elsewhere --mode 1080x2424@120Hz
+#
+# `--dir DIR` names the workspace to build in, defaulting to this one, so a
+# project that depends on iris can be run through the same rig. `--bin` runs a
+# crate binary rather than an example, and takes its own argv from
+# `$RUN_HEADLESS_ARGS`, word-split on purpose.
+#
+# `--mode` sets the output, for running something at a size other than a
+# desktop's -- a phone's, say. Set every run rather than only when it changes:
+# the compositor is reused between runs, so a default-shaped run after a
+# custom one would otherwise inherit the other's output and quietly screenshot
+# the wrong size.
+#
+# `--resize WxH@Hz` changes the output under the app once it is up, then
+# screenshots. A resize is its own case: what it has to match is a cold start
+# at that size, byte for byte, and nothing in `cargo test` can see it.
+#
+# `--replay FILE` drives a `.touch` recording into the window through
+# `replay-touch`, which reads it with the same parser `iris::harness` uses. A
+# recording is `<ms> down|move|up <x> <y>` in the output's own pixels. With
+# `--shot` it also writes `<shot>-before.png` from just before the gesture,
+# since "it moved" is a claim about two pictures.
+#
+# What it supplies is a compositor for winit to open a surface on: a headless
+# sway, and `grim` to screenshot it. Sway gets its own socket and runtime
+# directory rather than joining whatever else is running, because it tiles --
+# adding a window to someone else's compositor resizes theirs.
+set -eu
+
+need() {
+    command -v "$1" >/dev/null 2>&1 || {
+        echo "run-headless: $1 is not installed ($2)" >&2
+        exit 127
+    }
+}
+need sway "the compositor an example opens its window on"
+need swaymsg "sway's control socket"
+
+scripts=$(cd "$(dirname "$0")" && pwd)
+root=$(cd "$scripts/.." && pwd)
+workdir="$root"
+cd "$root"
+run="${XDG_RUNTIME_DIR:-/tmp}/iris-headless"
+seconds=3
+shot=""
+replay=""
+resize=""
+example=""
+kind=example
+mode=1920x1200@60Hz
+
+
+while [ $# -gt 0 ]; do
+    case "$1" in
+        --shot) shot=$2; shift 2 ;;
+        --seconds) seconds=$2; shift 2 ;;
+        --bin) kind=bin; shift ;;
+        --mode) mode=$2; shift 2 ;;
+        --resize) resize=$2; shift 2 ;;
+        --replay) replay=$2; shift 2 ;;
+        --dir) workdir=$(cd "$2" && pwd); shift 2 ;;
+        --) shift; break ;;
+        *) example=$1; shift ;;
+    esac
+done
+[ -n "$example" ] || { echo "usage: $0 NAME [--bin] [--dir DIR] [--mode WxH@Hz] [--resize WxH@Hz] [--replay TOUCH] [--shot PNG] [--seconds N] [-- cargo args]" >&2; exit 2; }
+[ -z "$replay" ] || [ -f "$replay" ] || { echo "run-headless: no touch script at $replay" >&2; exit 2; }
+[ -z "$shot" ] || need grim "the screenshot --shot writes"
+
+mkdir -p "$run"
+export SWAYSOCK="$run/sway.sock"
+
+# Named rather than left to sway's pid-based default, so a second run reuses
+# this compositor instead of starting another beside it.
+if ! swaymsg -t get_version >/dev/null 2>&1; then
+    rm -f "$SWAYSOCK"
+    WLR_BACKENDS=headless WLR_LIBINPUT_NO_DEVICES=1 LIBSEAT_BACKEND=noop \
+        setsid sway -c "$scripts/headless.conf" >"$run/sway.log" 2>&1 &
+    i=0
+    while [ $i -lt 20 ]; do
+        swaymsg -t get_version >/dev/null 2>&1 && break
+        i=$((i + 1)); sleep 0.5
+    done
+    swaymsg -t get_version >/dev/null 2>&1 || {
+        echo "run-headless: compositor did not start; see $run/sway.log" >&2
+        exit 1
+    }
+fi
+
+rm -f "$run/display"
+swaymsg exec -- "sh -c 'printf %s \"\$WAYLAND_DISPLAY\" > $run/display'" >/dev/null
+i=0
+while [ $i -lt 20 ]; do
+    [ -s "$run/display" ] && break
+    i=$((i + 1)); sleep 0.5
+done
+[ -s "$run/display" ] || { echo "run-headless: could not read WAYLAND_DISPLAY" >&2; exit 1; }
+WAYLAND_DISPLAY=$(cat "$run/display")
+export WAYLAND_DISPLAY
+
+echo "run-headless: $WAYLAND_DISPLAY (sway $(swaymsg -t get_version --raw | sed -n 's/.*"human_readable":"\([^"]*\)".*/\1/p'))" >&2
+
+# The extent `replay-touch` positions against, so a script's coordinates are
+# the output's own pixels. Set beside every mode change, since a gesture
+# scaled against a mode the output no longer has lands somewhere else and
+# still looks like a run that worked.
+set_mode() {
+    swaymsg output HEADLESS-1 mode "$1" >/dev/null
+    out_w=${1%x*}
+    out_h=${1#*x}; out_h=${out_h%@*}
+}
+set_mode "$mode"
+
+# Built before the app starts, so a compile error is not reported as a
+# window that failed to move.
+[ -z "$replay" ] || (cd "$root" && cargo build --bin replay-touch -p rig-input) >&2
+
+cd "$workdir"
+if [ "$kind" = bin ]; then
+    cargo build --bin "$example" "$@" >&2
+    bin="$workdir/target/debug/$example"
+else
+    cargo build --example "$example" "$@" >&2
+    bin="$workdir/target/debug/examples/$example"
+fi
+
+# Deliberately word-split: this is the binary's own argv, not a single path.
+# shellcheck disable=SC2086
+"$bin" ${RUN_HEADLESS_ARGS:-} >"$run/$example.log" 2>&1 &
+pid=$!
+trap 'kill "$pid" 2>/dev/null || true' EXIT INT TERM
+
+# Wait for the window to be mapped rather than for a number of seconds. A
+# fixed sleep took an all-black screenshot the first time this ran, when sway
+# had started in the same invocation and had not composited its output yet --
+# which is indistinguishable from an app that draws nothing.
+i=0
+while [ $i -lt 40 ]; do
+    kill -0 "$pid" 2>/dev/null || break
+    swaymsg -t get_tree --raw 2>/dev/null | grep -q "\"pid\":$pid," && break
+    i=$((i + 1)); sleep 0.25
+done
+
+i=0
+while [ $i -lt "$((seconds * 2))" ]; do
+    kill -0 "$pid" 2>/dev/null || break
+    i=$((i + 1)); sleep 0.5
+done
+
+if [ -n "$resize" ] && kill -0 "$pid" 2>/dev/null; then
+    set_mode "$resize"
+    echo "run-headless: resized to $resize" >&2
+    sleep 2
+fi
+
+if [ -n "$replay" ] && kill -0 "$pid" 2>/dev/null; then
+    if [ -n "$shot" ]; then
+        grim "${shot%.png}-before.png"
+        echo "run-headless: wrote ${shot%.png}-before.png (before the gesture)" >&2
+    fi
+    "$root/target/debug/replay-touch" "$out_w" "$out_h" "$replay"
+    # A fling outlives the finger: the gesture's own last sample is not
+    # when the list stops. Long enough for Android's spline to settle
+    # (`FlingCalculator::duration` tops out around a second and a half).
+    sleep 2
+fi
+
+if kill -0 "$pid" 2>/dev/null; then
+    [ -n "$shot" ] && grim "$shot" && echo "run-headless: wrote $shot" >&2
+    kill "$pid" 2>/dev/null || true
+    wait "$pid" 2>/dev/null || true
+    status=0
+else
+    wait "$pid" 2>/dev/null || status=$?
+    echo "run-headless: $example exited early (status ${status:-0})" >&2
+    status=${status:-1}
+fi
+
+echo "--- $example output ---" >&2
+cat "$run/$example.log" >&2
+exit "$status"

src/default/attr.rs

@@ -15,8 +15,11 @@ where
             let region = ctx.data.render.window_region(&id).unwrap();
             let id_pos = region.top_left;
             let container_pos = ctx.data.render.window_region(&container).unwrap().top_left;
-            let pos = ctx.data.pos + container_pos - id_pos;
+            // The two regions are on the grid and the pointer is not, so the
-            let size = region.size();
+            // step between them is taken there and the pointer keeps the
+            // precision the platform gave it.
+            let pos = ctx.data.pos + (container_pos - id_pos).to_f32();
+            let size = region.size().to_f32();
             select(
                 rsc,
                 ctx.data.render,
@@ -70,8 +73,8 @@ fn select(
     if let Some(region) = render.window_region(&id) {
         state.window.set_ime_allowed(true);
         state.window.set_ime_cursor_area(
-            LogicalPosition::<f32>::from(region.top_left.tuple()),
+            LogicalPosition::<f32>::from(region.top_left.to_f32().tuple()),
-            LogicalSize::<f32>::from(region.size().tuple()),
+            LogicalSize::<f32>::from(region.size().to_f32().tuple()),
         );
     }
     state.focus = Some(id);

src/default/mod.rs

@@ -1,10 +1,6 @@
 use crate::prelude::*;
 use arboard::Clipboard;
-use std::{
+use std::{marker::PhantomData, sync::Arc, time::Instant};
-    marker::{PhantomData, Sized},
-    sync::Arc,
-    time::Instant,
-};
 use winit::{
     event::{Ime, WindowEvent},
     event_loop::{ActiveEventLoop, EventLoopProxy},
@@ -29,7 +25,35 @@ pub use sense::*;
 pub use state::*;
 pub use task::*;
 
-pub type Proxy<Event> = EventLoopProxy<Event>;
+/// Sends an application's own events to its event loop. It wraps the proxy
+/// rather than being one because task updates travel the same way: what an
+/// application sends is its `Event`, not the loop's whole message type.
+pub struct Proxy<State: DefaultAppState>(EventLoopProxy<DefaultEvent<State>>);
+
+impl<State: DefaultAppState> Clone for Proxy<State> {
+    fn clone(&self) -> Self {
+        Self(self.0.clone())
+    }
+}
+
+impl<State: DefaultAppState> Proxy<State> {
+    pub fn send_event(&self, event: State::Event) {
+        let _ = self.0.send_event(DefaultEvent::User(event));
+    }
+}
+
+/// What the event loop carries: the application's own events, and the
+/// updates tasks send back to the ui thread.
+pub enum DefaultEvent<State: DefaultAppState> {
+    User(State::Event),
+    Update(Box<dyn TaskUpdate<DefaultRsc<State>>>),
+}
+
+impl<State: DefaultAppState> TaskQueue<DefaultRsc<State>> for Proxy<State> {
+    fn send(&self, update: Box<dyn TaskUpdate<DefaultRsc<State>>>) {
+        let _ = self.0.send_event(DefaultEvent::Update(update));
+    }
+}
 
 pub struct DefaultUiState {
     pub root: Option<StrongWidget>,
@@ -70,9 +94,8 @@ pub trait HasDefaultUiState: Sized + 'static {
 }
 
 pub trait DefaultAppState: HasDefaultUiState {
-    type Event = ();
+    type Event: Send = ();
-    fn new(ui_state: DefaultUiState, rsc: &mut DefaultRsc<Self>, proxy: Proxy<Self::Event>)
+    fn new(ui_state: DefaultUiState, rsc: &mut DefaultRsc<Self>, proxy: Proxy<Self>) -> Self;
-    -> Self;
     #[allow(unused_variables)]
     fn event(
         &mut self,
@@ -105,18 +128,14 @@ pub struct DefaultRsc<State: 'static> {
 }
 
 impl<State> DefaultRsc<State> {
-    fn init(window: Arc<Window>) -> (Self, TaskMsgReceiver<Self>) {
+    pub fn init(queue: Arc<dyn TaskQueue<Self>>) -> Self {
-        let (tasks, recv) = Tasks::init(window);
-        (
         Self {
             ui: Default::default(),
             events: Default::default(),
-                tasks,
+            tasks: Tasks::init(queue),
             state: Default::default(),
             _state: Default::default(),
-            },
+        }
-            recv,
-        )
     }
 
     pub fn create_state<T: 'static>(&mut self, id: impl IdLike, data: T) -> WeakState<T> {
@@ -181,43 +200,32 @@ pub struct DefaultApp<State: DefaultAppState> {
     rsc: DefaultRsc<State>,
     render: UiRenderState,
     state: State,
-    task_recv: TaskMsgReceiver<DefaultRsc<State>>,
 }
 
 impl<State: DefaultAppState> AppState for DefaultApp<State> {
-    type Event = State::Event;
+    type Event = DefaultEvent<State>;
 
     fn new(event_loop: &ActiveEventLoop, proxy: EventLoopProxy<Self::Event>) -> Self {
         let window = event_loop
             .create_window(State::window_attributes())
             .unwrap();
         let default_state = DefaultUiState::new(window);
-        let (mut rsc, task_recv) = DefaultRsc::init(default_state.window.clone());
+        let mut rsc = DefaultRsc::init(Arc::new(Proxy(proxy.clone())));
-        let state = State::new(default_state, &mut rsc, proxy);
+        let state = State::new(default_state, &mut rsc, Proxy(proxy));
         let render = UiRenderState::new();
-        Self {
+        Self { rsc, state, render }
-            rsc,
-            state,
-            render,
-            task_recv,
-        }
     }
 
     fn event(&mut self, event: Self::Event, _: &ActiveEventLoop) {
-        self.state.event(event, &mut self.rsc, &mut self.render);
+        match event {
+            DefaultEvent::User(event) => self.state.event(event, &mut self.rsc, &mut self.render),
+            DefaultEvent::Update(update) => update(&mut self.state, &mut self.rsc),
+        }
+        self.request_redraw_if_needed();
     }
 
     fn window_event(&mut self, event: WindowEvent, event_loop: &ActiveEventLoop) {
-        let Self {
+        let Self { rsc, render, state } = self;
-            rsc,
-            render,
-            state,
-            task_recv,
-        } = self;
-
-        for update in task_recv.try_iter() {
-            update(state, rsc);
-        }
 
         let ui_state = state.default_state_mut();
         let input_changed = ui_state.input.event(&event);
@@ -227,8 +235,7 @@ impl<State: DefaultAppState> AppState for DefaultApp<State> {
             ui_state.focus = None;
         }
         if input_changed {
-            let window_size = ui_state.window_size();
+            render.run_sensors(rsc, state, cursor_state);
-            render.run_sensors(rsc, state, cursor_state, window_size);
         }
         let ui_state = state.default_state_mut();
         if old != ui_state.focus
@@ -244,7 +251,7 @@ impl<State: DefaultAppState> AppState for DefaultApp<State> {
                 ui_state.renderer.draw();
             }
             WindowEvent::Resized(size) => {
-                render.resize((size.width, size.height));
+                render.resize((size.width, size.height), rsc.widgets_mut());
                 ui_state.renderer.resize(size)
             }
             WindowEvent::KeyboardInput { event, .. } => {
@@ -297,11 +304,8 @@ impl<State: DefaultAppState> AppState for DefaultApp<State> {
             _ => (),
         }
         state.window_event(event, rsc, render);
-        let ui_state = self.state.default_state_mut();
+        self.request_redraw_if_needed();
-        if render.needs_redraw(&ui_state.root, rsc.widgets()) {
+        self.state.default_state_mut().input.end_frame();
-            ui_state.renderer.window().request_redraw();
-        }
-        ui_state.input.end_frame();
     }
 
     fn exit(&mut self) {
@@ -309,6 +313,15 @@ impl<State: DefaultAppState> AppState for DefaultApp<State> {
     }
 }
 
+impl<State: DefaultAppState> DefaultApp<State> {
+    fn request_redraw_if_needed(&mut self) {
+        let ui_state = self.state.default_state_mut();
+        if self.render.needs_redraw(&ui_state.root, self.rsc.widgets()) {
+            ui_state.renderer.window().request_redraw();
+        }
+    }
+}
+
 pub trait RscIdx<Rsc> {
     type Output;
     fn get(self, rsc: &Rsc) -> &Self::Output;

src/default/render.rs

@@ -1,4 +1,4 @@
-use iris_core::{UiData, UiLimits, UiRenderNode, UiRenderState};
+use iris_core::{UiData, UiRenderNode, UiRenderState};
 use pollster::FutureExt;
 use std::sync::Arc;
 use wgpu::*;
@@ -22,7 +22,20 @@ impl UiRenderer {
     }
 
     pub fn draw(&mut self) {
-        let output = self.surface.get_current_texture().unwrap();
+        let (output, suboptimal) = match self.surface.get_current_texture() {
+            CurrentSurfaceTexture::Success(texture) => (texture, false),
+            // Used for this frame, and the swapchain rebuilt after it has
+            // been presented: configuring the surface while a texture it
+            // handed out is still alive panics.
+            CurrentSurfaceTexture::Suboptimal(texture) => (texture, true),
+            CurrentSurfaceTexture::Outdated | CurrentSurfaceTexture::Lost => {
+                self.surface.configure(&self.device, &self.config);
+                return;
+            }
+            CurrentSurfaceTexture::Timeout
+            | CurrentSurfaceTexture::Occluded
+            | CurrentSurfaceTexture::Validation => return,
+        };
         let view = output
             .texture
             .create_view(&TextureViewDescriptor::default());
@@ -46,7 +59,10 @@ impl UiRenderer {
 
         self.queue.submit(std::iter::once(encoder.finish()));
         self.window.pre_present_notify();
-        output.present();
+        self.queue.present(output);
+        if suboptimal {
+            self.surface.configure(&self.device, &self.config);
+        }
     }
 
     pub fn resize(&mut self, size: &PhysicalSize<u32>) {
@@ -65,9 +81,10 @@ impl UiRenderer {
     pub fn new(window: Arc<Window>) -> Self {
         let size = window.inner_size();
 
-        let instance = Instance::new(&InstanceDescriptor {
+        let instance = Instance::new(InstanceDescriptor {
             backends: Backends::PRIMARY,
-            ..Default::default()
+            display: Some(Box::new(window.clone())),
+            ..InstanceDescriptor::new_without_display_handle()
         });
 
         let surface = instance
@@ -79,22 +96,14 @@ impl UiRenderer {
                 power_preference: PowerPreference::default(),
                 compatible_surface: Some(&surface),
                 force_fallback_adapter: false,
+                apply_limit_buckets: false,
             })
             .block_on()
             .expect("Could not get adapter!");
 
-        let ui_limits = UiLimits::default();
-
         let (device, queue) = adapter
             .request_device(&DeviceDescriptor {
-                required_features: Features::TEXTURE_BINDING_ARRAY
-                    | Features::PARTIALLY_BOUND_BINDING_ARRAY
-                    | Features::SAMPLED_TEXTURE_AND_STORAGE_BUFFER_ARRAY_NON_UNIFORM_INDEXING,
                 required_limits: Limits {
-                    max_binding_array_elements_per_shader_stage: ui_limits
-                        .max_binding_array_elements_per_shader_stage(),
-                    max_binding_array_sampler_elements_per_shader_stage: ui_limits
-                        .max_binding_array_sampler_elements_per_shader_stage(),
                     max_buffer_size: 1 << 30,
                     ..Default::default()
                 },
@@ -114,6 +123,7 @@ impl UiRenderer {
         let config = SurfaceConfiguration {
             usage: TextureUsages::RENDER_ATTACHMENT,
             format: surface_format,
+            color_space: SurfaceColorSpace::Auto,
             width: size.width,
             height: size.height,
             present_mode: PresentMode::AutoVsync,
@@ -126,7 +136,7 @@ impl UiRenderer {
 
         let encoder = Self::create_encoder(&device);
 
-        let ui = UiRenderNode::new(&device, &queue, &config, ui_limits);
+        let ui = UiRenderNode::new(&device, &config);
 
         Self {
             surface,

src/default/sense.rs

@@ -4,14 +4,14 @@ use std::{
     rc::Rc,
 };
 
-#[derive(Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub enum CursorButton {
     Left,
     Right,
     Middle,
 }
 
