Make a declared length one that cannot carry a share

`declared_lens` filtered `leftover` out of both its sources and every
consumer then re-dropped it, so the rule lived in two filters and a comment.
A declaration is a `Len`: `LayoutLen::declared` states the rule once and both
sources go through it, and `Declared` replaces the bare two-element array on
`ActiveData` and in four signatures.

The two sources stay one value deliberately. A rule decides the child's box;
a hint only promises what it will report -- but `size_hint` is by contract an
exact answer with no painter context, and `hints_agree` fails a widget that
draws something else, so narrowing the box to a hint cannot change what is
drawn. Every consumer asks about the length, never which said it.

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

Cargo.lock

@@ -567,9 +567,9 @@ dependencies = [
 
 [[package]]
 name = "dlib"
-version = "0.5.2"
+version = "0.5.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "330c60081dcc4c72131f8eb70510f1ac07223e5d4163db481a04a0befcffa412"
+checksum = "ab8ecd87370524b461f8557c119c405552c396ed91fc0a8eec68679eab26f94a"
 dependencies = [
  "libloading",
 ]
@@ -2158,9 +2158,9 @@ checksum = "a993555f31e5a609f617c12db6250dedcac1b0a85076912c436e6fc9b2c8e6a3"
 
 [[package]]
 name = "quick-xml"
-version = "0.38.4"
+version = "0.41.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b66c2058c55a409d601666cffe35f04333cf1013010882cec174a7467cd4e21c"
+checksum = "e660451e55124f798a69a5af3f49ccfbefbd41910eefd25caf2393e1f3473ec1"
 dependencies = [
  "memchr",
 ]
@@ -2353,6 +2353,15 @@ version = "0.8.52"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0c6a884d2998352bb4daf0183589aec883f16a6da1f4dde84d8e2e9a5409a1ce"
 
+[[package]]
+name = "rig-input"
+version = "0.1.0"
+dependencies = [
+ "iris",
+ "wayland-client",
+ "wayland-protocols-wlr",
+]
+
 [[package]]
 name = "roxmltree"
 version = "0.21.1"
@@ -2909,9 +2918,9 @@ dependencies = [
 
 [[package]]
 name = "wayland-backend"
-version = "0.3.12"
+version = "0.3.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fee64194ccd96bf648f42a65a7e589547096dfa702f7cadef84347b66ad164f9"
+checksum = "38a91b4eaddff87b1cd1074985e3713da4af2c49742d1b356b2c01670a67a078"
 dependencies = [
  "cc",
  "downcast-rs",
@@ -2923,9 +2932,9 @@ dependencies = [
 
 [[package]]
 name = "wayland-client"
-version = "0.31.12"
+version = "0.31.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b8e6faa537fbb6c186cb9f1d41f2f811a4120d1b57ec61f50da451a0c5122bec"
+checksum = "e3c36a0f861ad76d0901f2800b46321410d9f73f2ea88aac0650d86c32688073"
 dependencies = [
  "bitflags 2.10.0",
  "rustix 1.1.3",
@@ -2957,9 +2966,9 @@ dependencies = [
 
 [[package]]
 name = "wayland-protocols"
-version = "0.32.10"
+version = "0.32.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "baeda9ffbcfc8cd6ddaade385eaf2393bd2115a69523c735f12242353c3df4f3"
+checksum = "23d0c813de3daa2ed6520af85a3bd49b0e722a3078506899aa9686fea58dc4b6"
 dependencies = [
  "bitflags 2.10.0",
  "wayland-backend",
@@ -2982,9 +2991,9 @@ dependencies = [
 
 [[package]]
 name = "wayland-protocols-wlr"
-version = "0.3.10"
+version = "0.3.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e9597cdf02cf0c34cd5823786dce6b5ae8598f05c2daf5621b6e178d4f7345f3"
+checksum = "eb04e52f7836d7c7976c78ca0250d61e33873c34156a2a1fc9474828ec268234"
 dependencies = [
  "bitflags 2.10.0",
  "wayland-backend",
@@ -2995,9 +3004,9 @@ dependencies = [
 
 [[package]]
 name = "wayland-scanner"
-version = "0.31.8"
+version = "0.31.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5423e94b6a63e68e439803a3e153a9252d5ead12fd853334e2ad33997e3889e3"
+checksum = "338e30461b3a2b67d70eb30a6d89f8e0c93a833e07d2ae89085cd070c4a00ac0"
 dependencies = [
  "proc-macro2",
  "quick-xml",
@@ -3006,9 +3015,9 @@ dependencies = [
 
 [[package]]
 name = "wayland-sys"
-version = "0.31.8"
+version = "0.31.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1e6dbfc3ac5ef974c92a2235805cc0114033018ae1290a72e474aa8b28cbbdfd"
+checksum = "d8eab23fefc9e41f8e841df4a9c707e8a8c4ed26e944ef69297184de2785e3be"
 dependencies = [
  "dlib",
  "log",

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 +23,13 @@ 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"]
+
+[profile.dev]
+debug = 1
+
+[profile.test]
+debug = "line-tables-only"
 
 [workspace.package]
 version = "0.1.0"
@@ -40,3 +49,5 @@ 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 +14,13 @@ 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