-#[derive(Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub enum CursorSense {
     PressStart(CursorButton),
     Pressing(CursorButton),
@@ -27,7 +27,7 @@ pub struct CursorSenses(Vec<CursorSense>);
 
 impl Event for CursorSenses {
     type Data<'a> = CursorData<'a>;
-    type State = SensorState;
+    type Global = Hovered;
     fn should_run<'a>(&self, data: &Self::Data<'a>) -> Option<Self::Data<'a>> {
         if let Some(sense) = should_run(self, &data.cursor, data.hover) {
             let mut data = data.clone();
@@ -37,6 +37,24 @@ impl Event for CursorSenses {
             None
         }
     }
+
+    /// A press or a scroll is used up by whatever answered it, so it stops
+    /// there. Hovering is not: a cursor resting somewhere goes on resting.
+    fn consumes(&self, data: &Self::Data<'_>) -> bool {
+        !data.sense.position_only()
+    }
+}
+
+/// Who the cursor was inside, before and after an input. The difference is
+/// whose hover has ended -- including a widget a higher layer has covered,
+/// which the walk stops before reaching.
+///
+/// Two buffers that swap rather than one rebuilt, so an input allocates
+/// nothing once they have grown.
+#[derive(Default)]
+pub struct Hovered {
+    was: Vec<WidgetId>,
+    now: Vec<WidgetId>,
 }
 
 impl CursorSense {
@@ -52,6 +70,12 @@ impl CursorSense {
     pub fn is_dragging(&self) -> bool {
         matches!(self, CursorSense::Pressing(CursorButton::Left))
     }
+
+    /// False if the sense is a button or a scroll, true if it is only about
+    /// where the cursor is.
+    fn position_only(&self) -> bool {
+        matches!(self, Self::HoverStart | Self::Hovering | Self::HoverEnd)
+    }
 }
 
 #[derive(Default, Clone)]
@@ -96,6 +120,12 @@ impl CursorButtons {
 }
 
 impl CursorState {
+    /// True if the cursor is only reporting where it is: no button and no
+    /// scroll this frame.
+    pub fn position_only(&self) -> bool {
+        self.scroll_delta == Vec2::ZERO && self.buttons.iter().all(|(_, state)| state.is_off())
+    }
+
     pub fn end_frame(&mut self) {
         self.buttons.end_frame();
         self.scroll_delta = Vec2::ZERO;
@@ -123,11 +153,6 @@ pub struct Sensor<Ctx: HasEvents, Data> {
 
 pub type SenseShape = UiRegion;
 
-#[derive(Default, Debug)]
-pub struct SensorState {
-    pub hover: ActivationState,
-}
-
 #[derive(Clone)]
 pub struct CursorData<'a> {
     /// where this widget was hit
@@ -147,7 +172,6 @@ pub trait SensorUi {
         rsc: &mut Rsc,
         state: &mut Rsc::State,
         cursor: CursorState,
-        window_size: Vec2,
     );
 }
 
@@ -157,47 +181,86 @@ impl SensorUi for UiRenderState {
         rsc: &mut Rsc,
         state: &mut Rsc::State,
         cursor: CursorState,
-        window_size: Vec2,
     ) {
         // in order to remove this take, need to store active list in UiRenderState somehow
         // this would probably be done through a generic parameter that adds yet another rsc /
         // state like thing, but local to render state, and is passed to UiRsc events so you can
         // update it there?
-        let mut active = std::mem::take(&mut rsc.events_mut().get_type::<CursorSense>().active);
+        let active = std::mem::take(&mut rsc.events_mut().get_type::<CursorSense>().active);
+        let mut hovered = std::mem::take(&mut rsc.events_mut().get_type::<CursorSense>().global);
+        hovered.now.clear();
+        let position_only = cursor.position_only();
+        let region_of = |id| self.window_region(&id);
+
         for layer in self.layers.indices().rev() {
-            let mut sensed = false;
+            let mut consumed = false;
-            for (id, sensor) in active.get_mut(&layer).into_flat_iter() {
+            for id in active.get(&layer).into_flat_iter().map(|(id, _)| *id) {
-                let shape = self.active.get(id).unwrap().region;
+                let Some(region) = region_of(id) else {
-                let region = shape.to_px(window_size);
+                    continue;
-                let in_shape = cursor.exists && region.contains(cursor.pos);
+                };
-                sensor.hover.update(in_shape);
+                if !cursor.exists || !region.contains(PxVec2::from_f32(cursor.pos)) {
-                if sensor.hover == ActivationState::Off {
                     continue;
                 }
-                sensed = true;
+                hovered.now.push(id);
-
+                let hover = match hovered.was.contains(&id) {
-                let cursor = cursor.clone();
+                    true => ActivationState::On,
-
+                    false => ActivationState::Start,
-                let data = CursorData {
-                    pos: cursor.pos - region.top_left,
-                    size: region.bot_right - region.top_left,
-                    scroll_delta: cursor.scroll_delta,
-                    hover: sensor.hover,
-                    cursor,
-                    // this does not have any meaning;
-                    // might wanna set up Event to have a prepare stage
-                    sense: CursorSense::Hovering,
-                    render: self,
                 };
-                rsc.run_event::<CursorSense>(*id, data, state);
+                // A press or a scroll stops where something answered it, so a
+                // button over a list does not swallow the list's scrolling.
+                consumed |= deliver(self, rsc, state, id, hover, &cursor, region);
+                // A cursor doing neither stops at whatever it is over, so
+                // hovering does not reach through.
+                consumed |= position_only;
             }
-            if sensed {
+            // Applied after the layer, never during it: senses on one layer do
+            // not block each other.
+            if consumed {
                 break;
             }
         }
-        rsc.events_mut().get_type::<CursorSense>().active = active;
+
+        // Whatever the cursor was inside and is not now, whether it left or a
+        // layer above took the input before the walk reached it. A widget that
+        // stopped being drawn has no region to report and is simply dropped.
+        for &id in &hovered.was {
+            if !hovered.now.contains(&id)
+                && let Some(region) = region_of(id)
+            {
+                deliver(self, rsc, state, id, ActivationState::End, &cursor, region);
             }
         }
+        std::mem::swap(&mut hovered.was, &mut hovered.now);
+
+        let senses = rsc.events_mut().get_type::<CursorSense>();
+        senses.active = active;
+        senses.global = hovered;
+    }
+}
+
+/// Runs one widget's cursor senses, and says whether they used up the input.
+fn deliver<Rsc: HasEvents>(
+    render: &UiRenderState,
+    rsc: &mut Rsc,
+    state: &mut Rsc::State,
+    id: WidgetId,
+    hover: ActivationState,
+    cursor: &CursorState,
+    region: PixelRegion,
+) -> bool {
+    let data = CursorData {
+        pos: cursor.pos - region.top_left.to_f32(),
+        size: region.size().to_f32(),
+        scroll_delta: cursor.scroll_delta,
+        hover,
+        cursor: cursor.clone(),
+        // this does not have any meaning;
+        // might wanna set up Event to have a prepare stage
+        sense: CursorSense::Hovering,
+        render,
+    };
+    rsc.run_event::<CursorSense>(id, data, state)
+}
 
 pub fn should_run(
     senses: &CursorSenses,
@@ -205,6 +268,11 @@ pub fn should_run(
     hover: ActivationState,
 ) -> Option<CursorSense> {
     for sense in senses.iter() {
+        // A widget the cursor is no longer inside senses only its position:
+        // the press that ended its hover landed on something else.
+        if !hover.is_on() && !sense.position_only() {
+            continue;
+        }
         if match sense {
             CursorSense::PressStart(button) => cursor.buttons.select(button).is_start(),
             CursorSense::Pressing(button) => cursor.buttons.select(button).is_on(),

src/default/task.rs

@@ -1,11 +1,5 @@
 use iris_core::HasState;
-use std::{
+use std::{pin::Pin, sync::Arc};
-    pin::Pin,
-    sync::{
-        Arc,
-        mpsc::{Receiver as SyncReceiver, Sender as SyncSender, channel as sync_channel},
-    },
-};
 use tokio::{
     runtime::Runtime,
     sync::mpsc::{
@@ -13,64 +7,52 @@ use tokio::{
         unbounded_channel as async_channel,
     },
 };
-use winit::window::Window;
-
-pub type TaskMsgSender<Rsc> = SyncSender<Box<dyn TaskUpdate<Rsc>>>;
-pub type TaskMsgReceiver<Rsc> = SyncReceiver<Box<dyn TaskUpdate<Rsc>>>;
 
 pub trait TaskUpdate<Rsc: HasState>: FnOnce(&mut Rsc::State, &mut Rsc) + Send {}
 impl<F: FnOnce(&mut Rsc::State, &mut Rsc) + Send, Rsc: HasState> TaskUpdate<Rsc> for F {}
 
+/// Hands an update from a task to the thread that owns the ui. Delivery and
+/// waking are one act: a host posts the update as a message its loop already
+/// carries, so nothing has to wake the loop separately, or claim a redraw to
+/// be looked at.
+pub trait TaskQueue<Rsc: HasState>: Send + Sync + 'static {
+    fn send(&self, update: Box<dyn TaskUpdate<Rsc>>);
+}
+
 pub struct Tasks<Rsc: HasState> {
     start: AsyncSender<BoxTask>,
-    window: Arc<Window>,
+    queue: Arc<dyn TaskQueue<Rsc>>,
-    msg_send: SyncSender<Box<dyn TaskUpdate<Rsc>>>,
 }
 
 pub struct TaskCtx<Rsc: HasState> {
-    send: TaskMsgSender<Rsc>,
+    queue: Arc<dyn TaskQueue<Rsc>>,
 }
 
 impl<Rsc: HasState> TaskCtx<Rsc> {
     pub fn update(&mut self, f: impl TaskUpdate<Rsc> + 'static) {
-        let _ = self.send.send(Box::new(f));
+        self.queue.send(Box::new(f));
-    }
-}
-impl<Rsc: HasState + 'static> TaskCtx<Rsc> {
-    fn new(send: TaskMsgSender<Rsc>) -> Self {
-        Self { send }
     }
 }
 
 type BoxTask = Pin<Box<dyn Future<Output = ()> + Send>>;
 
 impl<Rsc: HasState> Tasks<Rsc> {
-    pub fn init(window: Arc<Window>) -> (Self, TaskMsgReceiver<Rsc>) {
+    pub fn init(queue: Arc<dyn TaskQueue<Rsc>>) -> Self {
         let (start, start_recv) = async_channel();
-        let (msgs, msgs_recv) = sync_channel();
         std::thread::spawn(|| {
             let rt = Runtime::new().unwrap();
             rt.block_on(listen(start_recv))
         });
-        (
+        Self { start, queue }
-            Self {
-                start,
-                msg_send: msgs,
-                window,
-            },
-            msgs_recv,
-        )
     }
 
     pub fn spawn<F: AsyncFnOnce(TaskCtx<Rsc>) + 'static + std::marker::Send>(&mut self, task: F)
     where
         F::CallOnceFuture: Send,
     {
-        let send = self.msg_send.clone();
+        let queue = self.queue.clone();
-        let window = self.window.clone();
         let _ = self.start.send(Box::pin(async move {
-            task(TaskCtx::new(send)).await;
+            task(TaskCtx { queue }).await;
-            window.request_redraw();
         }));
     }
 }

src/harness.rs

@@ -0,0 +1,333 @@
+//! A ui with no window: build a tree, run frames, move a pointer, and read
+//! back where widgets landed.
+//!
+//! It does not draw. A claim about pixels still needs a real surface.
+
+use crate::prelude::*;
+use std::{
+    sync::{
+        Arc,
+        mpsc::{Receiver, SyncSender, sync_channel},
+    },
+    time::Duration,
+};
+
+/// There is no loop here to post to, so updates queue until the test asks
+/// for them.
+struct Queue(SyncSender<Box<dyn TaskUpdate<DefaultRsc<HarnessState>>>>);
+
+impl TaskQueue<DefaultRsc<HarnessState>> for Queue {
+    fn send(&self, update: Box<dyn TaskUpdate<DefaultRsc<HarnessState>>>) {
+        let _ = self.0.send(update);
+    }
+}
+
+/// `assert_eq!` for where a frame put a widget, written as its two corners.
+#[macro_export]
+macro_rules! assert_corners {
+    ($harness:expr, $id:expr, ($x0:expr, $y0:expr), ($x1:expr, $y1:expr)) => {
+        assert_eq!(
+            $harness.region(&$id).expect("widget drew nothing"),
+            $crate::core::PixelRegion {
+                top_left: $crate::core::PxVec2::new(
+                    $crate::core::Px::from_f32($x0 as f32),
+                    $crate::core::Px::from_f32($y0 as f32),
+                ),
+                bot_right: $crate::core::PxVec2::new(
+                    $crate::core::Px::from_f32($x1 as f32),
+                    $crate::core::Px::from_f32($y1 as f32),
+                ),
+            }
+        );
+    };
+}
+pub use crate::assert_corners;
+
+/// One replayed pointer sample, cut down to what a window delivers: where the
+/// pointer is, and whether the button changed.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum TouchAction {
+    Down,
+    Move,
+    Up,
+}
+
+impl TouchAction {
+    fn parse(word: &str) -> Option<Self> {
+        match word {
+            "down" => Some(Self::Down),
+            "move" => Some(Self::Move),
+            "up" => Some(Self::Up),
+            _ => None,
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug)]
+pub struct TouchSample {
+    pub t_ms: u64,
+    pub action: TouchAction,
+    pub pos: Vec2,
+}
+
+/// A recorded gesture, in the output's own pixels: `<ms> down|move|up <x> <y>`
+/// a line, `#` to end of line ignored.
+///
+/// One parser for both ways of replaying a recording -- into a harness, and
+/// into a real window -- so the two cannot read the same file differently.
+pub struct TouchScript {
+    pub samples: Vec<TouchSample>,
+}
+
+impl TouchScript {
+    pub fn parse(text: &str) -> Result<Self, String> {
+        let mut samples: Vec<TouchSample> = Vec::new();
+        for (i, line) in text.lines().enumerate() {
+            let line = line.split('#').next().unwrap_or("").trim();
+            if line.is_empty() {
+                continue;
+            }
+            let at = |what: &str| format!("touch script line {}: {what}: {line:?}", i + 1);
+            let mut words = line.split_whitespace();
+            let (Some(t), Some(action), Some(x), Some(y), None) = (
+                words.next(),
+                words.next(),
+                words.next(),
+                words.next(),
+                words.next(),
+            ) else {
+                return Err(at("expected `t_ms action x y`"));
+            };
+            let t_ms: u64 = t.parse().map_err(|_| at("t_ms is not a whole number"))?;
+            let action =
+                TouchAction::parse(action).ok_or_else(|| at("action is not down/move/up"))?;
+            let x: f32 = x.parse().map_err(|_| at("x is not a number"))?;
+            let y: f32 = y.parse().map_err(|_| at("y is not a number"))?;
+            if let Some(last) = samples.last()
+                && t_ms < last.t_ms
+            {
+                return Err(at("samples must be in time order"));
+            }
+            samples.push(TouchSample {
+                t_ms,
+                action,
+                pos: Vec2::new(x, y),
+            });
+        }
+        Ok(Self { samples })
+    }
+}
+
+#[derive(Default)]
+pub struct HarnessState {
+    pub root: Option<StrongWidget>,
+}
+
+impl HasRoot for HarnessState {
+    fn set_root(&mut self, root: StrongWidget) {
+        self.root = Some(root);
+    }
+}
+
+pub struct Harness {
+    pub rsc: DefaultRsc<HarnessState>,
+    pub render: UiRenderState,
+    pub state: HarnessState,
+    updates: Receiver<Box<dyn TaskUpdate<DefaultRsc<HarnessState>>>>,
+    cursor: CursorState,
+}
+
+impl Harness {
+    /// `size` is the output in physical pixels.
+    pub fn new(size: impl Into<Vec2>) -> Self {
+        // A `TaskQueue` must be `Sync`, which `mpsc::Sender` is not; the
+        // bound that comes with `SyncSender` is far past anything a test
+        // leaves unread.
+        let (send, updates) = sync_channel(1024);
+        let mut rsc = DefaultRsc::init(Arc::new(Queue(send)));
+        let mut render = UiRenderState::new();
+        render.resize(size, rsc.widgets_mut());
+        Self {
+            rsc,
+            render,
+            state: HarnessState::default(),
+            updates,
+            cursor: CursorState::default(),
+        }
+    }
+
+    pub fn size(&self) -> Vec2 {
+        self.render.output_size().to_f32()
+    }
+
+    pub fn resize(&mut self, size: impl Into<Vec2>) {
+        self.render.resize(size, self.rsc.widgets_mut());
+    }
+
+    /// Changes a length rule after the fact, the way `.width()` sets one.
+    pub fn set_len(&mut self, id: impl IdLike, axis: Axis, len: impl Into<LayoutLen>) {
+        self.rsc
+            .widgets_mut()
+            .set_size_rule(id, axis, SizeRule::Exact(len.into()));
+    }
+
+    /// Sets the root and lays it out, so a pointer event has something to hit.
+    pub fn set_root<T>(&mut self, widget: impl WidgetLike<DefaultRsc<HarnessState>, T>) {
+        widget.set_root(&mut self.rsc, &mut self.state);
+        self.frame();
+    }
+
+    pub fn needs_redraw(&self) -> bool {
+        self.render
+            .needs_redraw(&self.state.root, self.rsc.widgets())
+    }
+
+    pub fn apply_updates(&mut self) -> usize {
+        let mut applied = 0;
+        while let Ok(update) = self.updates.try_recv() {
+            update(&mut self.state, &mut self.rsc);
+            applied += 1;
+        }
+        applied
+    }
+
+    /// Waits for a task's first update, then applies everything waiting.
+    /// False if none arrived in time.
+    #[must_use]
+    pub fn await_update(&mut self, timeout: Duration) -> bool {
+        let Ok(update) = self.updates.recv_timeout(timeout) else {
+            return false;
+        };
+        update(&mut self.state, &mut self.rsc);
+        self.apply_updates();
+        true
+    }
+
+    /// Lays the tree out and builds its primitives.
+    pub fn frame(&mut self) {
+        self.apply_updates();
+        self.render.update(&self.state.root, &mut self.rsc);
+    }
+
+    /// Where the last frame put a widget, or `None` if it drew nothing.
+    pub fn region(&self, id: &impl IdLike) -> Option<PixelRegion> {
+        self.render.window_region(id)
+    }
+
+    pub fn move_to(&mut self, pos: impl Into<Vec2>) {
+        self.cursor.pos = pos.into();
+        self.cursor.exists = true;
+        self.sense();
+    }
+
+    pub fn leave(&mut self) {
+        self.cursor.exists = false;
+        self.sense();
+    }
+
+    pub fn press(&mut self, button: CursorButton) {
+        self.button(button).update(true);
+        self.sense();
+    }
+
+    pub fn release(&mut self, button: CursorButton) {
+        self.button(button).update(false);
+        self.sense();
+    }
+
+    /// A wheel carries no position, so this goes wherever the cursor was last
+    /// moved to -- nowhere, until it has been moved.
+    pub fn scroll(&mut self, delta: impl Into<Vec2>) {
+        self.cursor.scroll_delta = delta.into();
+        self.sense();
+    }
+
+    pub fn click(&mut self, pos: impl Into<Vec2>) {
+        self.move_to(pos);
+        self.press(CursorButton::Left);
+        self.release(CursorButton::Left);
+    }
+
+    /// Drives a recorded gesture through the harness.
+    pub fn replay(&mut self, script: &TouchScript) {
+        for sample in &script.samples {
+            match sample.action {
+                TouchAction::Down => {
+                    self.move_to(sample.pos);
+                    self.press(CursorButton::Left);
+                }
+                TouchAction::Move => self.move_to(sample.pos),
+                TouchAction::Up => {
+                    self.move_to(sample.pos);
+                    self.release(CursorButton::Left);
+                }
+            }
+        }
+    }
+
+    fn button(&mut self, button: CursorButton) -> &mut ActivationState {
+        let buttons = &mut self.cursor.buttons;
+        match button {
+            CursorButton::Left => &mut buttons.left,
+            CursorButton::Middle => &mut buttons.middle,
+            CursorButton::Right => &mut buttons.right,
+        }
+    }
+
+    /// Dispatches against the layout of the last frame, which is what a
+    /// window delivers input against too.
+    fn sense(&mut self) {
+        let cursor = self.cursor.clone();
+        self.render
+            .run_sensors(&mut self.rsc, &mut self.state, cursor);
+        self.cursor.end_frame();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn parse(text: &str) -> Result<Vec<(u64, TouchAction, f32, f32)>, String> {
+        Ok(TouchScript::parse(text)?
+            .samples
+            .iter()
+            .map(|s| (s.t_ms, s.action, s.pos.x, s.pos.y))
+            .collect())
+    }
+
+    #[test]
+    fn a_recording_is_time_action_and_a_point() {
+        assert_eq!(
+            parse("0 down 10 20\n16 move 10.5 24\n32 up 10.5 24").unwrap(),
+            [
+                (0, TouchAction::Down, 10.0, 20.0),
+                (16, TouchAction::Move, 10.5, 24.0),
+                (32, TouchAction::Up, 10.5, 24.0),
+            ]
+        );
+    }
+
+    #[test]
+    fn blank_lines_and_comments_are_not_samples() {
+        assert_eq!(
+            parse("# a flick\n\n 0 down 1 2   # the finger lands\n\n").unwrap(),
+            [(0, TouchAction::Down, 1.0, 2.0)]
+        );
+    }
+
+    #[test]
+    fn a_recording_that_goes_backwards_is_rejected() {
+        // Replay waits out the gap between samples, so time running backwards
+        // would silently become no wait at all.
+        let err = parse("16 down 1 2\n0 up 1 2").unwrap_err();
+        assert!(err.contains("time order"), "{err}");
+    }
+
+    #[test]
+    fn a_line_that_is_not_a_sample_says_which_line() {
+        let err = parse("0 down 1 2\n16 wiggle 1 2").unwrap_err();
+        assert!(err.contains("line 2"), "{err}");
+        assert!(err.contains("down/move/up"), "{err}");
+    }
+}