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/fixed.rs

@@ -0,0 +1,574 @@
+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
+    }
+
+    /// The same value on another grid, rounded where the new one is coarser.
+    pub const fn to_scale<const TO: u32>(self) -> Fixed<TO> {
+        Fixed(match TO >= SHIFT {
+            true => self.0 << (TO - SHIFT),
+            false => shift_round(self.0 as i64, SHIFT - TO) as i32,
+        })
+    }
+
+    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))
+    }
+}
+
+/// Back to a single step, rounding halves away from zero so that a value and
+/// its negation round to the same distance.
+const fn shift_round(v: i64, bits: u32) -> i64 {
+    let half = (1i64 << bits) >> 1;
+    match v < 0 {
+        true => -((-v + half) >> bits),
+        false => (v + half) >> bits,
+    }
+}
+
+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 a_coarser_grid_rounds_and_a_finer_one_does_not() {
+        // A third, which neither grid holds exactly.
+        let third = Rel::ONE / Rel::from_int(3);
+        assert_eq!(third.to_scale::<6>(), Fixed::<6>::from_raw(21));
+        let coarse = Fixed::<6>::from_raw(21);
+        assert_eq!(coarse.to_scale::<24>().to_scale::<6>(), coarse);
+    }
+
+    #[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 +1,485 @@
+//! 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, LayoutLen, PxVec2, Size, UiRegion, WidgetId};
+use std::{
+    cell::RefCell,
+    collections::{HashMap, HashSet},
+    fmt::Write,
+    time::Instant,
+};
+
+#[derive(Clone, Copy)]
+pub(crate) enum Counter {
+    Updates,
+    DrawRequests,
+    WidgetDraws,
+    RegionNodeDraws,
+    SizeReads,
+    HintHits,
+    HintMisses,
+    ReuseAttempts,
+    ReuseExact,
+    ReuseMoved,
+    ReuseDirty,
+    ReuseWrongParent,
+    ReuseRemapped,
+    ReuseOutside,
+    ReuseWrongLayer,
+    ReuseWrongNode,
+    QueuePops,
+    DepthReads,
+    LocalRedraws,
+    SizeChanges,
+    ReaderEdges,
+    PrimitiveWrites,
+    TextRenders,
+    TextShapeHits,
+    TextShapes,
+    TextBreaks,
+    GlyphPlacements,
+    OutsidePinnedLen,
+    OutsideRelBase,
+    OutsideRegion,
+}
+
+impl Counter {
+    const COUNT: usize = Self::OutsideRegion as usize + 1;
+
+    const NAMES: [&'static str; Self::COUNT] = [
+        "updates",
+        "draw requests",
+        "widget draws",
+        "region-node draws",
+        "draw-result size reads",
+        "hint hits",
+        "hint misses",
+        "reuse attempts",
+        "reuse exact",
+        "reuse moved",
+        "reuse: dirty",
+        "reuse: wrong parent",
+        "reuse remapped",
+        "reuse: outside what it holds for",
+        "reuse: another layer",
+        "reuse: region-node choice changed",
+        "redraw queue pops",
+        "depth reads",
+        "local redraws",
+        "size changes",
+        "reader edges",
+        "primitive writes",
+        "text renders",
+        "text shape hits",
+        "text shapes",
+        "text line breaks",
+        "glyph placements",
+        "reuse outside: the length it was pinned to",
+        "reuse outside: a rel base",
+        "reuse outside: a region length",
+    ];
+}
+
+#[derive(Clone, Copy)]
+pub(crate) enum TimerKind {
+    Update,
+    FullLayout,
+    IncrementalLayout,
+    TextRender,
+    TextShape,
+    TextBreak,
+    GlyphPlacement,
+}
+
+impl TimerKind {
+    const COUNT: usize = Self::GlyphPlacement as usize + 1;
+
+    const NAMES: [&'static str; Self::COUNT] = [
+        "update total",
+        "full layout",
+        "incremental layout",
+        "text render",
+        "text shape",
+        "text line break",
+        "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,
+    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,
+        pixel_size: 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,
+    pixel_size: PxVec2,
+    region_node: bool,
+) {
+    trace(
+        id,
+        TraceEvent::DrawRequest {
+            id,
+            parent,
+            region,
+            pixel_size,
+            region_node,
+        },
+    );
+}
+
+pub(crate) fn reuse(id: WidgetId, outcome: ReuseOutcome) {
+    trace(id, TraceEvent::Reuse { id, outcome });
+}
+
+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(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)]
-pub enum AxisAlign {
-    Neg,
-    Center,
-    Pos,
-}
+/// Where a widget sits in a box longer than it is. The default is the middle,
+/// because the two edges are the ones that assume a direction: which of them
+/// is the near one depends on the writing system and on which way a container
+/// runs, and the middle is the same either way.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct AxisAlign(Rel);
 
 impl AxisAlign {
-    pub const fn rel(&self) -> f32 {
-        match self {
-            Self::Neg => 0.0,
-            Self::Center => 0.5,
-            Self::Pos => 1.0,
+    pub const NEG: Self = Self::new(0.0);
+    pub const CENTER: Self = Self::new(0.5);
+    pub const POS: Self = Self::new(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 +66,46 @@ pub struct CardinalAlign {
 }
 
 impl CardinalAlign {
-    pub const LEFT: Self = Self::new(Axis::X, AxisAlign::Neg);
-    pub const H_CENTER: Self = Self::new(Axis::X, AxisAlign::Center);
-    pub const RIGHT: Self = Self::new(Axis::X, AxisAlign::Pos);
-    pub const TOP: Self = Self::new(Axis::Y, AxisAlign::Neg);
-    pub const V_CENTER: Self = Self::new(Axis::Y, AxisAlign::Center);
-    pub const BOT: Self = Self::new(Axis::Y, AxisAlign::Pos);
+    pub const LEFT: Self = Self::new(Axis::X, AxisAlign::NEG);
+    pub const H_CENTER: Self = Self::new(Axis::X, AxisAlign::CENTER);
+    pub const RIGHT: Self = Self::new(Axis::X, AxisAlign::POS);
+    pub const TOP: Self = Self::new(Axis::Y, AxisAlign::NEG);
+    pub const V_CENTER: Self = Self::new(Axis::Y, AxisAlign::CENTER);
+    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_CENTER: Self = Self::new(AxisAlign::Center, 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: Self = Self::new(AxisAlign::Center, 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_CENTER: Self = Self::new(AxisAlign::Center, AxisAlign::Pos);
-    pub const BOT_RIGHT: Self = Self::new(AxisAlign::Pos, AxisAlign::Pos);
+    /// Both axes at the near edge: the start of a box in its own orientation.
+    pub const NEAR: Self = Self {
+        x: AxisAlign::NEG,
+        y: AxisAlign::NEG,
+    };
+}
+
+impl RegionAlign {
+    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_RIGHT: Self = Self::new(AxisAlign::POS, AxisAlign::NEG);
+    pub const CENTER_LEFT: Self = Self::new(AxisAlign::NEG, 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 BOT_LEFT: Self = Self::new(AxisAlign::NEG, 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 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 +158,15 @@ impl Vec2 {
     }
 }
 