src/lib.rs

@@ -7,6 +7,8 @@
 
 pub mod default;
 pub mod event;
+pub mod harness;
+pub mod random;
 pub mod widget;
 
 pub use iris_core as core;

src/random.rs

@@ -0,0 +1,956 @@
+//! A seeded random widget tree, for tests and for looking at.
+//!
+//! One seed is one tree, on any machine and after any upgrade, so a test can
+//! grow the same tree twice and a failing seed is reproduced by its number.
+//! `examples/random.rs` draws one; `tests/generated.rs` checks that laying one
+//! out again lands where growing it from scratch would.
+
+use crate::prelude::*;
+use std::collections::HashMap;
+
+/// What a test changes between two trees grown from the same seed, so the
+/// warm one can be mutated and the cold one grown that way to begin with.
+#[derive(Default)]
+pub struct Edits {
+    /// Declared sizes, by the order the rules were put on.
+    pub sizes: HashMap<usize, SizeRules>,
+    /// Which children a span has, by the order the spans were made.
+    pub spans: HashMap<usize, SpanEdit>,
+    /// Alignments, by the order they were put on.
+    pub aligns: HashMap<usize, Align>,
+    /// Which widgets own a movable region, by the order they were offered
+    /// one. Region nodes change what a move writes and how deep a primitive's
+    /// chain is, so a tree that never grows one leaves both untested.
+    pub nodes: HashMap<usize, bool>,
+    /// Whether a [`Branch`] takes the side it would take at any measurement,
+    /// rather than the side the one it made says. The oracle wants the
+    /// measured side -- that is the whole point of a branch, and how a widget
+    /// believing a measurement a cold start would not have given it becomes a
+    /// different tree. A rig measuring cost wants this instead: a fixture
+    /// whose shape moves with the thing being measured cannot be compared
+    /// with itself across a change to it, and seed 1 at depth 8 went from 88
+    /// drawn widgets and 2,298 primitive writes a frame to 115 and 8,209
+    /// across fixed point, which is three and a half times the work behind a
+    /// number read as three and a half times the cost.
+    pub fixed_branches: bool,
+}
+
+#[derive(Default, Clone)]
+pub struct SpanEdit {
+    /// Children to leave out, by index among the ones grown.
+    pub detach: Vec<usize>,
+    /// How many of the span's spares are in it, appended in order.
+    pub attach: usize,
+}
+
+/// xorshift64, written out rather than taken from a crate so that a seed
+/// keeps meaning the same tree.
+pub struct Rng(u64);
+
+impl Rng {
+    pub fn new(seed: u64) -> Self {
+        Self(seed | 1)
+    }
+
+    pub fn bits(&mut self) -> u64 {
+        self.0 ^= self.0 << 13;
+        self.0 ^= self.0 >> 7;
+        self.0 ^= self.0 << 17;
+        self.0
+    }
+
+    pub fn below(&mut self, n: usize) -> usize {
+        (self.bits() % n as u64) as usize
+    }
+
+    pub fn chance(&mut self) -> bool {
+        self.bits() & 1 == 0
+    }
+}
+
+const COLORS: [UiColor; 6] = [
+    UiColor::RED,
+    UiColor::GREEN,
+    UiColor::BLUE,
+    UiColor::YELLOW,
+    UiColor::CYAN,
+    UiColor::MAGENTA,
+];
+
+/// Leaves grown beside every span, for a test to put into it.
+const SPARES: usize = 3;
+
+const WORDS: &str = "Wrapping shapes one source into as many lines as the box \
+    leaves room for, so a paragraph's height is an answer and not a setting.";
+
+/// What growing a tree gives back: every widget in creation order, so two
+/// trees from one seed line up index for index, and the declared sizes, which
+/// are what a test changes to watch the change propagate.
+#[derive(Default)]
+pub struct Tree {
+    pub ids: Vec<WidgetId>,
+    pub sized: Vec<WidgetId>,
+    pub aligned: Vec<WidgetId>,
+    pub nodes: Vec<WidgetId>,
+    pub spans: Vec<Spanned>,
+    pub scrolls: Vec<WeakWidget<Scroll>>,
+}
+
+/// Branches on a child's measured length. Comparing boxes catches a widget
+/// that moved; this catches one that believed a measurement a cold start
+/// would not have given it, by turning that into a different tree. Its own
+/// configuration never changes, so which side draws is a property of the
+/// layout alone.
+pub struct Branch {
+    pub probe: StrongWidget,
+    pub wide: StrongWidget,
+    pub narrow: StrongWidget,
+    pub threshold: f32,
+}
+
+impl Widget for Branch {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
+        let top = UiSpan::new(Len::ZERO, cut).shifted_desc();
+        let measured = painter
+            .widget_at(&self.probe, top.on_axis(Axis::Y))
+            .len(Axis::X);
+        let len = measured.apply_leftover();
+        let px = painter.to_px(len, Axis::X);
+        // The range it actually branched on, said the way a container says
+        // one: pinning the window instead would redraw this widget on every
+        // resize, which is a fixture that never exercises reuse.
+        let threshold = Px::from_f32(self.threshold);
+        let holds = match px > threshold {
+            true => Holds::from(threshold + Px::STEP..=Px::MAX),
+            false => Holds::from(Px::MIN..=threshold),
+        };
+        painter.window_holds(Axis::X, holds.through(len));
+
+        let below = UiSpan::new(cut, painter.region_len(Axis::Y)).shifted_desc();
+        let place = below.on_axis(Axis::Y);
+        match px > threshold {
+            true => painter.widget_at(&self.wide, place),
+            false => painter.widget_at(&self.narrow, place),
+        };
+        Size::LEFTOVER
+    }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
+}
+
+pub struct Spanned {
+    pub id: WeakWidget<Span>,
+    /// Everything made for this span that it does not hold -- spares never
+    /// attached and children detached alike. A widget belongs to one parent,
+    /// and one that belongs to nobody still has to be held here: dropping
+    /// the last share of it frees its id for the next widget to be given,
+    /// which puts two trees out of step.
+    pub spares: Vec<StrongWidget>,
+    /// How many children it was grown with, before any edit.
+    pub grown: usize,
+}
+
+/// A tree described rather than built: [`plan`] turns a seed into one of
+/// these and [`build`] turns it into widgets, where growing did both at once.
+///
+/// The split is what makes a counterexample readable. A failing seed used to
+/// be the entire record of one, because a grower that makes widgets as it
+/// draws leaves nothing to take apart -- a shrinker could only grow its own
+/// trees and hope to meet the same shape, which in practice it does not. A
+/// plan is reduced by [`Plan::smaller`] and built again, so any seed that
+/// fails can be cut down until what is left is small enough to read.
+#[derive(Clone, Debug, PartialEq)]
+pub struct Plan {
+    pub kind: Kind,
+    /// The declared size this widget carries. Whoever grows a widget offers
+    /// it one and the offer is taken or declined; a second offer to the same
+    /// widget is dropped, because two rules on one widget would settle in the
+    /// order they were applied rather than in grow order.
+    pub size: Option<SizeRules>,
+    /// The alignment it carries, under the same one-offer rule. An axis left
+    /// out takes the centered default, which is what [`RegionAlign`] reads it
+    /// as.
+    pub align: Option<Align>,
+    /// Whether it was offered a movable region of its own and what it
+    /// answered. `Some(false)` is an offer declined, which still uses up the
+    /// one offer, where `None` is an offer never made.
+    pub region_node: Option<bool>,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Kind {
+    /// Wrapped and unwrapped text, because only one of them reads the width
+    /// it is given and so only one has to be drawn again for a new one.
+    Wrapped,
+    OneLine,
+    Rect {
+        color: usize,
+        alpha: u8,
+    },
+    /// The one leaf whose own length is a number of pixels it knows before it
+    /// is drawn, which is the hint a rule beside it has to win over.
+    Image,
+    /// Scrolling reads the pixel length of its box, which nothing else here
+    /// does, and gives its child a box longer than its own.
+    Scroll {
+        axis: Axis,
+        inner: Box<Plan>,
+    },
+    /// All three sides are grown either way, so a tree that draws one has the
+    /// same ids as a tree that draws another.
+    Branch {
+        probe: Box<Plan>,
+        wide: Box<Plan>,
+        narrow: Box<Plan>,
+        threshold: f32,
+    },
+    /// Each side its own, since a padding that is the same all round hides
+    /// anything that treats one edge differently from another.
+    Pad {
+        padding: [i32; 4],
+        inner: Box<Plan>,
+    },
+    Stack {
+        children: Vec<Plan>,
+    },
+    Span {
+        dir: usize,
+        gap: i32,
+        /// Grown for this span, in the order they are made.
+        children: Vec<Plan>,
+        /// Grown beside it whether or not they end up in it, so the widget
+        /// after them has the same id in a tree that leaves them out as in
+        /// one that puts them in.
+        spares: Vec<Plan>,
+        /// Which of `children` then `spares` are actually in the span, and
+        /// in what order -- kept apart from the two lists above so that a
+        /// tree which detaches, attaches or reorders its children still
+        /// makes the same widgets in the same order, and two builds line up
+        /// index for index. Anything not named here is built and held
+        /// rather than dropped, since freeing an id hands it to the next
+        /// widget and puts two trees out of step.
+        order: Vec<usize>,
+    },
+}
+
+impl Plan {
+    /// A widget carrying nothing anybody has offered it yet.
+    fn bare(kind: Kind) -> Self {
+        Self {
+            kind,
+            size: None,
+            align: None,
+            region_node: None,
+        }
+    }
+
+    /// How many widgets building it makes, spares and detached children
+    /// included, since those are made either way.
+    pub fn size(&self) -> usize {
+        1 + match &self.kind {
+            Kind::Scroll { inner, .. } | Kind::Pad { inner, .. } => inner.size(),
+            Kind::Branch {
+                probe,
+                wide,
+                narrow,
+                ..
+            } => probe.size() + wide.size() + narrow.size(),
+            Kind::Stack { children } => children.iter().map(Plan::size).sum(),
+            Kind::Span {
+                children, spares, ..
+            } => children.iter().chain(spares).map(Plan::size).sum(),
+            _ => 0,
+        }
+    }
+
+    /// The trees to try instead of this one when reducing a counterexample,
+    /// biggest cut first: a shrinker takes the first that still fails, so
+    /// offering "this subtree alone" before "this subtree with one child
+    /// fewer" is what gets from six hundred widgets to six rather than to
+    /// five hundred and ninety.
+    ///
+    /// Every one of these is a tree the generator could have grown, so a
+    /// reduced plan is a counterexample in its own right rather than a
+    /// special case only the shrinker can make.
+    pub fn smaller(&self) -> Vec<Plan> {
+        let mut out = Vec::new();
+        // Standing in for the whole of it, which is the largest cut there is.
+        for kid in self.kids() {
+            out.push(kid.clone());
+        }
+        // Then what it carries, which costs nothing to put back if it was
+        // not the thing that mattered.
+        for dropped in [
+            self.region_node.map(|_| Plan {
+                region_node: None,
+                ..self.clone()
+            }),
+            self.align.map(|_| Plan {
+                align: None,
+                ..self.clone()
+            }),
+            self.size.map(|_| Plan {
+                size: None,
+                ..self.clone()
+            }),
+        ]
+        .into_iter()
+        .flatten()
+        {
+            out.push(dropped);
+        }
+        out.extend(self.kind.smaller().into_iter().map(|kind| Plan {
+            kind,
+            ..self.clone()
+        }));
+        out
+    }
+
+    /// Visits every widget in the order [`build`] makes them, so a count
+    /// kept by the visitor indexes the same widget as the matching [`Tree`]
+    /// vector does.
+    pub fn walk_mut(&mut self, at: &mut impl FnMut(&mut Plan)) {
+        match &mut self.kind {
+            Kind::Scroll { inner, .. } | Kind::Pad { inner, .. } => inner.walk_mut(at),
+            Kind::Branch {
+                probe,
+                wide,
+                narrow,
+                ..
+            } => {
+                probe.walk_mut(at);
+                wide.walk_mut(at);
+                narrow.walk_mut(at);
+            }
+            Kind::Stack { children } => {
+                for child in children {
+                    child.walk_mut(at);
+                }
+            }
+            Kind::Span {
+                children, spares, ..
+            } => {
+                for child in children.iter_mut().chain(spares) {
+                    child.walk_mut(at);
+                }
+            }
+            _ => {}
+        }
+        at(self);
+    }
+
+    /// The same tree with `edits` applied, by the indices the generator would
+    /// have used for them.
+    ///
+    /// [`plan`] resolves edits while drawing, which needs a seed. A scenario
+    /// needs them applied to a tree that already exists -- one it has built,
+    /// and one a shrinker may already have cut down, where no seed grows it
+    /// any more. Both routes take the same [`Edits`], so a case written
+    /// against one reads the same against the other.
+    pub fn edited(&self, edits: &Edits) -> Plan {
+        let mut out = self.clone();
+        let (mut sized, mut aligned, mut nodes, mut spans) = (0, 0, 0, 0);
+        out.walk_mut(&mut |plan| {
+            if let Kind::Span {
+                children,
+                spares,
+                order,
+                ..
+            } = &mut plan.kind
+            {
+                if let Some(edit) = edits.spans.get(&spans) {
+                    *order = span_edited(order, children.len(), spares.len(), edit);
+                }
+                spans += 1;
+            }
+            if let Kind::Branch { threshold, .. } = &mut plan.kind
+                && edits.fixed_branches
+            {
+                *threshold = f32::MIN;
+            }
+            if plan.size.is_some() {
+                if let Some(lens) = edits.sizes.get(&sized) {
+                    plan.size = Some(*lens);
+                }
+                sized += 1;
+            }
+            if plan.align.is_some() {
+                if let Some(align) = edits.aligns.get(&aligned) {
+                    plan.align = Some(*align);
+                }
+                aligned += 1;
+            }
+            if plan.region_node.is_some() {
+                if let Some(take) = edits.nodes.get(&nodes) {
+                    plan.region_node = Some(*take);
+                }
+                nodes += 1;
+            }
+        });
+        out
+    }
+
+    fn kids(&self) -> Vec<&Plan> {
+        match &self.kind {
+            Kind::Scroll { inner, .. } | Kind::Pad { inner, .. } => vec![inner],
+            Kind::Branch {
+                probe,
+                wide,
+                narrow,
+                ..
+            } => vec![probe, wide, narrow],
+            Kind::Stack { children } => children.iter().collect(),
+            Kind::Span { children, .. } => children.iter().collect(),
+            _ => Vec::new(),
+        }
+    }
+}
+
+impl Kind {
+    /// Simplifications of the shape alone, leaving what the widget carries to
+    /// [`Plan::smaller`]. Replacing a node with one of its children is there
+    /// rather than here, since it answers with a whole `Plan`.
+    fn smaller(&self) -> Vec<Kind> {
+        let mut out = Vec::new();
+        /// One child reduced at a time, rebuilt into the same shape. Every
+        /// answer has the same number of children as it was given, so it is
+        /// for the shapes whose child count is part of what they are.
+        fn reduced(kids: &[Plan], rebuild: &dyn Fn(Vec<Plan>) -> Kind) -> Vec<Kind> {
+            let mut out = Vec::new();
+            for (i, kid) in kids.iter().enumerate() {
+                for small in kid.smaller() {
+                    let mut next = kids.to_vec();
+                    next[i] = small;
+                    out.push(rebuild(next));
+                }
+            }
+            out
+        }
+
+        /// One child dropped, then [`reduced`]. For the shapes that hold any
+        /// number of children, where dropping one is the cut that matters.
+        fn each(kids: &[Plan], rebuild: &dyn Fn(Vec<Plan>) -> Kind) -> Vec<Kind> {
+            let mut out = Vec::new();
+            for i in 0..kids.len() {
+                if kids.len() > 1 {
+                    let mut less = kids.to_vec();
+                    less.remove(i);
+                    out.push(rebuild(less));
+                }
+            }
+            out.extend(reduced(kids, rebuild));
+            out
+        }
+        match self {
+            // The one leaf that reads the width it is given, then the one
+            // that does not, then the one that measures nothing at all. A
+            // picture measures nothing either, but its length is its own, so
+            // it steps to the leaf that takes whatever it is given.
+            Kind::Wrapped => out.push(Kind::OneLine),
+            Kind::OneLine | Kind::Image => out.push(Kind::Rect {
+                color: 0,
+                alpha: 255,
+            }),
+            Kind::Rect { .. } => {}
+            Kind::Scroll { axis, inner } => {
+                let axis = *axis;
+                out.extend(each(std::slice::from_ref(inner), &|mut k| Kind::Scroll {
+                    axis,
+                    inner: Box::new(k.remove(0)),
+                }));
+            }
+            Kind::Branch {
+                probe,
+                wide,
+                narrow,
+                threshold,
+            } => {
+                let threshold = *threshold;
+                // All three sides stay: a branch is the widget that draws
+                // one of two on a measurement, and one with a side missing
+                // is a different widget rather than a smaller one. Dropping
+                // the branch for a side is offered by `Plan::smaller`.
+                let sides = [(**probe).clone(), (**wide).clone(), (**narrow).clone()];
+                out.extend(reduced(&sides, &|k| Kind::Branch {
+                    probe: Box::new(k[0].clone()),
+                    wide: Box::new(k[1].clone()),
+                    narrow: Box::new(k[2].clone()),
+                    threshold,
+                }));
+            }
+            Kind::Pad { padding, inner } => {
+                let padding = *padding;
+                if padding != [0; 4] {
+                    out.push(Kind::Pad {
+                        padding: [0; 4],
+                        inner: inner.clone(),
+                    });
+                }
+                out.extend(each(std::slice::from_ref(inner), &|mut k| Kind::Pad {
+                    padding,
+                    inner: Box::new(k.remove(0)),
+                }));
+            }
+            Kind::Stack { children } => {
+                out.extend(each(children, &|children| Kind::Stack { children }))
+            }
+            Kind::Span {
+                dir,
+                gap,
+                children,
+                spares,
+                order,
+            } => {
+                let (dir, gap, n) = (*dir, *gap, children.len());
+                let span = |children: Vec<Plan>, spares: Vec<Plan>, order: Vec<usize>| Kind::Span {
+                    dir,
+                    gap,
+                    children,
+                    spares,
+                    order,
+                };
+                let identity: Vec<usize> = (0..n).collect();
+                // An order the generator did not choose is part of the tree,
+                // so take that off before taking the tree apart.
+                if *order != identity {
+                    out.push(span(children.clone(), spares.clone(), identity));
+                }
+                // Spares exist to be attached; with none attached they are
+                // widgets the span never holds.
+                if !spares.is_empty() && order.iter().all(|&i| i < n) {
+                    out.push(span(children.clone(), Vec::new(), order.clone()));
+                }
+                if gap != 0 {
+                    out.push(Kind::Span {
+                        dir,
+                        gap: 0,
+                        children: children.clone(),
+                        spares: spares.clone(),
+                        order: order.clone(),
+                    });
+                }
+                for k in 0..n {
+                    if n > 1 {
+                        let mut less = children.clone();
+                        less.remove(k);
+                        // Everything after it shifts down, spares included,
+                        // since they are indexed past the children.
+                        let order = order
+                            .iter()
+                            .filter(|&&i| i != k)
+                            .map(|&i| if i > k { i - 1 } else { i })
+                            .collect();
+                        out.push(span(less, spares.clone(), order));
+                    }
+                }
+                for (i, kid) in children.iter().enumerate() {
+                    for small in kid.smaller() {
+                        let mut next = children.clone();
+                        next[i] = small;
+                        out.push(span(next, spares.clone(), order.clone()));
+                    }
+                }
+            }
+        }
+        out
+    }
+}
+
+/// A [`SpanEdit`] applied to the order a span already holds its children in.
+///
+/// `detach` names positions in that order and `attach` takes from the front
+/// of what the span is not holding, both of which is what a test changing a
+/// live span does -- so an edit means the same thing said to a tree and said
+/// to the plan it was built from. On a span nobody has edited the order is
+/// the children in the order they were grown, and this is then "leave these
+/// out and put that many spares on the end".
+fn span_edited(order: &[usize], children: usize, spares: usize, edit: &SpanEdit) -> Vec<usize> {
+    let mut detach = edit.detach.clone();
+    detach.sort_unstable();
+    detach.dedup();
+    let mut next: Vec<usize> = order
+        .iter()
+        .enumerate()
+        .filter(|(at, _)| !detach.contains(at))
+        .map(|(_, &which)| which)
+        .collect();
+    // What the span is not holding, in the order it hands them back: what it
+    // was already not holding first, in the order the widgets were made, and
+    // what this edit takes out after that, highest position first. A child
+    // just detached goes to the back rather than straight back in, which is
+    // what makes detaching one and attaching one a trade.
+    let mut free: Vec<usize> = (0..children + spares)
+        .filter(|i| !order.contains(i))
+        .collect();
+    free.extend(detach.iter().rev().filter_map(|&at| order.get(at).copied()));
+    next.extend(free.into_iter().take(edit.attach));
+    next
+}
+
+/// Plans the tree `seed` describes, `edits` replacing what it would otherwise
+/// have given the widgets that carry them.
+///
+/// The edits are resolved here rather than at build time, so that a plan is
+/// the whole of what a tree is and building one has nothing left to decide.
+pub fn plan(seed: u64, depth: usize, edits: &Edits) -> Plan {
+    let mut sow = Sow {
+        rng: Rng::new(seed),
+        edits,
+        sized: 0,
+        aligned: 0,
+        nodes: 0,
+        spans: 0,
+    };
+    sow.node(depth)
+}
+
+/// Grows the tree `seed` describes, `edits` replacing the declared sizes it
+/// would otherwise have given those wrappers.
+pub fn grow<Rsc: UiRsc + 'static>(
+    rsc: &mut Rsc,
+    seed: u64,
+    depth: usize,
+    edits: &Edits,
+) -> (StrongWidget, Tree) {
+    build(rsc, &plan(seed, depth, edits))
+}
+
+/// Draws a plan out of the random stream. Every draw happens in the order it
+/// always has and before the decision it feeds, including the decisions that
+/// are then dropped, because a seed has to keep meaning the same tree.
+struct Sow<'a> {
+    rng: Rng,
+    edits: &'a Edits,
+    sized: usize,
+    aligned: usize,
+    nodes: usize,
+    spans: usize,
+}
+
+impl Sow<'_> {
+    fn leaf(&mut self) -> Plan {
+        Plan::bare(match self.rng.below(5) {
+            0 => Kind::Wrapped,
+            1 => Kind::OneLine,
+            2 => Kind::Image,
+            _ => {
+                let color = self.rng.below(COLORS.len());
+                let alpha = (self.rng.below(5) * 63) as u8;
+                Kind::Rect { color, alpha }
+            }
+        })
+    }
+
+    fn len(&mut self) -> Option<LayoutLen> {
+        match self.rng.below(4) {
+            0 => Some(LayoutLen::px(20.0 + self.rng.below(180) as f32)),
+            1 => Some(LayoutLen::LEFTOVER),
+            _ => None,
+        }
+    }
+
+    fn align(&mut self) -> Align {
+        let axis = |s: &mut Self| match s.rng.below(4) {
+            0 => None,
+            1 => Some(AxisAlign::NEG),
+            2 => Some(AxisAlign::CENTER),
+            _ => Some(AxisAlign::POS),
+        };
+        let (x, y) = (axis(self), axis(self));
+        // Aligning on neither axis leaves the branch unexercised.
+        match x.is_none() && y.is_none() {
+            true => Align {
+                x: Some(AxisAlign::CENTER),
+                y,
+            },
+            false => Align { x, y },
+        }
+    }
+
+    /// A declared size over half the tree, kept where a test can change it.
+    fn sized(&mut self, inner: &mut Plan) {
+        let take = self.rng.chance();
+        let lens = SizeRules {
+            x: self.len().into(),
+            y: self.len().into(),
+        };
+        if !take || inner.size.is_some() {
+            return;
+        }
+        let idx = self.sized;
+        self.sized += 1;
+        inner.size = Some(self.edits.sizes.get(&idx).copied().unwrap_or(lens));
+    }
+
+    /// An alignment over some of the tree, kept where a test can change it.
+    fn aligned(&mut self, inner: &mut Plan) {
+        let align = self.align();
+        if inner.align.is_some() {
+            return;
+        }
+        let idx = self.aligned;
+        self.aligned += 1;
+        inner.align = Some(self.edits.aligns.get(&idx).copied().unwrap_or(align));
+    }
+
+    /// A movable region of its own over some of the tree. What it changes is
+    /// how a move is written and how long a primitive's chain is, neither of
+    /// which any other branch here varies.
+    fn noded(&mut self, inner: &mut Plan) {
+        let take = self.rng.below(4) == 0;
+        if inner.region_node.is_some() {
+            return;
+        }
+        let idx = self.nodes;
+        self.nodes += 1;
+        inner.region_node = Some(self.edits.nodes.get(&idx).copied().unwrap_or(take));
+    }
+
+    fn offered(&mut self, inner: &mut Plan) {
+        self.sized(inner);
+        self.noded(inner);
+    }
+
+    fn node(&mut self, depth: usize) -> Plan {
+        if depth == 0 {
+            return self.leaf();
+        }
+        let positioned = self.rng.below(6);
+        if positioned == 0 {
+            let mut inner = self.node(depth - 1);
+            self.offered(&mut inner);
+            let axis = if self.rng.chance() { Axis::X } else { Axis::Y };
+            return Plan::bare(Kind::Scroll {
+                axis,
+                inner: Box::new(inner),
+            });
+        }
+        if positioned == 2 {
+            let probe = self.node(depth - 1);
+            let wide = self.node(depth - 1);
+            let narrow = self.node(depth - 1);
+            // Drawn either way, so the side a fixed branch takes is still a
+            // side the generator chose -- and it consumes the same randomness
+            // as a measured one, so the two grow the same ids.
+            let measured = self.rng.below(500) as f32;
+            let threshold = match self.edits.fixed_branches {
+                true => f32::MIN,
+                false => measured,
+            };
+            return Plan::bare(Kind::Branch {
+                probe: Box::new(probe),
+                wide: Box::new(wide),
+                narrow: Box::new(narrow),
+                threshold,
+            });
+        }
+        if positioned == 1 {
+            // Carries an alignment and makes no widget of its own, so the
+            // plan for it is the child it aligned.
+            let mut inner = self.node(depth - 1);
+            self.offered(&mut inner);
+            self.aligned(&mut inner);
+            return inner;
+        }
+        if self.rng.below(4) == 0 {
+            let mut inner = self.node(depth - 1);
+            self.offered(&mut inner);
+            let side = |s: &mut Self| s.rng.below(24) as i32;
+            let padding = [side(self), side(self), side(self), side(self)];
+            return Plan::bare(Kind::Pad {
+                padding,
+                inner: Box::new(inner),
+            });
+        }
+        let grown = 2 + self.rng.below(3);
+        let mut children = Vec::with_capacity(grown);
+        for _ in 0..grown {
+            let mut child = self.node(depth - 1);
+            self.offered(&mut child);
+            children.push(child);
+        }
+        if self.rng.chance() {
+            return Plan::bare(Kind::Stack { children });
+        }
+        let spares: Vec<Plan> = (0..SPARES).map(|_| self.leaf()).collect();
+        let idx = self.spans;
+        self.spans += 1;
+        let edit = self.edits.spans.get(&idx).cloned().unwrap_or_default();
+        let dir = self.rng.below(4);
+        let gap = self.rng.below(3) as i32 * 4;
+        let grown: Vec<usize> = (0..children.len()).collect();
+        let order = span_edited(&grown, children.len(), spares.len(), &edit);
+        Plan::bare(Kind::Span {
+            dir,
+            gap,
+            children,
+            spares,
+            order,
+        })
+    }
+}
+
+/// Builds a plan's widgets in the order it describes them, so two builds of
+/// one plan line up index for index and their boxes can be compared.
+pub fn build<Rsc: UiRsc + 'static>(rsc: &mut Rsc, plan: &Plan) -> (StrongWidget, Tree) {
+    let mut build = Build {
+        rsc,
+        tree: Tree::default(),
+        checkerboard: None,
+    };
+    let root = build.node(plan);
+    (root, build.tree)
+}
+
+struct Build<'a, Rsc> {
+    rsc: &'a mut Rsc,
+    tree: Tree,
+    /// The checkerboard, uploaded when the first image in this tree is built.
+    /// A handle is a reference to the texture, so every image after that one
+    /// clones this rather than uploading the same picture again.
+    checkerboard: Option<TextureHandle>,
+}
+
+impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
+    fn node(&mut self, plan: &Plan) -> StrongWidget {
+        let built = self.kind(&plan.kind);
+        let id = built.id();
+        if let Some(lens) = plan.size {
+            self.rsc.ui_mut().widgets.set_size_rules(id, lens.x, lens.y);
+            self.tree.sized.push(id);
+        }
+        if let Some(align) = plan.align {
+            let resolved = RegionAlign::from(align);
+            let widgets = &mut self.rsc.ui_mut().widgets;
+            for axis in Axis::BOTH {
+                widgets.set_alignment(id, axis, resolved[axis]);
+            }
+            self.tree.aligned.push(id);
+        }
+        if let Some(take) = plan.region_node {
+            self.rsc.ui_mut().widgets.set_region_node(id, take);
+            self.tree.nodes.push(id);
+        }
+        built
+    }
+
+    /// The one picture the generated trees draw: a 64x64 checkerboard of purple
+    /// and black in 8 px cells. Committed rather than drawn here, so that one
+    /// seed is one tree whatever anything else does, and included rather than
+    /// opened, so that growing a tree does not depend on a working directory.
+    fn checkerboard(&mut self) -> TextureHandle {
+        if self.checkerboard.is_none() {
+            let image = include_bytes!("assets/checkerboard.png")
+                .get_image()
+                .expect("the checkerboard is committed beside this file");
+            self.checkerboard = Some(self.rsc.ui_mut().textures.add(image));
+        }
+        self.checkerboard.clone().unwrap()
+    }
+
+    fn kind(&mut self, kind: &Kind) -> StrongWidget {
+        let id: StrongWidget = match kind {
+            Kind::Wrapped => wtext(WORDS).size(16).wrap(true).add_strong(self.rsc),
+            Kind::OneLine => wtext("one line, overflowing whatever it is given")
+                .size(16)
+                .wrap(false)
+                .add_strong(self.rsc),
+            Kind::Rect { color, alpha } => rect(COLORS[*color].alpha(*alpha)).add_strong(self.rsc),
+            Kind::Image => Image::new(self.checkerboard()).add_strong(self.rsc),
+            Kind::Scroll { axis, inner } => {
+                let inner = self.node(inner);
+                let id = Scroll::new(inner, *axis).add(self.rsc);
+                self.tree.scrolls.push(id);
+                self.tree.ids.push(id.id());
+                return id.add_strong(self.rsc);
+            }
+            Kind::Branch {
+                probe,
+                wide,
+                narrow,
+                threshold,
+            } => {
+                let probe = self.node(probe);
+                let wide = self.node(wide);
+                let narrow = self.node(narrow);
+                let id = Branch {
+                    probe,
+                    wide,
+                    narrow,
+                    threshold: *threshold,
+                }
+                .add(self.rsc);
+                self.tree.ids.push(id.id());
+                return id.add_strong(self.rsc);
+            }
+            Kind::Pad { padding, inner } => {
+                let inner = self.node(inner);
+                let [left, right, top, bottom] = padding.map(Px::from_int);
+                let padding = Padding {
+                    left,
+                    right,
+                    top,
+                    bottom,
+                };
+                Pad { padding, inner }.add_strong(self.rsc)
+            }
+            Kind::Stack { children } => {
+                let children = children.iter().map(|c| self.node(c)).collect();
+                Stack {
+                    children,
+                    size: StackSize::Child(0),
+                }
+                .add_strong(self.rsc)
+            }
+            Kind::Span {
+                dir,
+                gap,
+                children,
+                spares,
+                order,
+            } => {
+                let grown = children.len();
+                // Every one of them is made, in this order, whether or not
+                // the span ends up holding it.
+                let made: Vec<StrongWidget> = children
+                    .iter()
+                    .chain(spares)
+                    .map(|c| self.node(c))
+                    .collect();
+                let mut left: Vec<Option<StrongWidget>> = made.into_iter().map(Some).collect();
+                let children: Vec<StrongWidget> = order
+                    .iter()
+                    .filter_map(|&i| left.get_mut(i).and_then(Option::take))
+                    .collect();
+                // What the span does not hold is still held here: dropping
+                // the last share of a widget frees its id for the next one
+                // to be given, which puts two trees out of step.
+                let spares: Vec<StrongWidget> = left.into_iter().flatten().collect();
+                let dir = [Dir::RIGHT, Dir::DOWN, Dir::LEFT, Dir::UP][*dir % 4];
+                let id = Span {
+                    children,
+                    dir,
+                    gap: Px::from_int(*gap),
+                }
+                .add(self.rsc);
+                // A row takes the height it is given rather than its tallest
+                // child, which is a rule beside the span rather than anything
+                // it draws. Derived from `dir` rather than stored, so a plan
+                // that says the direction says this too.
+                if dir.axis == Axis::X {
+                    self.rsc
+                        .widgets_mut()
+                        .set_size_rules(id, None, Some(LayoutLen::rel(1.0)));
+                }
+                self.tree.ids.push(id.id());
+                self.tree.spans.push(Spanned { id, spares, grown });
+                return id.add_strong(self.rsc);
+            }
+        };
+        self.tree.ids.push(id.id());
+        id
+    }
+}

src/widget/image.rs

@@ -6,16 +6,22 @@ pub struct Image {
 }
 
 impl Widget for Image {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
-        painter.texture(&self.handle);
+        painter.primitive(&self.handle);
+        Size::px(self.handle.size())
     }
 
-    fn desired_width(&mut self, _: &mut SizeCtx) -> Len {
+    fn size_hint(&self, axis: Axis) -> Option<LayoutLen> {
-        Len::abs(self.handle.size().x)
+        Some(LayoutLen::px(self.handle.size()[axis]))
+    }
 }
 
-    fn desired_height(&mut self, _: &mut SizeCtx) -> Len {
+impl Image {
-        Len::abs(self.handle.size().y)
+    /// One texture already uploaded, for a caller holding its handle: [`image`]
+    /// uploads what it is given, and several widgets showing one picture want
+    /// one upload and one slot between them.
+    pub fn new(handle: TextureHandle) -> Self {
+        Self { handle }
     }
 }
 

src/widget/mask.rs

@@ -5,16 +5,18 @@ pub struct Masked {
 }
 
 impl Widget for Masked {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
-        painter.set_mask(painter.region());
+        painter.set_mask(UiRegion::FULL);
         painter.widget(&self.inner);
+        // What it occupies is its box, on both axes, for the reason `Scroll`
+        // reports the same: it clips what is inside to that box, so it can
+        // neither take less of one nor honestly ask for more. Passing the
+        // inner size up instead asks to be placed at a length it does not
+        // draw, and the framework would place the drawing it clipped away.
+        Size::LEFTOVER
     }
 
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
-        ctx.width(&self.inner)
+        Some(LayoutLen::LEFTOVER)
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        ctx.height(&self.inner)
     }
 }

src/widget/mod.rs

@@ -1,15 +1,15 @@
 mod image;
 mod mask;
 mod position;
-mod ptr;
 mod rect;
 mod text;
 mod trait_fns;
+mod wrapper;
 
 pub use image::*;
 pub use mask::*;
 pub use position::*;
-pub use ptr::*;
 pub use rect::*;
 pub use text::*;
 pub use trait_fns::*;
+pub use wrapper::*;

src/widget/position/align.rs

@@ -1,35 +0,0 @@
-use crate::prelude::*;
-
-pub struct Aligned {
-    pub inner: StrongWidget,
-    pub align: Align,
-}
-
-impl Widget for Aligned {
-    fn draw(&mut self, painter: &mut Painter) {
-        let region = match self.align.tuple() {
-            (Some(x), Some(y)) => painter
-                .size(&self.inner)
-                .to_uivec2()
-                .align(RegionAlign { x, y }),
-            (Some(x), None) => {
-                let x = painter.size_ctx().width(&self.inner).apply_rest().align(x);
-                UiRegion::new(x, UiSpan::FULL)
-            }
-            (None, Some(y)) => {
-                let y = painter.size_ctx().height(&self.inner).apply_rest().align(y);
-                UiRegion::new(UiSpan::FULL, y)
-            }
-            (None, None) => UiRegion::FULL,
-        };
-        painter.widget_within(&self.inner, region);
-    }
-
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
-        ctx.width(&self.inner)
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        ctx.height(&self.inner)
-    }
-}

src/widget/position/layer.rs

@@ -6,18 +6,10 @@ pub struct LayerOffset {
 }
 
 impl Widget for LayerOffset {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         for _ in 0..self.offset {
             painter.next_layer();
         }
-        painter.widget(&self.inner);
+        painter.widget(&self.inner).size()
-    }
-
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
-        ctx.width(&self.inner)
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        ctx.height(&self.inner)
     }
 }

src/widget/position/max_size.rs

@@ -1,48 +0,0 @@
-use crate::prelude::*;
-
-pub struct MaxSize {
-    pub inner: StrongWidget,
-    pub x: Option<Len>,
-    pub y: Option<Len>,
-}
-
-impl MaxSize {
-    fn apply_to_outer(&self, ctx: &mut SizeCtx) {
-        if let Some(x) = self.x {
-            ctx.outer.x.select_len(x.apply_rest());
-        }
-        if let Some(y) = self.y {
-            ctx.outer.y.select_len(y.apply_rest());
-        }
-    }
-}
-
-impl Widget for MaxSize {
-    fn draw(&mut self, painter: &mut Painter) {
-        painter.widget(&self.inner);
-    }
-
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
-        self.apply_to_outer(ctx);
-        let width = ctx.width(&self.inner);
-        if let Some(x) = self.x {
-            let width_px = width.apply_rest().to_abs(ctx.output_size().x);
-            let x_px = x.apply_rest().to_abs(ctx.output_size().x);
-            if width_px > x_px { x } else { width }
-        } else {
-            width
-        }
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        self.apply_to_outer(ctx);
-        let height = ctx.height(&self.inner);
-        if let Some(y) = self.y {
-            let height_px = height.apply_rest().to_abs(ctx.output_size().y);
-            let y_px = y.apply_rest().to_abs(ctx.output_size().y);
-            if height_px > y_px { y } else { height }
-        } else {
-            height
-        }
-    }
-}

src/widget/position/mod.rs

@@ -1,19 +1,13 @@
-mod align;
 mod layer;
-mod max_size;
 mod offset;
 mod pad;
 mod scroll;
-mod sized;
 mod span;
 mod stack;
 