-impl UiScalar {
+impl Len {
     pub const fn align(&self, align: AxisAlign) -> UiSpan {
         let rel = align.rel();
-        let mut start = UiScalar::rel(rel);
-        start.abs -= self.abs * rel;
-        start.rel -= self.rel * rel;
-        let mut end = UiScalar::rel(rel);
-        end.abs += self.abs * (1.0 - rel);
-        end.rel += self.rel * (1.0 - rel);
-        UiSpan { start, end }
+        let rest = Rel::ONE.sub(rel);
+        let at = Len::from_parts(rel, Px::ZERO);
+        UiSpan {
+            start: Len::from_parts(at.rel.sub(self.rel.mul(rel)), at.px.sub(self.px.mul(rel))),
+            end: Len::from_parts(at.rel.add(self.rel.mul(rest)), at.px.add(self.px.mul(rest))),
+        }
     }
 }
 
@@ -165,8 +182,8 @@ impl From<RegionAlign> for Align {
 impl From<Align> for RegionAlign {
     fn from(align: Align) -> Self {
         Self {
-            x: align.x.unwrap_or(AxisAlign::Center),
-            y: align.y.unwrap_or(AxisAlign::Center),
+            x: align.x.unwrap_or(AxisAlign::CENTER),
+            y: align.y.unwrap_or(AxisAlign::CENTER),
         }
     }
 }
@@ -189,7 +206,10 @@ 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 +218,5 @@ impl RegionAlign {
         UiVec2::from(self)
     }
 }
+
+impl_axis_index!(RegionAlign => AxisAlign);

core/src/orientation/axis.rs

@@ -1,11 +1,24 @@
 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 {
+    /// A per-axis pair with `aligned` on this axis and `ortho` on the other,
+    /// which is what `from_axis` does for a vector.
+    pub fn pair<T>(self, aligned: T, ortho: T) -> [T; 2] {
+        match self {
+            Self::X => [aligned, ortho],
+            Self::Y => [ortho, aligned],
+        }
+    }
+}
+
 impl std::ops::Not for Axis {
     type Output = Self;
 
@@ -40,21 +53,16 @@ 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 {
@@ -113,3 +121,6 @@ impl<T> BothAxis<T> {
         }
     }
 }
+
+impl_axis_index!({const SHIFT: u32} FixedVec2<SHIFT> => Fixed<SHIFT>);
+impl_axis_index!(Vec2 => f32);

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 y: Len,
+    pub x: LayoutLen,
+    pub y: LayoutLen,
 }
 
-#[derive(Debug, Clone, Copy, PartialEq)]
-pub struct Len {
-    pub abs: f32,
-    pub rel: f32,
-    pub rest: f32,
+/// What a widget asks for along one axis: a [`Len`] -- pixels and a fraction
+/// of the box it is given -- plus a share of whatever is left over once
+/// everything fixed has been taken. The parts add up rather than choosing
+/// between one another.
+///
+/// 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 {
-    fn from(value: Len) -> Self {
+/// A length with no share in it is a length a container does not have to
+/// 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,
-        y: Len::ZERO,
+        x: LayoutLen::ZERO,
+        y: LayoutLen::ZERO,
     };
 
-    pub const REST: Self = Self {
-        x: Len::REST,
-        y: Len::REST,
+    pub const LEFTOVER: Self = Self {
+        x: LayoutLen::LEFTOVER,
+        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),
-            y: Len::abs(v.y),
+            x: LayoutLen {
+                px: v.x,
+                ..LayoutLen::ZERO
+            },
+            y: LayoutLen {
+                px: v.y,
+                ..LayoutLen::ZERO
+            },
         }
     }
 
     pub fn rel(v: Vec2) -> Self {
         Self {
-            x: Len::rel(v.x),
-            y: Len::rel(v.y),
+            x: LayoutLen::rel(v.x),
+            y: LayoutLen::rel(v.y),
         }
     }
 
-    pub fn rest(v: Vec2) -> Self {
+    pub fn leftover(v: Vec2) -> Self {
         Self {
-            x: Len::rest(v.x),
-            y: Len::rest(v.y),
+            x: LayoutLen::leftover(v.x),
+            y: LayoutLen::leftover(v.y),
         }
     }
 
     pub fn to_uivec2(self) -> UiVec2 {
         UiVec2 {
-            x: self.x.apply_rest(),
-            y: self.y.apply_rest(),
+            x: self.x.apply_leftover(),
+            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,
-        rel: 0.0,
-        rest: 0.0,
+        px: Px::ZERO,
+        rel: Rel::ZERO,
+        leftover: Weight::ZERO,
     };
 
-    pub const REST: Self = Self {
-        abs: 0.0,
-        rel: 0.0,
-        rest: 1.0,
+    pub const LEFTOVER: Self = Self {
+        px: Px::ZERO,
+        rel: Rel::ZERO,
+        leftover: Weight::ONE,
     };
 