-pub use align::*;
 pub use layer::*;
-pub use max_size::*;
 pub use offset::*;
 pub use pad::*;
 pub use scroll::*;
-pub use sized::*;
 pub use span::*;
 pub use stack::*;

src/widget/position/offset.rs

@@ -6,16 +6,9 @@ pub struct Offset {
 }
 
 impl Widget for Offset {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
-        let region = UiRegion::FULL.offset(self.amt);
+        painter
-        painter.widget_within(&self.inner, region);
+            .widget_at(&self.inner, UiRegion::FULL.offset(self.amt))
-    }
+            .size()
-
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
-        ctx.width(&self.inner)
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        ctx.height(&self.inner)
     }
 }

src/widget/position/pad.rs

@@ -6,48 +6,50 @@ pub struct Pad {
 }
 
 impl Widget for Pad {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
-        painter.widget_within(&self.inner, self.padding.region());
+        // The inner's own alignment, not the near edge. This reports the
+        // inner's size plus the padding, so where the box is that answer the
+        // inset box is exactly the inner and alignment has no room to move
+        // it; where the box is bigger -- a share of a row, a rule over this
+        // widget -- the slack is the inner's to sit in, and forcing the near
+        // edge pinned it to a corner it had not asked for.
+        //
+        // Padding is an inset of both: it comes off the rel base, so `rel(1)`
+        // under it fills this widget rather than overflowing it by the
+        // padding, and it comes off the box, so what is drawn sits inside.
+        // The two stay distinct -- the box can be narrower still, where a row
+        // asked this widget in the room left, and a text wraps at that.
+        let inner = painter.widget_at(&self.inner, self.padding.region()).size();
+        Size {
+            x: LayoutLen {
+                px: inner.x.px + self.padding.left + self.padding.right,
+                ..inner.x
+            },
+            y: LayoutLen {
+                px: inner.y.px + self.padding.top + self.padding.bottom,
+                ..inner.y
+            },
         }
-
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
-        let width = self.padding.left + self.padding.right;
-        let height = self.padding.top + self.padding.bottom;
-        ctx.outer.x.abs -= width;
-        ctx.outer.y.abs -= height;
-        let mut size = ctx.width(&self.inner);
-        size.abs += width;
-        size
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        let width = self.padding.left + self.padding.right;
-        let height = self.padding.top + self.padding.bottom;
-        ctx.outer.x.abs -= width;
-        ctx.outer.y.abs -= height;
-        let mut size = ctx.height(&self.inner);
-        size.abs += height;
-        size
     }
 }
 
 pub struct Padding {
-    pub left: f32,
+    pub left: Px,
-    pub right: f32,
+    pub right: Px,
-    pub top: f32,
+    pub top: Px,
-    pub bottom: f32,
+    pub bottom: Px,
 }
 
 impl Padding {
     pub const ZERO: Self = Self {
-        left: 0.0,
+        left: Px::ZERO,
-        right: 0.0,
+        right: Px::ZERO,
-        top: 0.0,
+        top: Px::ZERO,
-        bottom: 0.0,
+        bottom: Px::ZERO,
     };
 
     pub fn uniform(amt: impl UiNum) -> Self {
-        let amt = amt.to_f32();
+        let amt = Px::from_num(amt);
         Self {
             left: amt,
             right: amt,
@@ -55,80 +57,82 @@ impl Padding {
             bottom: amt,
         }
     }
-    pub fn region(&self) -> UiRegion {
+    /// `region` less this padding on each side.
-        let mut region = UiRegion::FULL;
+    pub fn region_of(&self, mut region: UiRegion) -> UiRegion {
-        region.x.start.abs += self.left;
+        region.x.start.px += self.left;
-        region.y.start.abs += self.top;
+        region.y.start.px += self.top;
-        region.x.end.abs -= self.right;
+        region.x.end.px -= self.right;
-        region.y.end.abs -= self.bottom;
+        region.y.end.px -= self.bottom;
         region
     }
+
+    pub fn region(&self) -> UiRegion {
+        self.region_of(UiRegion::FULL)
+    }
     pub fn x(amt: impl UiNum) -> Self {
-        let amt = amt.to_f32();
+        let amt = Px::from_num(amt);
         Self {
             left: amt,
             right: amt,
-            top: 0.0,
+            ..Self::ZERO
-            bottom: 0.0,
         }
     }
     pub fn y(amt: impl UiNum) -> Self {
-        let amt = amt.to_f32();
+        let amt = Px::from_num(amt);
         Self {
-            left: 0.0,
-            right: 0.0,
             top: amt,
             bottom: amt,
+            ..Self::ZERO
         }
     }
 
     pub fn top(amt: impl UiNum) -> Self {
         let mut s = Self::ZERO;
-        s.top = amt.to_f32();
+        s.top = Px::from_num(amt);
         s
     }
 
     pub fn bottom(amt: impl UiNum) -> Self {
         let mut s = Self::ZERO;
-        s.bottom = amt.to_f32();
+        s.bottom = Px::from_num(amt);
         s
     }
 
     pub fn left(amt: impl UiNum) -> Self {
         let mut s = Self::ZERO;
-        s.left = amt.to_f32();
+        s.left = Px::from_num(amt);
         s
     }
 
     pub fn right(amt: impl UiNum) -> Self {
         let mut s = Self::ZERO;
-        s.right = amt.to_f32();
+        s.right = Px::from_num(amt);
         s
     }
 
     pub fn with_top(mut self, amt: impl UiNum) -> Self {
-        self.top = amt.to_f32();
+        self.top = Px::from_num(amt);
         self
     }
 
     pub fn with_bottom(mut self, amt: impl UiNum) -> Self {
-        self.bottom = amt.to_f32();
+        self.bottom = Px::from_num(amt);
         self
     }
 
     pub fn with_left(mut self, amt: impl UiNum) -> Self {
-        self.left = amt.to_f32();
+        self.left = Px::from_num(amt);
         self
     }
 
     pub fn with_right(mut self, amt: impl UiNum) -> Self {
-        self.right = amt.to_f32();
+        self.right = Px::from_num(amt);
         self
     }
 }
 
 impl<T: UiNum> From<T> for Padding {
     fn from(amt: T) -> Self {
-        Self::uniform(amt.to_f32())
+        Self::uniform(amt)
     }
 }

src/widget/position/scroll.rs

@@ -3,40 +3,76 @@ use crate::prelude::*;
 pub struct Scroll {
     inner: StrongWidget,
     axis: Axis,
-    amt: f32,
+    amt: Px,
     snap_end: bool,
-    container_len: f32,
+    container_len: Px,
-    content_len: f32,
+    content_len: Px,
 }
 
 impl Widget for Scroll {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
-        let output_len = painter.output_size().axis(self.axis);
+        let container_len = painter.px_len(self.axis);
-        let container_len = painter.region().axis(self.axis).len();
+        // Asked in the whole viewport, then put at the scrolled offset.
-        let content_len = painter
+        let answer_len = painter
-            .len_axis(&self.inner, self.axis)
+            .widget_at(&self.inner, PlaceDesc::WHOLE.fills())
-            .apply_rest()
+            .len(self.axis);
-            .within_len(container_len)
+        let answer_px = painter.to_px(answer_len.without_leftover(), self.axis);
-            .to_abs(output_len);
+        self.container_len = container_len;
-        self.container_len = container_len.to_abs(output_len);
+        self.content_len = answer_px.max(container_len);
-        self.content_len = content_len;
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
         }
         self.update_amt();
-
+        // Reading the box in pixels above holds this drawing to that one
-        let mut region = UiRegion::FULL.offset(Vec2::from_axis(self.axis, -self.amt, 0.0));
+        // length, so these two say where it holds more widely.
-        region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
+        //
-        painter.widget_within(&self.inner, region);
+        // Content of a fixed length that fits is handed the whole box below,
+        // and nothing here reads the box again, so every longer box gives the
+        // same drawing: it holds from the length the content needs upwards,
+        // and shrinking past that is what changes it. Where it sits in a box
+        // longer than itself is not this widget's to say -- placing its
+        // answer in the whole box is its own alignment, and that placement is
+        // a fraction of the box, so it holds at every length too.
+        //
+        // One scrolled part way sits where it is until the box shrinks past
+        // what is left of it. Kept to the end, it moves with every length.
+        let answer_is_px = answer_len.is_px();
+        if answer_is_px && self.content_len <= self.container_len {
+            painter.holds(self.axis, answer_px..=Px::MAX);
+        } else if answer_is_px && !self.snap_end {
+            let left = self.content_len - self.amt;
+            painter.holds(self.axis, Px::MIN..=left);
         }
 
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
+        // Content that fills the viewport is the viewport, and is handed back
-        ctx.width(&self.inner)
+        // as it came -- it has nothing to scroll through, so the clamp above
+        // has already put `amt` at zero. Writing the same box as its own
+        // length in pixels is the same box in another form, and the two do
+        // not round alike: a part centred in `rel 1` lands a step from one
+        // centred in `px 900`, since halving a difference is not halving each
+        // part of it.
+        let content = match self.content_len > self.container_len {
+            true => {
+                let start = Len::from_parts(Rel::ZERO, -self.amt);
+                UiSpan::new(start, start.offset(self.content_len)).shifted_desc()
+            }
+            false => PlaceDescAxis::WHOLE,
+        };
+        // The viewport is the inner's rel base, so a fraction it declares or
+        // reports is a fraction of what is on screen rather than of the
+        // content box its own answer decided. Where it goes is the content
+        // box, scrolled: its drawing moved there, not made again there.
+        painter.place_at(&self.inner, content.on_axis(self.axis).fills());
+        // What it occupies is its box, on both axes: it clips its content to
+        // that box, so it can neither take less of one nor honestly ask for
+        // more. The content's length is what it scrolls through, not what it
+        // is.
+        Size::LEFTOVER
     }
 
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
-        ctx.height(&self.inner)
+        Some(LayoutLen::LEFTOVER)
     }
 }
 
@@ -45,22 +81,24 @@ impl Scroll {
         Self {
             inner,
             axis,
-            amt: 0.0,
+            amt: Px::ZERO,
             snap_end: true,
-            container_len: 0.0,
+            container_len: Px::ZERO,
-            content_len: 0.0,
+            content_len: Px::ZERO,
         }
     }
 
     pub fn update_amt(&mut self) {
-        self.amt = self.amt.max(0.0);
+        self.amt = self.amt.max(Px::ZERO);
-        let len = (self.content_len - self.container_len).max(0.0);
+        let len = (self.content_len - self.container_len).max(Px::ZERO);
         self.amt = self.amt.min(len);
         self.snap_end = self.amt == len;
     }
 
+    /// Scrolled by a distance the platform measures, which is the last place
+    /// a wheel notch or a finger is a float.
     pub fn scroll(&mut self, amt: f32) {
-        self.amt -= amt;
+        self.amt -= Px::from_f32(amt);
         self.update_amt();
     }
 }

src/widget/position/sized.rs

@@ -1,34 +0,0 @@
-use crate::prelude::*;
-
-pub struct Sized {
-    pub inner: StrongWidget,
-    pub x: Option<Len>,
-    pub y: Option<Len>,
-}
-
-impl Sized {
-    fn apply_to_outer(&self, ctx: &mut SizeCtx) {
-        if let Some(x) = self.x {
-            ctx.outer.x.select_len(x.apply_rest());
-        }
-        if let Some(y) = self.y {
-            ctx.outer.y.select_len(y.apply_rest());
-        }
-    }
-}
-
-impl Widget for Sized {
-    fn draw(&mut self, painter: &mut Painter) {
-        painter.widget(&self.inner);
-    }
-
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
-        self.apply_to_outer(ctx);
-        self.x.unwrap_or_else(|| ctx.width(&self.inner))
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        self.apply_to_outer(ctx);
-        self.y.unwrap_or_else(|| ctx.height(&self.inner))
-    }
-}

src/widget/position/span.rs

@@ -4,61 +4,164 @@ use std::marker::PhantomData;
 pub struct Span {
     pub children: Vec<StrongWidget>,
     pub dir: Dir,
-    pub gap: f32,
+    pub gap: Px,
 }
 
 impl Widget for Span {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
-        let total = self.len_sum(&mut painter.size_ctx());
+        let axis = self.dir.axis;
-        let mut start = UiScalar::rel_min();
+        // The row this span lays its children out along, as a length of the
+        // rel base they are laid out against. Where it starts is nothing's
+        // business -- a slot is a length from there -- so what this reads is
+        // the length alone.
+        let row = painter.region_len(axis);
+        // A length for every child before their final slots are chosen: from
+        // a hint where one says, and from drawing otherwise. The rel base passes
+        // through unchanged, so `rel(0.5)` is half the area this span was
+        // given whatever else is in it and wherever this child sits among
+        // them; what a drawn child is asked in is the room left from the
+        // cursor, because a text has to wrap at the width actually there.
+        let mut cursor = Len::ZERO;
+        let mut lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let mut span = UiSpan::FULL;
+            let len = match painter.size_hint(child, axis) {
-            span.start = start;
+                Some(len) => len,
-            let len = painter.len_axis(child, self.dir.axis);
+                None => {
-            if len.rest > 0.0 {
+                    // Across itself the child sits where its own alignment
-                let offset = UiScalar::new(total.rel, total.abs);
+                    // says, in the whole of the row: a span is what contains
-                let rel_end = UiScalar::rel(len.rest / total.rest);
+                    // its children there, and nothing divides that axis.
-                let end = (UiScalar::rel_max() + start) - offset;
+                    let room = self.slot(row, cursor, row).shifted_desc().on_axis(axis);
-                start = rel_end.within(&start.to(end));
+                    painter.widget_at(child, room).len(axis)
-            }
-            start.abs += len.abs;
-            start.rel += len.rel;
-            span.end = start;
-            let mut child_region = UiRegion::from_axis(self.dir.axis, span, UiSpan::FULL);
-            if self.dir.sign == Sign::Neg {
-                child_region.flip(self.dir.axis);
-            }
-            painter.widget_within(child, child_region);
-            start.abs += self.gap;
                 }
+            };
+            cursor += len.without_leftover();
+            cursor.px += self.gap;
+            lens.push(len);
         }
 
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
+        let gaps = self
-        match self.dir.axis {
+            .gap
-            Axis::X => self.desired_len(ctx),
+            .mul_int(self.children.len().saturating_sub(1) as i32);
-            Axis::Y => self.desired_ortho(ctx),
+        let total = lens.iter().fold(
+            LayoutLen {
+                px: gaps,
+                ..LayoutLen::ZERO
+            },
+            |sum, len| sum + *len,
+        );
+
+        // What is left for the shares to divide: the row less everything
+        // fixed, as a length of the rel base rather than a number of pixels.
+        let all_fixed = total.without_leftover();
+        let room = row - all_fixed;
+        // The three cases a rounded division needed -- the fixed parts
+        // growing slower than the box, faster, or exactly with it -- are the
+        // sign of `room.rel`, which the range `longer_than` keeps already
+        // reads. What the generated oracle checks is the consequence, since
+        // which children exist at all turns on this.
+        let any_leftover = total.leftover > Weight::ZERO;
+        let has_room = any_leftover && painter.longer_than(row, all_fixed, axis);
+
+        // Across itself a span is as long as its longest child -- unless a
+        // rule beside it gives that length outright, and then reading them
+        // answers nothing and makes its size depend on theirs for it. A rule
+        // that only bounds the length does not count: the answer is still
+        // this span's to give.
+        let shrinks = !painter.has_exact_size(!axis);
+        // What the fixed parts and the gaps before here take, which is a sum
+        // of lengths and exact, and how much of the leftover weight is
+        // spoken for. Both ends of a slot are read from those two rather
+        // than stepped from the last child: the share of the room is
+        // rounded, and taking each end from the one before it would carry
+        // every rounding along the row.
+        let mut fixed = Len::ZERO;
+        let mut taken = Weight::ZERO;
+        let mut ortho = LayoutLen::ZERO;
+        // Nothing divides the room where no child asked for any of it, and a
+        // ratio of a whole of nothing has no answer.
+        let reached = |fixed: Len, taken: Weight| match any_leftover {
+            false => fixed,
+            true => fixed + room.scale(Rel::ratio(taken, total.leftover)),
+        };
+        for (child, &len) in self.children.iter().zip(&lens) {
+            // A child asking for nothing but a part of what is left over,
+            // when nothing is, is not drawn at all. One that also asked for
+            // pixels or a fraction keeps those and overflows.
+            if len.is_only_leftover() && !has_room {
+                painter.undraw(child);
+                fixed.px += self.gap;
+                continue;
             }
+            let from = reached(fixed, taken);
+            if len.leftover > Weight::ZERO && has_room {
+                taken += len.leftover;
+            }
+            fixed += len.without_leftover();
+            let to = reached(fixed, taken);
+            // Along the row the span says where the child goes, and that slot
+            // is the child's box outright rather than something to place an
+            // answer inside again. A share is decided here and nowhere
+            // else: its slot narrows its rel base, and the child is asked in
+            // it, since a text wraps at the width it is actually given. A
+            // fixed child's slot is its own answer, so a drawing made in the
+            // room is put there as it is, and one not made yet is made here.
+            let slot = self.slot(row, from, to);
+            let mut place = slot.shifted_desc().fills().on_axis(axis);
+            if len.leftover > Weight::ZERO && has_room {
+                place = place.rel_base(axis, slot.len());
+            }
+            let used = painter.place_at(child, place).len(!axis);
+            if shrinks {
+                // Choosing between a fixed and a relative length from the
+                // span's own eventual width admits multiple fixed points.
+                // A scalable child therefore makes the span scalable too;
+                // only fixed children are compared with one another.
+                if !used.is_px() {
+                    ortho = LayoutLen::LEFTOVER;
+                } else if ortho.leftover == Weight::ZERO {
+                    ortho.px = ortho.px.max(used.px);
+                }
+            }
+            fixed.px += self.gap;
         }
 
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
+        // Carried whole rather than collapsed to one share: a span that sizes
-        match self.dir.axis {
+        // from its children does not resolve `leftover`, it passes the weight up,
-            Axis::X => self.desired_ortho(ctx),
+        // so nesting spans divides the same space rather than re-dividing a
-            Axis::Y => self.desired_len(ctx),
+        // share of it. Four `leftover(1)` children under two spans under one span
-        }
+        // get a quarter each, which collapsing to `leftover(1)` per level does
+        // not give. Resolution happens at the nearest ancestor with a length,
+        // and the root always has one.
+        let ortho = match shrinks {
+            true => ortho,
+            false => LayoutLen::rel(1.0),
+        };
+        Size::from_axis(axis, total, ortho)
     }
 }
 
 impl Span {
+    /// The stretch of the row between two distances from where this span
+    /// starts laying children out, as a span of its own box. A negative
+    /// direction lays out from the far end, so the same two distances mirror
+    /// in a row `row` long.
+    fn slot(&self, row: Len, from: Len, to: Len) -> UiSpan {
+        match self.dir.sign {
+            Sign::Pos => from.to(to),
+            Sign::Neg => (row - to).to(row - from),
+        }
+    }
+
     pub fn empty(dir: Dir) -> Self {
         Self {
             children: Vec::new(),
             dir,
-            gap: 0.0,
+            gap: Px::ZERO,
         }
     }
 
     pub fn gap(mut self, gap: impl UiNum) -> Self {
-        self.gap = gap.to_f32();
+        self.gap = Px::from_num(gap);
         self
     }
 
@@ -69,93 +172,12 @@ impl Span {
     pub fn pop(&mut self) -> Option<StrongWidget> {
         self.children.pop()
     }
-
-    fn len_sum(&mut self, ctx: &mut SizeCtx) -> Len {
-        let gap = self.gap * self.children.len().saturating_sub(1) as f32;
-        self.children.iter().fold(Len::abs(gap), |mut s, id| {
-            // it's tempting to subtract the abs & rel from the ctx outer,
-            // but that would create inconsistent sizing if you put
-            // a rest first vs last & only speed up in one direction.
-            // I think this is only solvable by restricting how you can
-            // compute size, bc currently you need child to define parent's
-            // sectioning and you need parent's sectioning to define child.
-            // Fortunately, that doesn't matter in most cases
-            let len = ctx.len_axis(id, self.dir.axis);
-            s += len;
-            s
-        })
-    }
-
-    fn desired_len(&mut self, ctx: &mut SizeCtx) -> Len {
-        let len = self.len_sum(ctx);
-        if len.rest == 0.0 && len.rel == 0.0 {
-            len
-        } else {
-            Len::default()
-        }
-    }
-
-    fn desired_ortho(&mut self, ctx: &mut SizeCtx) -> Len {
-        // this is a weird hack to get text wrapping to work properly when in a downward span
-        // the correct solution here is to add a function to widget that lets them
-        // request that ctx.outer has an axis "resolved" before checking the other,
-        // and panicking or warning if two request opposite axis (unsolvable in that case)
-        let outer = ctx.outer.axis(self.dir.axis);
-        if self.dir.axis == Axis::X {
-            // so....... this literally copies draw so that the lengths are correctly set in the
-            // context, which makes this slow and not cool
-            let total = self.len_sum(ctx);
-            let mut start = UiScalar::rel_min();
-            let mut ortho_len = Len::ZERO;
-            for child in &self.children {
-                let mut span = UiSpan::FULL;
-                span.start = start;
-                let len = ctx.len_axis(child, self.dir.axis);
-                if len.rest > 0.0 {
-                    let offset = UiScalar::new(total.rel, total.abs);
-                    let rel_end = UiScalar::rel(len.rest / total.rest);
-                    let end = (UiScalar::rel_max() + start) - offset;
-                    start = rel_end.within(&start.to(end));
-                }
-                start.abs += len.abs;
-                start.rel += len.rel;
-                span.end = start;
-
-                let scalar = span.len();
-                *ctx.outer.axis_mut(self.dir.axis) = outer.select_len(scalar);
-                let ortho = ctx.len_axis(child, !self.dir.axis);
-                // TODO: rel shouldn't do this, but no easy way before actually calculating pixels
-                if ortho.rel > 0.0 || ortho.rest > 0.0 {
-                    ortho_len.rest = 1.0;
-                    ortho_len.abs = 0.0;
-                    break;
-                }
-                ortho_len.abs = ortho_len.abs.max(ortho.abs);
-                start.abs += self.gap;
-            }
-            ortho_len
-        } else {
-            let mut ortho_len = Len::ZERO;
-            let ortho = !self.dir.axis;
-            for child in &self.children {
-                let len = ctx.len_axis(child, ortho);
-                // TODO: rel shouldn't do this, but no easy way before actually calculating pixels
-                if len.rel > 0.0 || len.rest > 0.0 {
-                    ortho_len.rest = 1.0;
-                    ortho_len.abs = 0.0;
-                    break;
-                }
-                ortho_len.abs = ortho_len.abs.max(len.abs);
-            }
-            ortho_len
-        }
-    }
 }
 
 pub struct SpanBuilder<State, const LEN: usize, Wa: WidgetArrLike<State, LEN, Tag>, Tag> {
     pub children: Wa,
     pub dir: Dir,
-    pub gap: f32,
+    pub gap: Px,
     _pd: PhantomData<(State, Tag)>,
 }
 