-    pub fn apply_rest(&self) -> UiScalar {
-        UiScalar {
-            rel: self.rel + if self.rest > 0.0 { 1.0 } else { 0.0 },
-            abs: self.abs,
-        }
+    /// The whole of what is left over counts as the whole box, which is what
+    /// a length means to something that is not dividing a box between
+    /// siblings -- a scroll asking how long its content is.
+    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(),
-            rel: 0.0,
-            rest: 0.0,
+            px: part.px,
+            rel: part.rel,
+            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.to_f32(),
-            rest: 0.0,
+            rel: Rel::from_num(rel),
+            ..Self::ZERO
         }
     }
-    pub fn rest(ratio: impl UiNum) -> Self {
+    pub fn leftover(ratio: impl UiNum) -> Self {
         Self {
-            abs: 0.0,
-            rel: 0.0,
-            rest: ratio.to_f32(),
+            leftover: Weight::from_num(ratio),
+            ..Self::ZERO
         }
     }
 }
@@ -141,38 +208,26 @@ impl Len {
 pub mod len_fns {
     use super::*;
 
-    pub fn abs(abs: impl UiNum) -> Len {
-        Len {
-            abs: abs.to_f32(),
-            rel: 0.0,
-            rest: 0.0,
+    pub fn px(px: impl UiNum) -> LayoutLen {
+        LayoutLen::px(px)
     }
+    pub fn rel(rel: impl UiNum) -> LayoutLen {
+        LayoutLen::rel(rel)
     }
-    pub fn rel(rel: impl UiNum) -> Len {
-        Len {
-            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(),
-        }
+    pub fn leftover(ratio: impl UiNum) -> LayoutLen {
+        LayoutLen::leftover(ratio)
     }
 }
 
-impl_op!(Len Add add; abs rel rest);
-impl_op!(Len Sub sub; abs rel rest);
+impl_op!(same LayoutLen Add add; px rel leftover);
+impl_op!(same LayoutLen Sub sub; px rel leftover);
 
-impl_op!(Size Add add; x y);
-impl_op!(Size Sub sub; x y);
+impl_op!(same Size Add add; 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 {
-            write!(f, "{} abs;", self.abs)?;
+        if self.px != Px::ZERO {
+            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 {
-            write!(f, "{} rest;", self.rest)?;
+        if self.leftover != Weight::ZERO {
+            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::{
-    UiNum,
-    util::{LerpUtil, impl_op},
-};
+use crate::{Px, PxVec2, Rel, UiNum, util::impl_op};
 
 #[repr(C)]
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, bytemuck::Pod, bytemuck::Zeroable, Default)]
 pub struct UiVec2 {
-    pub x: UiScalar,
-    pub y: UiScalar,
+    pub x: Len,
+    pub y: Len,
 }
 
 impl UiVec2 {
     pub const ZERO: Self = Self {
-        x: UiScalar::ZERO,
-        y: UiScalar::ZERO,
+        x: Len::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 {
-        let abs = abs.into();
+    pub const fn px(px: impl const Into<Vec2>) -> Self {
+        let px = px.into();
         Self {
-            x: UiScalar::abs(abs.x),
-            y: UiScalar::abs(abs.y),
+            x: Len::px(px.x),
+            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),
-            y: UiScalar::rel(rel.y),
+            x: Len::rel(rel.x),
+            y: Len::rel(rel.y),
         }
     }
 
@@ -56,30 +62,15 @@ impl UiVec2 {
         }
     }
 
-    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),
-        }
+    /// Resolved against a box of `size`, which is where a fraction stops
+    /// being one and becomes a place.
+    pub fn to_px(&self, size: PxVec2) -> PxVec2 {
+        PxVec2::new(self.x.to_px(size.x), self.y.to_px(size.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,
@@ -92,34 +83,27 @@ impl UiVec2 {
         }
     }
 
-    pub fn get_abs(&self) -> Vec2 {
-        (self.x.abs, self.y.abs).into()
+    pub fn get_px(&self) -> Vec2 {
+        (self.x.px.to_f32(), self.y.px.to_f32()).into()
     }
 
     pub fn get_rel(&self) -> Vec2 {
-        (self.x.rel, self.y.rel).into()
-    }
-
-    pub fn abs_mut(&mut self) -> Vec2View<'_> {
-        Vec2View {
-            x: &mut self.x.abs,
-            y: &mut self.y.abs,
-        }
+        (self.x.rel.to_f32(), self.y.rel.to_f32()).into()
     }
 }
 
 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!(UiVec2 Sub sub; x y);
+impl_op!(same UiVec2 Add add; x y);
+impl_op!(same UiVec2 Sub sub; x y);
 
 const impl From<Vec2> for UiVec2 {
-    fn from(abs: Vec2) -> Self {
-        Self::abs(abs)
+    fn from(px: Vec2) -> Self {
+        Self::px(px)
     }
 }
 
@@ -127,135 +111,149 @@ const impl<T: const UiNum, U: const UiNum> From<(T, U)> for UiVec2
 where
     (T, U): const Destruct,
 {
-    fn from(abs: (T, U)) -> Self {
-        Self::abs(abs)
+    fn from(px: (T, U)) -> Self {
+        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)]
-pub struct UiScalar {
-    pub rel: f32,
-    pub abs: f32,
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, bytemuck::Pod, Default, bytemuck::Zeroable)]
+pub struct Len {
+    pub rel: Rel,
+    pub px: Px,
 }
 
-impl Eq for UiScalar {}
-impl Hash for UiScalar {
-    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
-        state.write_u32(self.rel.to_bits());
-        state.write_u32(self.abs.to_bits());
+impl_op!(same Len Add add; rel px);
+impl_op!(same Len Sub sub; rel px);
+
+impl Len {
+    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);
-impl_op!(UiScalar Sub sub; rel abs);
-
-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 }
+    /// From parts already on the grid, rather than numbers to be put on it.
+    pub const fn from_parts(rel: Rel, px: Px) -> Self {
+        Self { rel, px }
     }
 
     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 {
-        Self { rel: 0.0, abs }
+    pub const fn px(px: f32) -> Self {
+        Self::from_parts(Rel::ZERO, Px::from_f32(px))
     }
 
     pub const fn rel_min() -> Self {
-        Self::new(0.0, 0.0)
+        Self::ZERO
     }
 
     pub const fn rel_max() -> Self {
-        Self::new(1.0, 0.0)
+        Self::FULL
     }
 
     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 {
-        self.abs += amt;
+    /// Both parts by the same fraction, which is what a part of a length
+    /// 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,
-            abs: offset,
+            rel: self.rel.lerp(span.start.rel, span.end.rel),
+            px: self.px.add(self.rel.lerp(span.start.px, span.end.px)),
         }
     }
 
-    /// Undoes `within`, and `None` where the span has a fixed length: every
-    /// fraction of it lands on the same `rel`, so none can be told apart.
-    pub fn outside(&self, span: &UiSpan) -> Option<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);
-        Some(Self { rel, abs })
-    }
-
-    pub fn within_len(&self, len: UiScalar) -> Self {
+    pub const fn within_len(&self, len: Len) -> Self {
         self.within(&UiSpan {
-            start: UiScalar::ZERO,
+            start: Len::ZERO,
             end: len,
         })
     }
 
-    pub fn select_len(&self, len: UiScalar) -> Self {
+    pub fn select_len(&self, len: Len) -> Self {
         len.within_len(*self)
     }
 
     pub const fn flip(&mut self) {
-        self.rel = 1.0 - self.rel;
-        self.abs = -self.abs;
+        self.rel = Rel::ONE.sub(self.rel);
+        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 {
-        self.rel * rel + self.abs
+    /// Resolved against a box of `len`, which is the only place a fraction
+    /// 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 end: UiScalar,
+    pub start: Len,
+    pub end: Len,
 }
 
 impl UiSpan {
     pub const FULL: Self = Self {
-        start: UiScalar::ZERO,
-        end: UiScalar::FULL,
+        start: Len::ZERO,
+        end: Len::FULL,
     };
 
     pub const fn rel(rel: f32) -> Self {
         Self {
-            start: UiScalar::rel(rel),
-            end: UiScalar::rel(rel),
+            start: Len::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 }
     }
 
@@ -263,14 +261,19 @@ 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),
@@ -278,16 +281,27 @@ impl UiSpan {
         }
     }
 
-    pub fn outside(&self, parent: &Self) -> Option<Self> {
-        Some(Self {
-            start: self.start.outside(parent)?,
-            end: self.end.outside(parent)?,
-        })
+    /// A box `len` long inside this one, on the side `align` says. Both must
+    /// be lengths of the same rel base: it subtracts one from the other
+    /// rather than composing it in, which is what keeps a fraction the same
+    /// 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
     }
+
+    /// Both ends by the same amount, which is what moving a box without
+    /// changing its length does to every part of it.
+    pub const fn translated(self, by: Len) -> Self {
+        Self {
+            start: self.start + by,
+            end: self.end + by,
+        }
+    }
 }
 
 #[repr(C)]
@@ -298,6 +312,17 @@ pub struct UiRegion {
 }
 
 impl UiRegion {
+    /// Every part of the box by the same amount on each axis. Done to the
+    /// whole region rather than an end at a time, because that is what it is
+    /// -- and because four adds in a row are four adds, where four asked for
+    /// separately are four sequences.
+    pub const fn translated(self, x: Len, y: Len) -> Self {
+        Self {
+            x: self.x.translated(x),
+            y: self.y.translated(y),
+        }
+    }
+
     pub const FULL: Self = Self {
         x: UiSpan::FULL,
         y: UiSpan::FULL,
@@ -319,20 +344,6 @@ impl UiRegion {
             y: self.y.within(&parent.y),
         }
     }
-    pub const fn axis(&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(),
@@ -351,10 +362,10 @@ 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(),
-            bot_right: self.bot_right().get_rel() * size + self.bot_right().get_abs(),
+            top_left: self.top_left().to_px(size),
+            bot_right: self.bot_right().to_px(size),
         }
     }
 
@@ -397,50 +408,6 @@ impl UiRegion {
     }
 }
 
-/// Taking a drawing out of one box and putting it in another, checked once
-/// for a whole subtree so that applying it cannot fail.
-///
-/// A box of a fixed length holds each part as an offset from its start rather
-/// than as a fraction of it, so those parts can be carried to a box of the
-/// same length but never stretched to a different one.
-#[derive(Debug, Copy, Clone, PartialEq)]
-pub struct Remap {
-    from: UiRegion,
-    to: UiRegion,
-}
-
-impl Remap {
-    pub fn new(from: UiRegion, to: UiRegion) -> Option<Self> {
-        [Axis::X, Axis::Y]
-            .into_iter()
-            .all(|axis| {
-                let (from, to) = (from.axis(axis), to.axis(axis));
-                from.start.rel != from.end.rel || from.len() == to.len()
-            })
-            .then_some(Self { from, to })
-    }
-
-    pub fn apply(&self, region: UiRegion) -> UiRegion {
-        UiRegion {
-            x: Self::span(region.x, self.from.x, self.to.x),
-            y: Self::span(region.y, self.from.y, self.to.y),
-        }
-    }
-
-    fn span(span: UiSpan, from: UiSpan, to: UiSpan) -> UiSpan {
-        match span.outside(&from) {
-            Some(out) => out.within(&to),
-            // `new` admits this only where the two are the same length, so
-            // the difference between their starts is the whole move.
-            None => {
-                let mut span = span;
-                span.shift(to.start - from.start);
-                span
-            }
-        }
-    }
-}
-
 impl Display for UiRegion {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(
@@ -453,21 +420,21 @@ impl Display for UiRegion {
     }
 }
 