@@ -181,13 +203,13 @@ impl<State, const LEN: usize, Wa: WidgetArrLike<State, LEN, Tag>, Tag>
         Self {
             children,
             dir,
-            gap: 0.0,
+            gap: Px::ZERO,
             _pd: PhantomData,
         }
     }
 
     pub fn gap(mut self, gap: impl UiNum) -> Self {
-        self.gap = gap.to_f32();
+        self.gap = Px::from_num(gap);
         self
     }
 }

src/widget/position/stack.rs

@@ -8,29 +8,52 @@ pub struct Stack {
 }
 
 impl Widget for Stack {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
-        let mut iter = self.children.iter();
+        let sizing = match self.size {
-        if let Some(child) = iter.next() {
+            StackSize::Default => None,
-            painter.child_layer();
+            StackSize::Child(i) => Some(i),
-            painter.widget(child);
+        };
+        // Whichever child sizes the stack is given the stack's whole box --
+        // the stack is the length that child asked for, so placing that
+        // answer inside the box it decided would apply it twice.
+        let size = match sizing.and_then(|i| self.children.get(i).map(|c| (i, c))) {
+            // On the layer that child ends up on, so the ask below is a reuse
+            // rather than a second drawing of it somewhere else: a retained
+            // drawing belongs to the layer it was made on.
+            Some((i, child)) => {
+                painter.child_layer_at(i);
+                painter.widget_at(child, PlaceDesc::WHOLE.fills()).size()
             }
-        for child in iter {
+            None => Size::LEFTOVER,
-            painter.next_layer();
+        };
-            painter.widget(child);
+        // Every other child gets the box the sizing child decided: the
+        // stack is that length, so that is the box they are asked in, and a
+        // fraction under them is a fraction of it. A share leaves the axis
+        // to whoever gave the stack its box. Where a child sits in a box
+        // bigger than itself is its own business.
+        let place = PlaceDesc::from_axes(|axis| {
+            let len = size[axis];
+            match len.leftover == Weight::ZERO {
+                true => len.without_leftover().as_desc().fills(),
+                false => PlaceDescAxis::WHOLE,
             }
+        });
+        for (i, child) in self.children.iter().enumerate() {
+            if sizing == Some(i) {
+                continue;
+            }
+            painter.child_layer_at(i);
+            painter.widget_at(child, place);
+        }
+        size
     }
 
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
+    /// Without a sizing child a stack is whatever box it is given, which it
+    /// can say without drawing anything.
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
         match self.size {
-            StackSize::Default => Len::default(),
+            StackSize::Default => Some(LayoutLen::LEFTOVER),
-            StackSize::Child(i) => ctx.width(&self.children[i]),
+            StackSize::Child(_) => None,
-        }
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        match self.size {
-            StackSize::Default => Len::default(),
-            StackSize::Child(i) => ctx.height(&self.children[i]),
         }
     }
 }

src/widget/ptr.rs

@@ -1,57 +0,0 @@
-use crate::prelude::*;
-use std::marker::{Sized, Unsize};
-
-pub struct WidgetPtr {
-    pub inner: Option<StrongWidget>,
-}
-
-impl Widget for WidgetPtr {
-    fn draw(&mut self, painter: &mut Painter) {
-        if let Some(id) = &self.inner {
-            painter.widget(id);
-        }
-    }
-
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
-        if let Some(id) = &self.inner {
-            ctx.width(id)
-        } else {
-            Len::ZERO
-        }
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        if let Some(id) = &self.inner {
-            ctx.height(id)
-        } else {
-            Len::ZERO
-        }
-    }
-}
-
-impl WidgetPtr {
-    pub fn new() -> Self {
-        Self::default()
-    }
-    pub fn empty() -> Self {
-        Self {
-            inner: Default::default(),
-        }
-    }
-    pub fn set<W: ?Sized + Unsize<dyn Widget>>(&mut self, to: StrongWidget<W>) {
-        self.inner = Some(to)
-    }
-
-    pub fn replace<W: ?Sized + Unsize<dyn Widget>>(
-        &mut self,
-        to: StrongWidget<W>,
-    ) -> Option<StrongWidget> {
-        self.inner.replace(to)
-    }
-}
-
-impl Default for WidgetPtr {
-    fn default() -> Self {
-        Self::empty()
-    }
-}

src/widget/rect.rs

@@ -28,21 +28,18 @@ impl Rect {
 }
 
 impl Widget for Rect {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         painter.primitive(RectPrimitive {
             color: self.color,
             radius: self.radius,
             thickness: self.thickness,
             inner_radius: self.inner_radius,
         });
+        Size::LEFTOVER
     }
 
-    fn desired_width(&mut self, _: &mut SizeCtx) -> Len {
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
-        Len::rest(1)
+        Some(LayoutLen::LEFTOVER)
-    }
-
-    fn desired_height(&mut self, _: &mut SizeCtx) -> Len {
-        Len::rest(1)
     }
 }
 

src/widget/text/build.rs

@@ -1,5 +1,5 @@
 use crate::prelude::*;
-use std::marker::{PhantomData, Sized};
+use std::marker::PhantomData;
 
 pub struct TextBuilder<State, O = TextOutput, H: WidgetOption<State> = ()> {
     pub content: String,

src/widget/text/edit.rs

@@ -55,44 +55,43 @@ impl TextEdit {
 }
 
 impl Widget for TextEdit {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let base = painter.layer;
         painter.child_layer();
-        self.view.draw(painter);
+        let (_, size) = self.view.draw(painter);
         painter.layer = base;
         let region = self.region();
 
         let Some(selection) = self.selection else {
-            return;
+            return size;
         };
         let layout = self.view.buf.layout();
 
         // parley reports selection as boxes in layout space, so bidi and
         // wrapped lines come out right without this code knowing about either.
         for (rect, _) in selection.geometry(layout) {
-            let size = vec2(rect.width() as f32, rect.height() as f32);
+            let rect_size = vec2(rect.width() as f32, rect.height() as f32);
             let top_left = vec2(rect.x0 as f32, rect.y0 as f32);
             painter.primitive_within(
                 RectPrimitive::color(Color::SKY),
-                size.align(Align::TOP_LEFT).offset(top_left).within(&region),
+                rect_size
+                    .align(Align::TOP_LEFT)
+                    .offset(top_left)
+                    .within(&region),
             );
         }
 
         let caret = selection.focus().geometry(layout, CARET_WIDTH);
-        let size = vec2(caret.width() as f32, caret.height() as f32);
+        let caret_size = vec2(caret.width() as f32, caret.height() as f32);
         let top_left = vec2(caret.x0 as f32, caret.y0 as f32);
         painter.primitive_within(
             RectPrimitive::color(Color::WHITE),
-            size.align(Align::TOP_LEFT).offset(top_left).within(&region),
+            caret_size
+                .align(Align::TOP_LEFT)
+                .offset(top_left)
+                .within(&region),
         );
-    }
+        size
-
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
-        self.view.desired_width(ctx)
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        self.view.desired_height(ctx)
     }
 }
 
@@ -127,7 +126,6 @@ impl<'a> TextEditCtx<'a> {
     pub fn set(&mut self, text: &str) {
         let text = self.string(text);
         self.text.view.buf.set_text(text);
-        self.text.view.buf.changed = true;
         self.text.selection = None;
     }
 
@@ -174,7 +172,6 @@ impl<'a> TextEditCtx<'a> {
         };
         let at = at.min(self.text.view.buf.text().len());
         self.text.view.buf.edit().insert_str(at, text);
-        self.text.view.buf.changed = true;
         self.set_caret(at + text.len());
     }
 
@@ -187,7 +184,6 @@ impl<'a> TextEditCtx<'a> {
         }
         let range = sel.text_range();
         self.text.view.buf.edit().replace_range(range.clone(), "");
-        self.text.view.buf.changed = true;
         self.set_caret(range.start);
         true
     }
@@ -265,7 +261,6 @@ impl<'a> TextEditCtx<'a> {
 
     fn delete_range(&mut self, start: usize, end: usize) {
         self.text.view.buf.edit().replace_range(start..end, "");
-        self.text.view.buf.changed = true;
         self.set_caret(start);
     }
 
@@ -281,7 +276,13 @@ impl<'a> TextEditCtx<'a> {
     }
 
     pub fn select(&mut self, pos: Vec2, size: Vec2, drag: bool, recent: bool) {
-        let pos = pos - self.text.region().top_left().to_abs(size);
+        let pos = pos
+            - self
+                .text
+                .region()
+                .top_left()
+                .to_px(PxVec2::from_f32(size))
+                .to_f32();
         let prev_sel = self.text.selection;
         let prev_hit = self.text.double_hit;
 
@@ -320,12 +321,10 @@ impl<'a> TextEditCtx<'a> {
         let old = (self.text.view.buf.text().to_string(), self.text.selection);
         let mut undo = false;
         let res = self.apply_event_inner(event, modifiers, &mut undo);
-        if undo {
+        if undo && let Some((old, selection)) = self.text.history.pop() {
-            if let Some((old, selection)) = self.text.history.pop() {
             self.set(&old);
             self.text.selection = selection;
             self.clamp_selection_to_layout();
-            }
         } else if self.text.view.buf.text() != old.0 {
             self.text.history.push(old);
         }

src/widget/text/mod.rs

@@ -14,11 +14,8 @@ pub struct Text {
 }
 
 pub struct TextView {
-    pub attrs: MutDetect<TextAttrs>,
+    pub attrs: TextAttrs,
-    pub buf: MutDetect<TextBuffer>,
+    pub buf: TextBuffer,
-    // cache
-    tex: Option<RenderedText>,
-    width: Option<f32>,
     pub hint: Option<StrongWidget>,
 }
 
@@ -28,19 +25,13 @@ impl TextView {
     }
 
     pub fn wrap_width(&self) -> Option<f32> {
-        self.width
+        self.buf.wrap_width()
     }
 }
 
 impl TextView {
     pub fn new(buf: TextBuffer, attrs: TextAttrs, hint: Option<StrongWidget>) -> Self {
-        Self {
+        Self { attrs, buf, hint }
-            attrs: attrs.into(),
-            buf: buf.into(),
-            tex: None,
-            width: None,
-            hint,
-        }
     }
 
     /// region where the text should be draw
@@ -52,56 +43,45 @@ impl TextView {
             .align(self.align)
     }
 
-    fn render(&mut self, ctx: &mut SizeCtx) -> &RenderedText {
+    /// The text shaped for the width it is drawn in. The buffer keeps its
-        let width = if self.attrs.wrap {
+    /// answers under the attrs too, so changing those asks a new question
-            Some(ctx.px_size().x)
+    /// rather than invalidating anything.
-        } else {
+    fn render(&mut self, painter: &mut Painter) -> &RenderedText {
-            None
+        let width = self.attrs.wrap.then(|| painter.px_len(Axis::X));
-        };
+        // The shaper measures in floats, which is where a glyph advance comes
-        if width != self.width || self.tex.is_none() || self.attrs.changed || self.buf.changed {
+        // from; what it answers goes back on the grid.
-            self.width = width;
+        painter.render_text(&mut self.buf, &self.attrs, width.map(Px::to_f32));
-            self.tex = Some(ctx.draw_text(&mut self.buf, &self.attrs, width));
+        if width.is_some() {
-            self.attrs.changed = false;
+            painter.holds(Axis::X, self.buf.width_holds());
-            self.buf.changed = false;
         }
-        self.tex.as_ref().unwrap()
+        self.buf.rendered().expect("render_text placed the glyphs")
-    }
-    pub fn tex(&self) -> Option<&RenderedText> {
-        self.tex.as_ref()
-    }
-    pub fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
-        if self.is_empty()
-            && let Some(hint) = &self.hint
-        {
-            ctx.width(hint)
-        } else {
-            Len::abs(self.render(ctx).size.x)
-        }
-    }
-    pub fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        if self.is_empty()
-            && let Some(hint) = &self.hint
-        {
-            ctx.height(hint)
-        } else {
-            Len::abs(self.render(ctx).size.y)
-        }
-    }
-    pub fn draw(&mut self, painter: &mut Painter) -> UiRegion {
-        let align = self.align;
-        if self.is_empty() && self.hint.is_some() {
-            let region = self.render(&mut painter.size_ctx()).size.align(align);
-            if let Some(hint) = &self.hint {
-                painter.widget(hint);
-            }
-            return region;
     }
 
-        let tex = self.render(&mut painter.size_ctx());
+    pub fn tex(&self) -> Option<&RenderedText> {
+        self.buf.rendered()
+    }
+    /// Draws the text, and says where the glyphs went and what they use.
+    pub fn draw(&mut self, painter: &mut Painter) -> (UiRegion, Size) {
+        let align = self.align;
+        if self.is_empty() && self.hint.is_some() {
+            let region = self.render(painter).size.align(align);
+            let size = match &self.hint {
+                Some(hint) => painter.widget(hint).size(),
+                None => Size::ZERO,
+            };
+            return (region, size);
+        }
+
+        let tex = self.render(painter);
         let region = tex.size.align(align);
-        let within = region.within(&painter.region());
+        // The step at or above what the shaper measured, so a parent that
-        painter.glyphs(tex, within);
+        // hands back the length this reports hands back a box the longest
-        region
+        // line fits in. Rounded to the nearest step it is half the time a
+        // hair under that line, and the break made in it is not the break a
+        // cold layout makes there.
+        let size = Size::from_px(PxVec2::ceil_from_f32(tex.size));
+        painter.glyphs(tex, region);
+        (region, size)
     }
 
     pub fn content(&self) -> String {
@@ -117,7 +97,7 @@ impl Text {
             content: content.into(),
         }
     }
-    fn update_buf(&mut self, _ctx: &mut SizeCtx) {
+    fn update_buf(&mut self) {
         if self.content.changed {
             self.content.changed = false;
             self.view.buf.set_text(self.content.as_str());
@@ -126,19 +106,9 @@ impl Text {
 }
 
 impl Widget for Text {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
-        self.update_buf(&mut painter.size_ctx());
+        self.update_buf();
-        self.view.draw(painter);
+        self.view.draw(painter).1
-    }
-
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
-        self.update_buf(ctx);
-        self.view.desired_width(ctx)
-    }
-
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
-        self.update_buf(ctx);
-        self.view.desired_height(ctx)
     }
 }
 

src/widget/trait_fns.rs

@@ -12,14 +12,23 @@ widget_trait! {
         }
     }
 
-    fn align(self, align: impl Into<Align>) -> impl WidgetFn<Rsc, Aligned> {
+    fn align(self, align: impl Into<Align>) -> impl WidgetIdFn<Rsc, WL::Widget> {
-        move |state| Aligned {
+        // An axis left out keeps whatever it had, which is centered unless
-            inner: self.add_strong(state),
+        // something else set it.
-            align: align.into(),
+        let align = align.into();
+        move |state| {
+            let id = self.add(state);
+            let widgets = &mut state.ui_mut().widgets;
+            for axis in Axis::BOTH {
+                if let Some(align) = align[axis] {
+                    widgets.set_alignment(id, axis, align);
+                }
+            }
+            id
         }
     }
 
-    fn center(self) -> impl WidgetFn<Rsc, Aligned> {
+    fn center(self) -> impl WidgetIdFn<Rsc, WL::Widget> {
         self.align(Align::CENTER)
     }
 
@@ -31,48 +40,46 @@ widget_trait! {
         }
     }
 
-    fn sized(self, size: impl Into<Size>) -> impl WidgetFn<Rsc, Sized> {
+    fn region_node(self) -> impl WidgetIdFn<Rsc, WL::Widget> {
+        |state| {
+            let id = self.add(state);
+            state.ui_mut().widgets.set_region_node(id, true);
+            id
+        }
+    }
+
+    fn sized(self, size: impl Into<Size>) -> impl WidgetIdFn<Rsc, WL::Widget> {
         let size = size.into();
-        move |state| Sized {
+        move |state| {
-            inner: self.add_strong(state),
+            let id = self.add(state);
-            x: Some(size.x),
+            let widgets = &mut state.ui_mut().widgets;
-            y: Some(size.y),
+            widgets.set_size_rule(id, Axis::X, SizeRule::Exact(size.x));
+            widgets.set_size_rule(id, Axis::Y, SizeRule::Exact(size.y));
+            id
         }
     }
 
-    fn max_width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, MaxSize> {
+    fn width(self, len: impl Into<LayoutLen>) -> impl WidgetIdFn<Rsc, WL::Widget> {
         let len = len.into();
-        move |state| MaxSize {
+        move |state| {
-            inner: self.add_strong(state),
+            let id = self.add(state);
-            x: Some(len),
+            state
-            y: None,
+                .ui_mut()
+                .widgets
+                .set_size_rule(id, Axis::X, SizeRule::Exact(len));
+            id
         }
     }
 
-    fn max_height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, MaxSize> {
+    fn height(self, len: impl Into<LayoutLen>) -> impl WidgetIdFn<Rsc, WL::Widget> {
         let len = len.into();
-        move |state| MaxSize {
+        move |state| {
-            inner: self.add_strong(state),
+            let id = self.add(state);
-            x: None,
+            state
-            y: Some(len),
+                .ui_mut()
-        }
+                .widgets
-    }
+                .set_size_rule(id, Axis::Y, SizeRule::Exact(len));
-
+            id
-    fn width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, Sized> {
-        let len = len.into();
-        move |state| Sized {
-            inner: self.add_strong(state),
-            x: Some(len),
-            y: None,
-        }
-    }
-
-    fn height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, Sized> {
-        let len = len.into();
-        move |state| Sized {
-            inner: self.add_strong(state),
-            x: None,
-            y: Some(len),
         }
     }
 
@@ -85,7 +92,9 @@ widget_trait! {
 
     fn scrollable(self) -> impl WidgetIdFn<Rsc, Scroll> where Rsc: HasEvents {
         move |state| {
-            Scroll::new(self.add_strong(state), Axis::Y)
+            let inner = self.add(state);
+            state.ui_mut().widgets.set_region_node(inner, true);
+            Scroll::new(inner.upgrade(state), Axis::Y)
                 .on(CursorSense::Scroll, |ctx, rsc| {
                     let delta = ctx.data.scroll_delta.y * 50.0;
                     ctx.widget(rsc).scroll(delta);
@@ -125,9 +134,13 @@ widget_trait! {
         |state| self.add(state)
     }
 
-    fn set_ptr(self, ptr: WeakWidget<WidgetPtr>, state: &mut Rsc) {
+    // Named for the type it makes rather than as `wrapped`, which would read
-        let id = self.add_strong(state);
+    // as the text setting. `widget_trait!` takes no attributes, so what it is
-        state.ui_mut().widgets[ptr].inner = Some(id);
+    // for is on `Wrapper` itself.
+    fn wrapper(self) -> impl WidgetFn<Rsc, Wrapper> {
+        |state| Wrapper {
+            inner: Some(self.add_strong(state)),
+        }
     }
 }
 

src/widget/wrapper.rs

@@ -0,0 +1,41 @@
+use crate::prelude::*;
+use std::marker::Unsize;
+
+/// One widget in a box of its own, doing as little as possible on the way:
+/// it draws its child in the whole of its box and reports back what the child
+/// said. It exists because a length and an alignment are properties of one
+/// widget, so a widget cannot both be 100 wide and take two shares of a row
+/// -- the two lengths need two widgets, and this is the smaller one.
+///
+/// Its child is optional so it can also be the swappable slot a tab bar
+/// needs, which is what it was written for.
+#[derive(Default)]
+pub struct Wrapper {
+    pub inner: Option<StrongWidget>,
+}
+
+impl Widget for Wrapper {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        match &self.inner {
+            Some(id) => painter.widget(id).size(),
+            None => Size::default(),
+        }
+    }
+}
+
+impl Wrapper {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn set<W: ?Sized + Unsize<dyn Widget>>(&mut self, to: StrongWidget<W>) {
+        self.inner = Some(to)
+    }
+
+    pub fn replace<W: ?Sized + Unsize<dyn Widget>>(
+        &mut self,
+        to: StrongWidget<W>,
+    ) -> Option<StrongWidget> {
+        self.inner.replace(to)
+    }
+}

tests/cases/determinism.rs

@@ -0,0 +1,103 @@
+//! A measurement that decides control flow.
+//!
+//! Comparing boxes catches a widget that moved. It does not catch a widget
+//! that measured a child, believed a different answer from the one a cold
+//! start would give, and took the other branch -- which is the same defect
+//! arriving somewhere it cannot be ignored. A widget here branches on what it
+//! measured, so a disagreement shows up as a different tree.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+/// Measures `probe` across `axis` and draws one of two children on the
+/// answer. Its own configuration never changes, so which child is drawn is a
+/// property of the layout alone.
+struct BranchesOnMeasurement {
+    probe: StrongWidget,
+    wide: StrongWidget,
+    narrow: StrongWidget,
+    threshold: f32,
+}
+
+impl Widget for BranchesOnMeasurement {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
+        let top = UiSpan::new(Len::ZERO, cut).shifted_desc();
+        let measured = painter
+            .widget_at(&self.probe, top.on_axis(Axis::Y))
+            .len(Axis::X);
+        let px = painter.to_px(measured.apply_leftover(), Axis::X);
+
+        let below = UiSpan::new(cut, painter.region_len(Axis::Y)).shifted_desc();
+        let place = below.on_axis(Axis::Y);
+        match px > Px::from_f32(self.threshold) {
+            true => painter.widget_at(&self.wide, place),
+            false => painter.widget_at(&self.narrow, place),
+        };
+        Size::LEFTOVER
+    }
+}
+
+fn plant(h: &mut Harness, threshold: f32) -> (WidgetId, WidgetId) {
+    let words = "the quick brown fox jumps over the lazy dog and keeps running";
+    let probe = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let wide = rect(Color::RED).add(&mut h.rsc);
+    let narrow = rect(Color::BLUE).add(&mut h.rsc);
+    let branch = BranchesOnMeasurement {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold,
+    }
+    .add(&mut h.rsc);
+    let side = rect(Color::GREEN).width(120).add(&mut h.rsc);
+    h.set_root((side, branch).span(Dir::RIGHT));
+    (wide.id(), narrow.id())
+}
+
+/// Which of the two branches drew, as a pair a test can compare.
+fn taken(h: &Harness, wide: WidgetId, narrow: WidgetId) -> (bool, bool) {
+    (h.region(&wide).is_some(), h.region(&narrow).is_some())
+}
+
+#[test]
+fn a_branch_taken_on_a_measurement_holds_across_repaints() {
+    for threshold in [0.0, 200.0, 400.0, 600.0, 779.0, 780.0, 781.0, 2000.0] {
+        let mut h = Harness::new((900, 600));
+        let (wide, narrow) = plant(&mut h, threshold);
+        let first = taken(&h, wide, narrow);
+        assert_ne!(first, (false, false), "threshold {threshold}: neither drew");
+
+        for frame in 0..4 {
+            h.rsc.widgets_mut().mark_for_redraw(wide);
+            h.rsc.widgets_mut().mark_for_redraw(narrow);
+            h.frame();
+            assert_eq!(
+                taken(&h, wide, narrow),
+                first,
+                "threshold {threshold}, repaint {frame}: the branch moved when nothing did"
+            );
+        }
+    }
+}
+
+#[test]
+fn a_branch_taken_on_a_measurement_is_the_one_a_cold_start_takes() {
+    for threshold in [0.0, 200.0, 400.0, 600.0, 779.0, 780.0, 781.0, 2000.0] {
+        let mut warm = Harness::new((900, 600));
+        let (wide, narrow) = plant(&mut warm, threshold);
+        warm.resize((640, 480));
+        warm.frame();
+        warm.rsc.widgets_mut().mark_for_redraw(wide);
+        warm.frame();
+
+        let mut cold = Harness::new((640, 480));
+        let (cwide, cnarrow) = plant(&mut cold, threshold);
+
+        assert_eq!(
+            taken(&warm, wide, narrow),
+            taken(&cold, cwide, cnarrow),
+            "threshold {threshold}: warm and cold took different branches"
+        );
+    }
+}

tests/cases/drift.rs

@@ -0,0 +1,45 @@
+//! What a retained drawing costs in accuracy when it is moved instead of made
+//! again. A subtree's stored regions are the only record of where it is, so a
+//! move that works from the last answer rather than from the box it is now in
+//! integrates its own rounding, and nothing later recomputes it. Re-expressing
+//! each part as the same fraction of the new box is what keeps a long-lived
+//! layout on the one a cold start produces.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+/// A row of a fixed height under a bar, so changing the bar's height moves the
+/// row without changing the box it is given: the move path, repeatedly.
+fn plant(h: &mut Harness, bar_height: f32) -> (WeakWidget<Rect>, WeakWidget<Rect>) {
+    let bar = rect(Color::RED).height(bar_height).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let row = (inner, rect(Color::GREEN)).span(Dir::RIGHT).height(100);
+    h.set_root((bar, row).span(Dir::DOWN));
+    (bar, inner)
+}
+
+/// Enough moves to pass the 0.05 physical pixels layout treats as the same
+/// place, for a move that adds an offset to the last answer. Measured on this
+/// fixture on 2026-09-15: adding the offset to both ends of a span shortened
+/// the row by 0.071 over this many moves and by 0.712 over ten times as many,
+/// growing with the count rather than settling. Placing the far end from the
+/// near one instead left 0.069, because the length is re-derived either way.
+const MOVES: usize = 20_000;
+
+#[test]
+fn a_subtree_moved_many_times_stays_where_a_cold_layout_puts_it() {
+    let mut warm = Harness::new((640, 900));
+    let (bar, inner) = plant(&mut warm, 40.0);
+    let mut height = 40.0;
+    for step in 0..MOVES {
+        height = 40.0 + (step % 300) as f32 * 0.37;
+        warm.set_len(bar, Axis::Y, height);
+        warm.frame();
+    }
+
+    let mut cold = Harness::new((640, 900));
+    let (_, cold_inner) = plant(&mut cold, height);
+    cold.frame();
+
+    assert_eq!(warm.region(&inner), cold.region(&cold_inner));
+}

tests/cases/idempotence.rs

@@ -0,0 +1,29 @@
+//! Whether measuring a widget and then giving it the length it reported is a
+//! fixed point, which is what a span that sizes to its children needs.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+#[test]
+fn a_wrapping_text_in_a_span_settles_on_one_width() {
+    let mut h = Harness::new((900, 600));
+    let words = "the quick brown fox jumps over the lazy dog and keeps on running \
+                 until it reaches the end of a rather long line of text";
+    let t = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let filler = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((t, filler).span(Dir::RIGHT));
+
+    let mut widths = Vec::new();
+    for _ in 0..6 {
+        let r = h.region(&t.id()).unwrap();
+        widths.push(r.bot_right.x - r.top_left.x);
+        // Redrawing it changes nothing about the state, so nothing may move.
+        h.rsc.widgets_mut().mark_for_redraw(t.id());
+        h.frame();
+    }
+    println!("widths over six frames: {widths:?}");
+    assert!(
+        widths.windows(2).all(|w| w[0] == w[1]),
+        "a repaint that changed nothing moved it: {widths:?}"
+    );
+}

tests/cases/plan.rs

@@ -0,0 +1,156 @@
+//! The tree a seed describes, as a value rather than as widgets.
+//!
+//! Two things have to hold for a plan to be worth having. Editing a plan has
+//! to mean what growing with those edits means, or a scenario reads one thing
+//! and the oracle another. And reducing a plan has to end, or a shrinker
+//! searching for the smallest counterexample never returns.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+use iris::random::{Edits, Kind, Plan, Rng, SpanEdit, grow, plan};
+use std::collections::HashMap;
+
+fn some_edits(seed: u64, of: &Plan) -> Edits {
+    let mut rng = Rng::new(seed);
+    let (mut sized, mut aligned, mut nodes, mut spans) = (0, 0, 0, 0);
+    let mut of = of.clone();
+    of.walk_mut(&mut |p| {
+        if matches!(p.kind, Kind::Span { .. }) {
+            spans += 1;
+        }
+        sized += p.size.is_some() as usize;
+        aligned += p.align.is_some() as usize;
+        nodes += p.region_node.is_some() as usize;
+    });
+    let pick =
+        |n: usize, rng: &mut Rng| -> Vec<usize> { (0..n).filter(|_| rng.chance()).collect() };
+    Edits {
+        sizes: pick(sized, &mut rng)
+            .into_iter()
+            .map(|i| {
+                (
+                    i,
+                    SizeRules {
+                        x: SizeRule::Exact(LayoutLen::LEFTOVER),
+                        y: SizeRule::Free,
+                    },
+                )
+            })
+            .collect(),
+        aligns: pick(aligned, &mut rng)
+            .into_iter()
+            .map(|i| {
+                (
+                    i,
+                    Align {
+                        x: Some(AxisAlign::POS),
+                        y: None,
+                    },
+                )
+            })
+            .collect(),
+        nodes: pick(nodes, &mut rng)
+            .into_iter()
+            .map(|i| (i, true))
+            .collect(),
+        spans: pick(spans, &mut rng)
+            .into_iter()
+            .map(|i| {
+                (
+                    i,
+                    SpanEdit {
+                        detach: vec![0],
+                        attach: 2,
+                    },
+                )
+            })
+            .collect::<HashMap<_, _>>(),
+        fixed_branches: false,
+    }
+}
+
+/// The two routes to an edited tree are one tree. `plan` resolves edits out
+/// of the random stream as it draws; `edited` puts them on a tree that
+/// already exists, which is the only route a shrunk plan has, since no seed
+/// grows one. A scenario written against either has to read the same.
+#[test]
+fn editing_a_plan_is_growing_one_with_those_edits() {
+    for seed in 1..=60 {
+        let bare = plan(seed, 5, &Edits::default());
+        let edits = some_edits(seed, &bare);
+        assert_eq!(
+            bare.edited(&edits),
+            plan(seed, 5, &edits),
+            "seed {seed}: edited and grown-with-edits disagree"
+        );
+    }
+}
+
+/// No simplification is larger, which is the half of "the shrinker stops" a
+/// widget count can see. Most are not smaller either -- a dropped alignment
+/// and a simpler leaf both keep the count -- so what rules out circling is
+/// that those are one-way too: a `Some` becomes a `None`, and a kind steps
+/// down a ladder with no way back up.
+#[test]
+fn no_simplification_of_a_plan_is_larger_than_it() {
+    for seed in 1..=60 {
+        let tree = plan(seed, 4, &Edits::default());
+        let mut queue = vec![tree];
+        let mut seen = 0;
+        while let Some(node) = queue.pop() {
+            seen += 1;
+            if seen > 400 {
+                break;
+            }
+            for small in node.smaller() {
+                assert!(
+                    small.size() <= node.size(),
+                    "seed {seed}: a simplification grew from {} to {}",
+                    node.size(),
+                    small.size()
+                );
+                if small.size() < node.size() {
+                    queue.push(small);
+                }
+            }
+        }
+    }
+}
+
+/// Reducing until nothing reduces ends, and ends at something small enough to
+/// read rather than at the tree it started from.
+#[test]
+fn reducing_a_plan_all_the_way_ends() {
+    for seed in 1..=30 {
+        let mut node = plan(seed, 5, &Edits::default());
+        let grown = node.size();
+        let mut steps = 0;
+        while let Some(next) = node.smaller().into_iter().next() {
+            node = next;
+            steps += 1;
+            assert!(steps < 10_000, "seed {seed}: reducing did not end");
+        }
+        assert!(
+            node.size() < grown.max(2),
+            "seed {seed}: reduced {grown} widgets to {}",
+            node.size()
+        );
+    }
+}
+
+/// Every image in a tree is the same picture, and a handle is a reference to
+/// the texture rather than a copy of it, so one upload and one slot serve all
+/// of them however many a tree grows -- and the trees are grown in hundreds.
+#[test]
+fn a_tree_of_images_uploads_one_texture() {
+    let mut images = 0;
+    let mut tree = plan(1, 4, &Edits::default());
+    tree.walk_mut(&mut |p| images += (p.kind == Kind::Image) as usize);
+    assert!(images > 1, "a tree of {images} images tests nothing");
+
+    let mut h = Harness::new((900, 1200));
+    let (root, _) = grow(&mut h.rsc, 1, 4, &Edits::default());
+    h.state.root = Some(root);
+    h.frame();
+    assert_eq!(h.rsc.ui().textures.count(), 1);
+}

tests/cases/pointer.rs

@@ -0,0 +1,85 @@
+//! Which widget an input reaches.
+
+use std::{cell::RefCell, rc::Rc};
+
+use iris::harness::{Harness, TouchScript};
+use iris::prelude::*;
+
+#[test]
+fn a_press_reaches_only_the_widget_under_the_cursor() {
+    let mut h = Harness::new((400, 200));
+    let clicks = Rc::new(RefCell::new(Vec::new()));
+
+    let (on_left, on_right) = (clicks.clone(), clicks.clone());
+    let left = rect(Color::RED)
+        .width(100)
+        .on(CursorSense::click(), move |_, _| {
+            on_left.borrow_mut().push("left")
+        })
+        .add(&mut h.rsc);
+    let right = rect(Color::BLUE)
+        .on(CursorSense::click(), move |_, _| {
+            on_right.borrow_mut().push("right")
+        })
+        .add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+
+    h.click((50, 100));
+    assert_eq!(*clicks.borrow(), ["left"]);
+
+    h.click((300, 100));
+    assert_eq!(*clicks.borrow(), ["left", "right"]);
+}
+
+#[test]
+fn hover_ends_when_the_cursor_leaves_the_window() {
+    let mut h = Harness::new((400, 200));
+    let hovered = Rc::new(RefCell::new(0));
+    let ended = Rc::new(RefCell::new(0));
+
+    let (h_count, e_count) = (hovered.clone(), ended.clone());
+    let widget = rect(Color::RED)
+        .on(CursorSense::HoverStart, move |_, _| {
+            *h_count.borrow_mut() += 1
+        })
+        .on(CursorSense::HoverEnd, move |_, _| {
+            *e_count.borrow_mut() += 1
+        })
+        .add(&mut h.rsc);
+    h.set_root(widget);
+
+    h.move_to((200, 100));
+    assert_eq!((*hovered.borrow(), *ended.borrow()), (1, 0));
+
+    // A second sample inside the same widget is not a second hover.
+    h.move_to((210, 100));
+    assert_eq!((*hovered.borrow(), *ended.borrow()), (1, 0));
+
+    h.leave();
+    assert_eq!((*hovered.borrow(), *ended.borrow()), (1, 1));
+}
+
+#[test]
+fn a_recorded_gesture_presses_where_it_says() {
+    let mut h = Harness::new((400, 200));
+    let clicks = Rc::new(RefCell::new(Vec::new()));
+
+    let (on_left, on_right) = (clicks.clone(), clicks.clone());
+    let left = rect(Color::RED)
+        .width(100)
+        .on(CursorSense::click(), move |_, _| {
+            on_left.borrow_mut().push("left")
+        })
+        .add(&mut h.rsc);
+    let right = rect(Color::BLUE)
+        .on(CursorSense::click(), move |_, _| {
+            on_right.borrow_mut().push("right")
+        })
+        .add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+
+    let script = TouchScript::parse("0 down 300 100\n80 up 300 100").unwrap();
+    h.replay(&script);
+
+    assert_eq!(*clicks.borrow(), ["right"]);
+}

tests/cases/pointer_routing.rs

@@ -0,0 +1,225 @@
+//! Input across layers: what stops at a layer, what passes through it, and
+//! where hovering stops.
+
+use std::{cell::RefCell, rc::Rc};
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+const WINDOW: f32 = 100.0;
+
+/// Every sense that has fired on one widget since it was last read.
+#[derive(Default, Clone)]
+struct Fired(Rc<RefCell<Vec<CursorSense>>>);
+
+impl Fired {
+    fn take(&self) -> Vec<CursorSense> {
+        std::mem::take(&mut self.0.borrow_mut())
+    }
+}
+
+/// A widget filling whatever it is given, recording the senses it is sent.
+fn listener(h: &mut Harness, senses: impl Into<CursorSenses>) -> (WeakWidget<Rect>, Fired) {
+    let fired = Fired::default();
+    let record = fired.clone();
+    let id = rect(Color::WHITE)
+        .on(senses.into(), move |ctx, _| {
+            record.0.borrow_mut().push(ctx.data.sense)
+        })
+        .add(&mut h.rsc);
+    (id, fired)
+}
+
+/// A widget with no senses of its own, to leave a gap beside one that has.
+fn blank(h: &mut Harness) -> WeakWidget<Rect> {
+    rect(Color::WHITE).add(&mut h.rsc)
+}
+
+fn harness() -> Harness {
+    Harness::new((WINDOW, WINDOW))
+}
+
+#[test]
+fn hover_stops_at_the_topmost_widget() {
+    let mut h = harness();
+    let (bottom, bottom_hover) = listener(&mut h, CursorSense::HoverStart);
+    let (middle, middle_hover) = listener(&mut h, CursorSense::HoverStart);
+    let (top, top_hover) = listener(&mut h, CursorSense::HoverStart);
+    h.set_root((bottom, middle, top).stack());
+
+    h.move_to((50, 50));
+
+    assert_eq!(top_hover.take(), [CursorSense::HoverStart]);
+    assert_eq!(
+        middle_hover.take(),
+        [],
+        "hover is not shared with a layer below"
+    );
+    assert_eq!(bottom_hover.take(), []);
+}
+
+#[test]
+fn a_scroll_passes_through_every_widget_that_does_not_want_it() {
+    let mut h = harness();
+    let (list, scrolled) = listener(&mut h, CursorSense::Scroll);
+    let (button, clicked) = listener(&mut h, CursorSense::click());
+    let (overlay, overlay_clicked) = listener(&mut h, CursorSense::click());
+    h.set_root((list, button, overlay).stack());
+
+    h.move_to((50, 50));
+    h.scroll((0, 10));
+
+    assert_eq!(
+        scrolled.take(),
+        [CursorSense::Scroll],
+        "two layers of click-only widgets do not stop a scroll"
+    );
+    assert_eq!(clicked.take(), []);
+    assert_eq!(overlay_clicked.take(), []);
+}
+
+#[test]
+fn hovering_a_button_above_does_not_stop_a_later_scroll() {
+    let mut h = harness();
+    let (list, scrolled) = listener(&mut h, CursorSense::Scroll);
+    let (button, _clicked) = listener(&mut h, CursorSense::click());
+    h.set_root((list, button).stack());
+
+    // The hover arrives in its own frame, as a window delivers it.
+    h.move_to((50, 50));
+    assert_eq!(scrolled.take(), []);
+
+    h.scroll((0, 10));
+    assert_eq!(
+        scrolled.take(),
+        [CursorSense::Scroll],
+        "a hover already resting on the button must not consume the wheel"
+    );
+}
+
+#[test]
+fn only_the_topmost_listener_takes_a_press() {
+    let mut h = harness();
+    let (below, below_clicked) = listener(&mut h, CursorSense::click());
+    let (above, above_clicked) = listener(&mut h, CursorSense::click());
+    h.set_root((below, above).stack());
+
+    h.click((50, 50));
+
+    assert_eq!(above_clicked.take(), [CursorSense::click()]);
+    assert_eq!(below_clicked.take(), [], "one press goes to one widget");
+}
+
+#[test]
+fn a_press_beside_the_button_reaches_the_layer_below() {
+    let mut h = harness();
+    let (list, list_clicked) = listener(&mut h, CursorSense::click());
+    // The row above the list covers it, but only its left half is the button.
+    let (button, button_clicked) = listener(&mut h, CursorSense::click());
+    let row = (button, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((list, row).stack());
+
+    h.click((20, 50));
+    assert_eq!(button_clicked.take(), [CursorSense::click()]);
+    assert_eq!(list_clicked.take(), []);
+
+    h.click((80, 50));
+    assert_eq!(button_clicked.take(), [], "the cursor is not on the button");
+    assert_eq!(
+        list_clicked.take(),
+        [CursorSense::click()],
+        "a press beside the button belongs to what is under it"
+    );
+}
+
+#[test]
+fn leaving_a_widget_still_ends_its_hover() {
+    let mut h = harness();
+    let (widget, hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    h.set_root(widget);
+
+    h.move_to((50, 50));
+    assert_eq!(hover.take(), [CursorSense::HoverStart]);
+
+    h.leave();
+    assert_eq!(hover.take(), [CursorSense::HoverEnd]);
+}
+
+#[test]
+fn leaving_a_widget_does_not_block_the_layer_below() {
+    let mut h = harness();
+    let (below, below_hover) = listener(&mut h, CursorSense::HoverStart);
+    // Only the left half of the layer above is a widget, so the cursor can
+    // leave it without leaving the one underneath.
+    let (above, above_hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    let row = (above, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((below, row).stack());
+
+    h.move_to((20, 50));
+    assert_eq!(above_hover.take(), [CursorSense::HoverStart]);
+    assert_eq!(below_hover.take(), [], "the layer above is over it");
+
+    h.move_to((80, 50));
+    assert_eq!(above_hover.take(), [CursorSense::HoverEnd]);
+    assert_eq!(
+        below_hover.take(),
+        [CursorSense::HoverStart],
+        "ending a hover above must not stop the hover below"
+    );
+}
+
+#[test]
+fn covering_a_widget_ends_its_hover() {
+    let mut h = harness();
+    let (below, below_hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    let (above, above_hover) = listener(&mut h, CursorSense::HoverStart);
+    let row = (above, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((below, row).stack());
+
+    h.move_to((80, 50));
+    assert_eq!(below_hover.take(), [CursorSense::HoverStart]);
+
+    h.move_to((20, 50));
+    assert_eq!(above_hover.take(), [CursorSense::HoverStart]);
+    assert_eq!(
+        below_hover.take(),
+        [CursorSense::HoverEnd],
+        "a widget covered by one that took the input is no longer hovered"
+    );
+
+    h.move_to((80, 50));
+    assert_eq!(
+        below_hover.take(),
+        [CursorSense::HoverStart],
+        "uncovering it hovers it again"
+    );
+}
+
+#[test]
+fn hover_starts_and_ends_once_each() {
+    let mut h = harness();
+    // Only the left half is the widget, so the cursor can leave it without
+    // leaving the window.
+    let (widget, hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    let row = (widget, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root(row);
+
+    h.move_to((20, 50));
+    assert_eq!(hover.take(), [CursorSense::HoverStart]);
+
+    h.move_to((30, 50));
+    assert_eq!(hover.take(), [], "staying inside is not a second start");
+
+    h.move_to((80, 50));
+    assert_eq!(hover.take(), [CursorSense::HoverEnd]);
+
+    h.move_to((90, 50));
+    assert_eq!(hover.take(), [], "an ended hover does not end again");
+
+    h.move_to((20, 50));
+    assert_eq!(
+        hover.take(),
+        [CursorSense::HoverStart],
+        "re-entering starts it"
+    );
+}