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct PixelRegion {
-    pub top_left: Vec2,
-    pub bot_right: Vec2,
+    pub top_left: PxVec2,
+    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
     }
 }
@@ -478,19 +445,5 @@ impl Display for PixelRegion {
     }
 }
 
-pub struct Vec2View<'a> {
-    pub x: &'a mut f32,
-    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;
-    }
-}
+impl_axis_index!(UiVec2 => Len);
+impl_axis_index!(UiRegion => UiSpan);

core/src/primitive/layer.rs

@@ -120,6 +120,10 @@ impl<T: Default> Layers<T> {
 }
 
 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

@@ -1,11 +1,17 @@
+#[cfg(feature = "layout-diagnostics")]
+use crate::layout_diagnostics::{self as diag, Counter, TimerKind};
 use crate::{
-    Align, GlyphAtlas, GlyphEntry, GlyphKey, PlacedGlyph, RegionAlign, UiColor, util::Vec2,
+    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},
@@ -17,8 +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 {
@@ -26,6 +56,7 @@ impl Default for TextData {
             layout_ctx: LayoutContext::new(),
             scale_ctx: ScaleContext::new(),
             atlas: GlyphAtlas::default(),
+            spare: VecDeque::new(),
         }
     }
 }
@@ -81,6 +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)]
@@ -95,6 +129,7 @@ impl TextBuffer {
             text: text.into(),
             layout: Layout::new(),
             layout_key: None,
+            placed: None,
         }
     }
 
@@ -119,15 +154,45 @@ 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())
     }
@@ -138,8 +203,62 @@ 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);
@@ -150,10 +269,13 @@ 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);
     }
 }
 
@@ -196,9 +318,9 @@ impl TextData {
                     };
                     placed.push(PlacedGlyph {
                         entry,
-                        offset: Vec2::new(
-                            glyph.x.floor() + entry.left as f32,
-                            glyph.y.floor() - entry.top as f32,
+                        offset: PxVec2::new(
+                            Px::from_int(glyph.x.floor() as i32 + entry.left),
+                            Px::from_int(glyph.y.floor() as i32 - entry.top),
                         ),
                     });
                 }
@@ -265,18 +387,54 @@ 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>,
-    ) -> RenderedText {
+    ) -> &'b 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);
+        // 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/render/atlas.rs

@@ -1,5 +1,5 @@
 use crate::{
-    PatchRect,
+    PatchRect, PxVec2,
     util::{HashMap, Vec2},
 };
 use image::RgbaImage;
@@ -241,5 +241,7 @@ 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

@@ -1,11 +1,10 @@
-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 height: f32,
+    pub dim: Vec2,
 }
 
 #[repr(C)]
@@ -13,15 +12,19 @@ pub struct WindowUniform {
 pub struct PrimitiveInstance {
     pub region: UiRegion,
     pub mask_idx: MaskIdx,
+    pub move_idx: MoveIdx,
 }
 
 impl PrimitiveInstance {
-    const ATTRIBS: [VertexAttribute; 5] = vertex_attr_array![
-        0 => Float32x2,
-        1 => Float32x2,
-        2 => Float32x2,
-        3 => Float32x2,
+    // The region's four scalars, each a `Rel` beside a `Px`: whole counts
+    // that the shader decodes, rather than the numbers themselves.
+    const ATTRIBS: [VertexAttribute; 6] = vertex_attr_array![
+        0 => Sint32x2,
+        1 => Sint32x2,
+        2 => Sint32x2,
+        3 => Sint32x2,
         4 => Uint32,
+        5 => Uint32,
     ];
 
     pub fn desc() -> VertexBufferLayout<'static> {
@@ -43,4 +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

@@ -17,11 +17,22 @@ mod texture;
 mod util;
 
 pub use atlas::*;
-pub use data::{Mask, MaskIdx};
+pub use data::{Mask, MaskIdx, MoveIdx, MoveOffset};
 pub use primitive::*;
 
 const PRELUDE: &str = include_str!("./shader/prelude.wgsl");
 
+fn module_source(wgsl: &str) -> String {
+    // The steps come from the same constants the CPU counts in, rather than
+    // a second copy of them written into the shader: a grid the two disagree
+    // about puts every coordinate somewhere else.
+    format!(
+        "const PX_STEP: f32 = 1.0 / {}.0;\nconst REL_STEP: f32 = 1.0 / {}.0;\n{PRELUDE}\n{wgsl}",
+        1u32 << crate::PX_SHIFT,
+        1u32 << crate::REL_SHIFT,
+    )
+}
+
 pub struct UiRenderNode {
     shared_layout: BindGroupLayout,
     shared_group: BindGroup,
@@ -34,6 +45,7 @@ pub struct UiRenderNode {
     active: Vec<usize>,
     window_buffer: Buffer,
     masks: ArrBuf<Mask>,
+    moves: ArrBuf<MoveOffset>,
 }
 
 struct RenderLayer {
@@ -94,7 +106,8 @@ impl UiRenderNode {
             self.active.push(i);
             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 {
+                        let h = &mut primitive.handle;
                         if h.layer == i && h.kind == change.kind && h.inst_idx == change.old {
                             h.inst_idx = change.new;
                             break;
@@ -127,32 +140,35 @@ impl UiRenderNode {
         for primitive in &mut self.primitives {
             primitive.render.update(ui);
         }
+        let mut regroup = false;
         if ui.masks.changed {
             ui.masks.changed = false;
-            if self.masks.update(device, queue, &ui.masks[..]) {
+            regroup |= self.masks.update(device, queue, &ui.masks[..]);
+        }
+        if ui_render.moves.changed {
+            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 {
-            width: size.x,
-            height: size.y,
-        }];
+        let slice = &[WindowUniform { dim: size }];
         queue.write_buffer(&self.window_buffer, 0, bytemuck::cast_slice(slice));
     }
 
     pub fn new(device: &Device, config: &SurfaceConfiguration) -> Self {
         let window_uniform = WindowUniform {
-            width: config.width as f32,
-            height: config.height as f32,
+            dim: Vec2::new(config.width as f32, config.height as f32),
         };
         let window_buffer = device.create_buffer_init(&BufferInitDescriptor {
             label: Some("window"),
@@ -166,7 +182,13 @@ impl UiRenderNode {
             BufferUsages::STORAGE | BufferUsages::COPY_DST,
             "ui masks",
         );
-        let shared_group = Self::shared_group(device, &shared_layout, &window_buffer, &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);
 
         Self {
             shared_layout,
@@ -177,6 +199,7 @@ impl UiRenderNode {
             layers: HashMap::default(),
             active: Vec::new(),
             masks,
+            moves,
         }
     }
 
@@ -211,7 +234,7 @@ impl UiRenderNode {
     ) -> RenderPipeline {
         let module = device.create_shader_module(ShaderModuleDescriptor {
             label: Some(label),
-            source: ShaderSource::Wgsl(format!("{PRELUDE}\n{wgsl}").into()),
+            source: ShaderSource::Wgsl(module_source(wgsl).into()),
         });
         device.create_render_pipeline(&RenderPipelineDescriptor {
             label: Some(label),
@@ -252,7 +275,8 @@ impl UiRenderNode {
         })
     }
 
-    /// What every draw in the ui is given: the window and the masks.
+    /// What every draw in the ui is given: the window, the masks and the
+    /// move chain every position is resolved through.
     fn shared_layout(device: &Device) -> BindGroupLayout {
         device.create_bind_group_layout(&BindGroupLayoutDescriptor {
             entries: &[
@@ -276,6 +300,16 @@ impl UiRenderNode {
                     },
                     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 shared"),
         })
@@ -286,6 +320,7 @@ impl UiRenderNode {
         layout: &BindGroupLayout,
         window: &Buffer,
         masks: &ArrBuf<Mask>,
+        moves: &ArrBuf<MoveOffset>,
     ) -> BindGroup {
         device.create_bind_group(&BindGroupDescriptor {
             layout,
@@ -298,6 +333,10 @@ impl UiRenderNode {
                     binding: 1,
                     resource: masks.buffer.as_entire_binding(),
                 },
+                BindGroupEntry {
+                    binding: 2,
+                    resource: moves.buffer.as_entire_binding(),
+                },
             ],
             label: Some("ui shared"),
         })
@@ -374,3 +413,35 @@ impl ListBuffers {
         }
     }
 }
+
+#[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/primitive.rs

@@ -3,7 +3,7 @@ use std::{any::TypeId, marker::PhantomData};
 use crate::{
     Color, TextureHandle, UiData, UiRegion, WidgetId,
     render::{
-        data::{MaskIdx, PrimitiveInstance},
+        data::{MaskIdx, MoveIdx, PrimitiveInstance},
         page::GlyphRender,
         texture::ImageRender,
     },
@@ -246,6 +246,7 @@ impl LayerDraws {
             primitive,
             region,
             mask_idx,
+            move_idx,
         }: PrimitiveInst<P>,
     ) -> PrimitiveHandle {
         self.updated = true;
@@ -258,7 +259,11 @@ impl LayerDraws {
             .get_or_insert_with(InstanceList::new::<P>)
             .push(
                 id,
-                PrimitiveInstance { region, mask_idx },
+                PrimitiveInstance {
+                    region,
+                    mask_idx,
+                    move_idx,
+                },
                 bytemuck::bytes_of(&primitive),
             );
         PrimitiveHandle {
@@ -304,6 +309,7 @@ pub struct PrimitiveInst<P> {
     pub primitive: P,
     pub region: UiRegion,
     pub mask_idx: MaskIdx,
+    pub move_idx: MoveIdx,
 }
 
 pub struct PrimitiveChange {
@@ -347,7 +353,7 @@ impl RectPrimitive {
 
 /// `color` is multiplied by the atlas alpha for a mask glyph; a colour glyph
 /// takes the texel unchanged, which `GlyphEntry::IS_COLORED` selects.
-#[repr(C, align(8))]
+#[repr(C)]
 #[derive(Debug, Copy, Clone)]
 pub struct GlyphPrimitive {
     pub uv_min: Vec2,
@@ -358,8 +364,8 @@ pub struct GlyphPrimitive {
     pub flags: u32,
 }
 
-// Manual rather than derived: the align(8) leaves four bytes of padding, which
-// is how WGSL lays the struct out.
+// Manual rather than derived: `Vec2`'s alignment leaves four bytes of padding
+// here, which is how WGSL lays the struct out.
 unsafe impl bytemuck::Pod for GlyphPrimitive {}
 unsafe impl bytemuck::Zeroable for GlyphPrimitive {}
 impl Primitive for GlyphPrimitive {

core/src/render/shader/prelude.wgsl

@@ -7,32 +7,121 @@
 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` and `REL_STEP` are prepended from `iris_core`'s own constants:
+// what it stores is a whole count of each, both 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,
 }
 
-struct UiSpan {
-    start: UiScalar,
-    end: UiScalar,
+const MOVE_NONE: u32 = 4294967295u;
+// Keep in step with `iris_core::CHAIN_LIMIT`. It bounds a malformed cycle
+// rather than any real tree, and the CPU walk uses the same number so both
+// resolve a deep one the same way.
+const CHAIN_LIMIT: u32 = 64u;
+
+// 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),
+    );
 }
 