tests/cases/scroll.rs

@@ -0,0 +1,163 @@
+//! Scrolling moves content and stops at its ends.
+
+use iris::harness::{Harness, assert_corners};
+use iris::prelude::*;
+
+#[test]
+fn scrollable_enables_a_region_node_but_raw_scroll_does_not() {
+    let mut h = Harness::new((100, 100));
+    let default_child = ().add(&mut h.rsc);
+    let _default = default_child.scrollable().add(&mut h.rsc);
+    assert!(h.rsc.widgets().is_region_node(default_child));
+    h.rsc.widgets_mut().set_region_node(default_child, false);
+    assert!(!h.rsc.widgets().is_region_node(default_child));
+
+    let raw_child = ().add(&mut h.rsc);
+    let _raw = Scroll::new(raw_child.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    assert!(!h.rsc.widgets().is_region_node(raw_child));
+
+    let explicit = ().region_node().add(&mut h.rsc);
+    assert!(h.rsc.widgets().is_region_node(explicit));
+}
+
+#[test]
+fn a_scrollable_child_can_drop_its_region_node() {
+    let mut h = Harness::new((400, 200));
+    let top = rect(Color::RED).height(200).add(&mut h.rsc);
+    let bottom = rect(Color::BLUE).height(200).add(&mut h.rsc);
+    let content = (top, bottom).span(Dir::DOWN).add(&mut h.rsc);
+    h.set_root(content.scrollable());
+    h.rsc.widgets_mut().set_region_node(content, false);
+    h.frame();
+
+    h.move_to((200, 100));
+    h.scroll((0, 1));
+    h.frame();
+
+    assert!(!h.rsc.widgets().is_region_node(content));
+    assert_corners!(h, top, (0, -150), (400, 50));
+}
+
+#[test]
+fn a_wheel_scrolls_the_content_and_stops_at_its_end() {
+    let mut h = Harness::new((400, 200));
+    // Twice the window's height, so there is 200 to scroll.
+    let top = rect(Color::RED).height(200).add(&mut h.rsc);
+    let bottom = rect(Color::BLUE).height(200).add(&mut h.rsc);
+    h.set_root((top, bottom).span(Dir::DOWN).scrollable());
+    h.move_to((200, 100));
+
+    // `Scroll` starts snapped to the end.
+    assert_corners!(h, top, (0, -200), (400, 0));
+
+    // The handler scales a wheel line by 50.
+    h.scroll((0, 1));
+    h.frame();
+    assert_corners!(h, top, (0, -150), (400, 50));
+
+    h.scroll((0, 10));
+    h.frame();
+    assert_corners!(h, top, (0, 0), (400, 200));
+}
+
+#[test]
+fn fixed_content_and_a_share_fill_one_viewport() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED)
+        .width(LayoutLen {
+            px: Px::from_int(600),
+            rel: Rel::ZERO,
+            leftover: Weight::ONE,
+        })
+        .add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (0, 0), (900, 100));
+}
+
+#[test]
+fn fixed_content_wider_than_the_viewport_still_scrolls() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED).width(1200).add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (-300, 0), (900, 100));
+}
+
+#[test]
+fn a_lone_share_fills_without_scrolling() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED).width(LayoutLen::LEFTOVER).add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (0, 0), (900, 100));
+}
+
+#[test]
+fn wrapping_content_beside_a_fixed_length_is_stable_warm_and_cold() {
+    fn plant(h: &mut Harness) -> (WidgetId, WidgetId) {
+        let fixed = rect(Color::RED).width(600).add(&mut h.rsc);
+        let text = wtext("Wrapping shapes one source into as many lines as the box leaves room for, so a paragraph's height is an answer and not a setting.")
+            .size(16)
+            .wrap(true)
+            .width(LayoutLen::LEFTOVER)
+            .add(&mut h.rsc);
+        let content = (fixed, text).span(Dir::RIGHT).add(&mut h.rsc);
+        let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+        h.set_root(scroll);
+        (text.id(), content.id())
+    }
+
+    let mut warm = Harness::new((900, 300));
+    let (text, content) = plant(&mut warm);
+    warm.rsc.widgets_mut().mark_for_redraw(text);
+    warm.frame();
+
+    let mut cold = Harness::new((900, 300));
+    let (cold_text, cold_content) = plant(&mut cold);
+
+    assert_eq!(warm.region(&text), cold.region(&cold_text));
+    assert_eq!(warm.region(&content), cold.region(&cold_content));
+}
+
+/// A widget that clips to its box may not report more than the box: its
+/// parent would place the part it cut off, and the framework would put a
+/// drawing longer than its box somewhere. `Masked` is the second of these
+/// after `Scroll`, and the assertion in `draw_at` is what says so.
+#[test]
+#[should_panic = "clips to"]
+fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
+    struct Clipper(StrongWidget);
+
+    impl Widget for Clipper {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.set_mask(UiRegion::FULL);
+            painter.widget(&self.0).size()
+        }
+    }
+
+    let mut h = Harness::new((100, 100));
+    let tall = rect(Color::RED).height(400).add_strong(&mut h.rsc);
+    let clipper = Clipper(tall).add(&mut h.rsc);
+    // `set_root` lays the tree out, so this is where it is caught.
+    h.set_root(clipper);
+}
+
+/// Content that fits sits in the viewport, not in a box of the window's
+/// length anchored at the viewport's start. `Part::From` takes window
+/// lengths, so a `rel(1.0)` span in one is the window, and only a scroll
+/// filling the window would land right.
+#[test]
+fn content_that_fits_is_placed_in_the_viewport_and_not_in_the_window() {
+    let mut h = Harness::new((400, 400));
+    let head = rect(Color::RED).height(100).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).height(50).add(&mut h.rsc);
+    let scroll = Scroll::new(inner.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    h.set_root((head, scroll).span(Dir::DOWN));
+
+    assert_corners!(h, scroll, (0, 100), (400, 400));
+    assert_corners!(h, inner, (0, 225), (400, 275));
+}

tests/cases/tasks.rs

@@ -0,0 +1,23 @@
+//! What a background task can change, and how it gets back to the ui.
+
+use std::time::Duration;
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+#[test]
+fn a_task_update_reaches_the_tree() {
+    let mut h = Harness::new((400, 200));
+    let widget = rect(Color::RED).add(&mut h.rsc);
+    h.set_root(widget.task_on(CursorSense::click(), async move |mut ctx| {
+        ctx.update(move |_, rsc| widget(rsc).color = Color::BLUE);
+    }));
+
+    h.click((200, 100));
+
+    assert!(
+        h.await_update(Duration::from_secs(5)),
+        "the task sent no update"
+    );
+    assert_eq!(h.rsc[widget].color, Color::BLUE);
+}

tests/chain_cost.rs

@@ -0,0 +1,199 @@
+//! What the vertex shader's move-chain walk costs, against how many nested
+//! region nodes a primitive resolves through.
+//!
+//!     cargo test --release --test chain_cost -- --ignored --nocapture
+//!
+//! Timed on the GPU with timestamp queries rather than by the clock: wall time
+//! here varied by 2x between runs of one unchanged binary. The pass is
+//! submitted and waited on, so this is the GPU's cost and not the recording
+//! loop's -- which is what `draw_cost.rs` measures instead.
+//!
+//! The instances are two pixels wide so that vertex work dominates; a chain
+//! walk that does not show up against small quads will not show up against
+//! anything.
+
+use iris::prelude::*;
+use iris_core::{
+    Len, MaskIdx, MoveIdx, PrimitiveInst, RectPrimitive, UiData, UiRegion, UiRenderNode,
+    UiRenderState, UiSpan,
+};
+use wgpu::{Color as GpuColor, *};
+
+#[path = "gpu/mod.rs"]
+mod gpu;
+
+const SIZE: u32 = 1024;
+const INSTANCES: usize = 200_000;
+const FRAMES: u32 = 20;
+/// Reported as the best of this many batches, since the mean moves by more
+/// than the thing being measured.
+const BATCHES: u32 = 8;
+
+fn gpu() -> Option<(Device, Queue, f32)> {
+    let adapter = gpu::adapter()?;
+    if !adapter.features().contains(Features::TIMESTAMP_QUERY) {
+        println!("no timestamp queries on {:?}", adapter.get_info().name);
+        return None;
+    }
+    println!("adapter: {:?}", adapter.get_info().name);
+    let (device, queue) = pollster::block_on(adapter.request_device(&DeviceDescriptor {
+        required_features: Features::TIMESTAMP_QUERY,
+        ..Default::default()
+    }))
+    .ok()?;
+    let period = queue.get_timestamp_period();
+    Some((device, queue, period))
+}
+
+/// A chain `depth` slots long, and instances that all resolve through its end.
+fn fill(ui: &mut UiData, render: &mut UiRenderState, depth: usize) {
+    let kind = ui.primitives.kind::<RectPrimitive>();
+    let id = ui.widgets.add_strong(Rect::new(UiColor::WHITE)).id();
+
+    let mut slot = MoveIdx::NONE;
+    for _ in 0..depth {
+        slot = render.moves.push(slot, UiRegion::FULL);
+    }
+
+    let px = |v: f32| Len::px(v);
+    for i in 0..INSTANCES {
+        let x = (i % (SIZE as usize / 2)) as f32 * 2.0;
+        let y = (i / (SIZE as usize / 2)) as f32;
+        render.layers.write(
+            0,
+            PrimitiveInst {
+                kind,
+                id,
+                primitive: RectPrimitive::color(UiColor::WHITE),
+                region: UiRegion::new(
+                    UiSpan::new(px(x), px(x + 2.0)),
+                    UiSpan::new(px(y), px(y + 1.0)),
+                ),
+                mask_idx: MaskIdx::NONE,
+                move_idx: slot,
+            },
+        );
+    }
+}
+
+/// Nanoseconds the pass took on the GPU, best of `BATCHES`.
+fn pass_cost(device: &Device, queue: &Queue, period: f32, depth: usize) -> f64 {
+    let format = TextureFormat::Bgra8Unorm;
+    let mut node = UiRenderNode::new(device, &gpu::config(format, SIZE));
+    let mut ui = UiData::default();
+    let mut render = UiRenderState::new();
+    fill(&mut ui, &mut render, depth);
+    node.update(device, queue, &mut ui, &mut render);
+
+    let target = device.create_texture(&TextureDescriptor {
+        label: Some("chain cost"),
+        size: Extent3d {
+            width: SIZE,
+            height: SIZE,
+            depth_or_array_layers: 1,
+        },
+        mip_level_count: 1,
+        sample_count: 1,
+        dimension: TextureDimension::D2,
+        format,
+        usage: TextureUsages::RENDER_ATTACHMENT,
+        view_formats: &[],
+    });
+    let view = target.create_view(&TextureViewDescriptor::default());
+
+    let queries = device.create_query_set(&QuerySetDescriptor {
+        label: Some("chain cost"),
+        ty: QueryType::Timestamp,
+        count: 2,
+    });
+    let resolved = device.create_buffer(&BufferDescriptor {
+        label: Some("resolved"),
+        size: 16,
+        usage: BufferUsages::QUERY_RESOLVE | BufferUsages::COPY_SRC,
+        mapped_at_creation: false,
+    });
+    let readback = device.create_buffer(&BufferDescriptor {
+        label: Some("readback"),
+        size: 16,
+        usage: BufferUsages::MAP_READ | BufferUsages::COPY_DST,
+        mapped_at_creation: false,
+    });
+
+    let frame = || {
+        let mut encoder = device.create_command_encoder(&CommandEncoderDescriptor::default());
+        {
+            let pass = &mut encoder.begin_render_pass(&RenderPassDescriptor {
+                label: None,
+                color_attachments: &[Some(RenderPassColorAttachment {
+                    view: &view,
+                    resolve_target: None,
+                    ops: Operations {
+                        load: LoadOp::Clear(GpuColor::BLACK),
+                        store: StoreOp::Store,
+                    },
+                    depth_slice: None,
+                })],
+                depth_stencil_attachment: None,
+                timestamp_writes: Some(RenderPassTimestampWrites {
+                    query_set: &queries,
+                    beginning_of_pass_write_index: Some(0),
+                    end_of_pass_write_index: Some(1),
+                }),
+                occlusion_query_set: None,
+                multiview_mask: None,
+            });
+            node.draw(pass);
+        }
+        encoder.resolve_query_set(&queries, 0..2, &resolved, 0);
+        encoder.copy_buffer_to_buffer(&resolved, 0, &readback, 0, 16);
+        queue.submit(Some(encoder.finish()));
+
+        let slice = readback.slice(..);
+        slice.map_async(MapMode::Read, |_| {});
+        let _ = device.poll(PollType::Wait {
+            submission_index: None,
+            timeout: None,
+        });
+        let ns = {
+            let view = slice.get_mapped_range().expect("timestamps did not map");
+            let stamps: [u64; 2] = [
+                u64::from_le_bytes(view[..8].try_into().unwrap()),
+                u64::from_le_bytes(view[8..16].try_into().unwrap()),
+            ];
+            (stamps[1].saturating_sub(stamps[0])) as f64 * period as f64
+        };
+        readback.unmap();
+        ns
+    };
+
+    frame();
+    let mut best = f64::MAX;
+    for _ in 0..BATCHES {
+        let mut total = 0.0;
+        for _ in 0..FRAMES {
+            total += frame();
+        }
+        best = best.min(total / FRAMES as f64);
+    }
+    best
+}
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn chain_cost_by_depth() {
+    let Some((device, queue, period)) = gpu() else {
+        println!("no gpu with timestamps; nothing measured");
+        return;
+    };
+    println!("{INSTANCES} instances, {SIZE}x{SIZE}, best of {BATCHES} batches");
+    let mut base = None;
+    for depth in [1, 2, 4, 8, 16, 32, 64] {
+        let ns = pass_cost(&device, &queue, period, depth);
+        let base = *base.get_or_insert(ns);
+        println!(
+            "depth {depth:>3}: {:>9.1} us   {:+6.1}% against depth 1",
+            ns / 1000.0,
+            (ns - base) / base * 100.0
+        );
+    }
+}

tests/draw_cost.rs

@@ -0,0 +1,179 @@
+//! What one frame of `UiRenderNode::draw` costs on the CPU, against the number
+//! of layers it walks. Recording only: the pass is built and dropped without
+//! being submitted, so this is the loop's cost and not the GPU's.
+//!
+//!     cargo test --release --test draw_cost -- --ignored --nocapture
+//!
+//! Wall time is the wrong number to read for anything under a few percent --
+//! it varied by 2x between runs of one unchanged binary where instructions
+//! retired varied by 0.1%. Count those instead:
+//!
+//!     perf stat -e instructions:u target/release/.../draw_cost-* --ignored
+//!
+//! That is how `PrimitiveRender` was measured against a match in the renderer:
+//! 6 instructions per list drawn, against the ~5,400 wgpu spends recording
+//! one.
+
+use std::time::Instant;
+
+use iris::prelude::*;
+use iris_core::{
+    GlyphPrimitive, MaskIdx, MoveIdx, PrimitiveInst, RectPrimitive, TextureHandle,
+    TexturePrimitive, UiData, UiRegion, UiRenderNode, UiRenderState,
+};
+use wgpu::{Color as GpuColor, *};
+
+#[path = "gpu/mod.rs"]
+mod gpu;
+
+const SIZE: u32 = 1024;
+const FRAMES: u32 = 200;
+/// Reported as the best of this many batches, since the mean moves by more
+/// than the thing being measured.
+const BATCHES: u32 = 8;
+
+fn gpu() -> Option<(Device, Queue)> {
+    let adapter = gpu::adapter()?;
+    println!("adapter: {:?}", adapter.get_info());
+    pollster::block_on(adapter.request_device(&DeviceDescriptor::default())).ok()
+}
+
+/// Every layer draws all three primitives, so the renderer takes a different
+/// path for each list it walks -- which is the case a single-primitive layer
+/// would never exercise. Images are bound per instance, so there are few.
+fn fill(
+    ui: &mut UiData,
+    render: &mut UiRenderState,
+    layers: usize,
+    per_layer: usize,
+) -> Vec<TextureHandle> {
+    let rect = ui.primitives.kind::<RectPrimitive>();
+    let glyph = ui.primitives.kind::<GlyphPrimitive>();
+    let texture = ui.primitives.kind::<TexturePrimitive>();
+    let id = ui.widgets.add_strong(Rect::new(UiColor::WHITE)).id();
+    let handles: Vec<_> = (0..4)
+        .map(|_| ui.textures.add(image::RgbaImage::new(4, 4)))
+        .collect();
+
+    let mut layer = 0;
+    for _ in 0..layers {
+        for _ in 0..per_layer {
+            render.layers.write(
+                layer,
+                PrimitiveInst {
+                    kind: rect,
+                    id,
+                    primitive: RectPrimitive::color(UiColor::WHITE),
+                    region: UiRegion::FULL,
+                    mask_idx: MaskIdx::NONE,
+                    move_idx: MoveIdx::NONE,
+                },
+            );
+            render.layers.write(
+                layer,
+                PrimitiveInst {
+                    kind: glyph,
+                    id,
+                    primitive: GlyphPrimitive {
+                        uv_min: vec2(0.0, 0.0),
+                        uv_max: vec2(1.0, 1.0),
+                        layer: 0,
+                        color: UiColor::WHITE,
+                        flags: 0,
+                    },
+                    region: UiRegion::FULL,
+                    mask_idx: MaskIdx::NONE,
+                    move_idx: MoveIdx::NONE,
+                },
+            );
+        }
+        for h in &handles[..2] {
+            render.layers.write(
+                layer,
+                PrimitiveInst {
+                    kind: texture,
+                    id,
+                    primitive: TexturePrimitive::from(h),
+                    region: UiRegion::FULL,
+                    mask_idx: MaskIdx::NONE,
+                    move_idx: MoveIdx::NONE,
+                },
+            );
+        }
+        layer = render.layers.next(layer);
+    }
+    handles
+}
+
+fn frame_cost(device: &Device, queue: &Queue, layers: usize, per_layer: usize) -> f64 {
+    let format = TextureFormat::Bgra8Unorm;
+    let mut node = UiRenderNode::new(device, &gpu::config(format, SIZE));
+    let mut ui = UiData::default();
+    let mut render = UiRenderState::new();
+    let _handles = fill(&mut ui, &mut render, layers, per_layer);
+    node.update(device, queue, &mut ui, &mut render);
+
+    let target = device.create_texture(&TextureDescriptor {
+        label: Some("draw cost"),
+        size: Extent3d {
+            width: SIZE,
+            height: SIZE,
+            depth_or_array_layers: 1,
+        },
+        mip_level_count: 1,
+        sample_count: 1,
+        dimension: TextureDimension::D2,
+        format,
+        usage: TextureUsages::RENDER_ATTACHMENT,
+        view_formats: &[],
+    });
+    let view = target.create_view(&TextureViewDescriptor::default());
+
+    let record = |frames: u32| {
+        let start = Instant::now();
+        for _ in 0..frames {
+            let mut encoder = device.create_command_encoder(&CommandEncoderDescriptor::default());
+            {
+                let pass = &mut encoder.begin_render_pass(&RenderPassDescriptor {
+                    color_attachments: &[Some(RenderPassColorAttachment {
+                        view: &view,
+                        resolve_target: None,
+                        ops: Operations {
+                            load: LoadOp::Clear(GpuColor::BLACK),
+                            store: StoreOp::Store,
+                        },
+                        depth_slice: None,
+                    })],
+                    ..Default::default()
+                });
+                node.draw(pass);
+            }
+            drop(encoder.finish());
+        }
+        start.elapsed().as_secs_f64() / frames as f64
+    };
+    record(FRAMES / 4);
+    (0..BATCHES)
+        .map(|_| record(FRAMES))
+        .fold(f64::MAX, f64::min)
+}
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn draw_cost_by_layer_count() {
+    let Some((device, queue)) = gpu() else {
+        panic!("no wgpu device; see the this-machine-graphics notes");
+    };
+    println!(
+        "layers, each 8 rects + 8 glyphs + 2 images: us/frame (us per layer), best of {BATCHES}"
+    );
+    let base = frame_cost(&device, &queue, 1, 8) * 1e6;
+    for layers in [8, 64, 256, 1024] {
+        let per_frame = frame_cost(&device, &queue, layers, 8) * 1e6;
+        // Net of the empty pass, which is the same in any version of this.
+        println!(
+            "{layers:>5}: {per_frame:8.1} us ({:.3} us)",
+            (per_frame - base).max(0.0) / layers as f64
+        );
+    }
+}