-struct UiScalar {
+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,
-    abs: f32,
+    px: 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(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 {
@@ -52,13 +141,18 @@ fn vs_main(
 ) -> 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 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 = floor(top_left_rel * window.dim) + floor(top_left_abs);
-    let bot_right = floor(bot_right_rel * window.dim) + floor(bot_right_abs);
+    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>(
@@ -81,13 +175,16 @@ fn masked(in: VertexOutput, color: vec4<f32>) -> vec4<f32> {
         return color;
     }
     let mask = masks[in.mask_idx];
-    let tl = vec2(mask.x.start.rel, mask.y.start.rel);
-    let tl_abs = vec2(mask.x.start.abs, mask.y.start.abs);
-    let br = vec2(mask.x.end.rel, mask.y.end.rel);
-    let br_abs = vec2(mask.x.end.abs, mask.y.end.abs);
+    // 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 = floor(tl * window.dim) + floor(tl_abs);
-    let bot_right = floor(br * window.dim) + floor(br_abs);
+    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;

core/src/ui/active.rs

@@ -1,20 +1,87 @@
-use crate::{LayerId, MaskIdx, PrimitiveHandle, Size, 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,
-    /// What the widget said it used of `region`, the last time it drew.
+    /// The measured answer and its dependencies. A hint-only dependency or
+    /// a widget first encountered during placement has no measurement yet.
+    pub answer: Option<(Size, LayoutHolds)>,
+    /// 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 children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
-    /// Whether it read the output's size, and so is wrong when that changes.
-    pub reads_output: bool,
+    /// 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(|(size, _)| size)
+    }
+}

core/src/ui/holds.rs

@@ -0,0 +1,184 @@
+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 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 +1,109 @@
+use crate::util::impl_axis_index;
+use crate::{Axis, Holds, Len, Px, PxVec2, UiRegion, UiVec2};
+
+const AXES: [Axis; 2] = [Axis::X, Axis::Y];
+
+/// 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 {
+        AXES.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

@@ -1,13 +1,26 @@
 use crate::{
-    Mask, PrimitiveRegistry, TextData, Textures, WeakWidget, WidgetId, Widgets, util::TrackedArena,
+    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 holds;
+mod layout_holds;
 mod painter;
+mod place;
 mod render_state;
 
 pub use active::*;
+pub use holds::*;
+pub use layout_holds::*;
 pub use painter::{Painter, PrimitiveLike};
+pub use place::*;
 pub use render_state::*;
 
 #[derive(Default)]
@@ -20,6 +33,94 @@ pub struct UiData {
     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

@@ -1,27 +1,61 @@
+#[cfg(feature = "layout-diagnostics")]
+use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Len, RenderedText, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
-    UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, WidgetId,
+    Axis, Declared, Holds, LayoutHolds, LayoutLen, Len, PlaceDesc, Px, PxVec2, RegionAlign, Rel,
+    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
+    TextureHandle, UiRegion, UiRenderState, UiRsc, UiVec2, Weight, WidgetId, Widgets,
     render::{
-        GlyphPrimitive, Mask, MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst, PrimitiveKind,
+        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
         TexturePrimitive,
     },
-    util::Vec2,
+    ui::render_state::{DrawInfo, Placing},
 };
+const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 
 /// makes your surfaces look pretty
 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>,
-    pub(super) reads_output: bool,
+    /// 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,
 }
 
@@ -33,119 +67,310 @@ impl<'a> Painter<'a> {
 
     /// 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(h);
+        self.push_primitive(RetainedPrimitive { handle: h, region });
     }
 
-    fn push_primitive(&mut self, h: PrimitiveHandle) {
+    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(&mut self, primitive: impl PrimitiveLike) {
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, self.region)
+        self.primitive_at(primitive, UiRegion::FULL)
     }
 
+    /// Writes a primitive in a part of this widget's own box, in that box's
+    /// coordinates.
     pub fn primitive_within(&mut self, primitive: impl PrimitiveLike, region: UiRegion) {
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, region.within(&self.region));
+        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
+    /// base forwarded unchanged: what a container that is only a wrapper
+    /// 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, self.region)
+        self.widget_at(id, UiRegion::FULL)
     }
 
-    /// Draws a widget somewhere within this one. Drawing one a second time
-    /// gives it a new box, keeping the drawing it already has where it can.
-    pub fn widget_within<'s, W: ?Sized>(
-        &'s mut self,
-        id: &'s StrongWidget<W>,
-        region: UiRegion,
-    ) -> DrawResult<'s, 'a, W> {
-        let region = region.within(&self.region);
-        self.widget_at(id, region)
+    /// Resolves what the place says about the child's rel base into a length,
+    /// where that is this widget's own narrowed the way the region is. An
+    /// axis the region leaves whole is not read at all, so a wrapper that
+    /// only moves its child does not pin its drawing to a rel base.
+    fn state_rel_base(&mut self, mut place: PlaceDesc) -> PlaceDesc {
+        for axis in AXES {
+            if let Some(span) = place[axis].narrows_rel_base() {
+                let len = span.len();
+                let stated = (len != Len::FULL).then(|| len.within_len(self.rel_base(axis)));
+                place[axis] = place[axis].with_rel_base(stated);
+            }
+        }
+        place
     }
 
-    fn widget_at<'s, W: ?Sized>(
+    /// Asks a child, saying what its fractions are of and where it is asked.
+    ///
+    /// `place` says where the child goes and what its fractions are of:
+    /// see [`PlaceDesc`]. A `UiRegion` converts into the common case, which
+    /// 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>,
-        region: UiRegion,
+        place: impl Into<PlaceDesc>,
     ) -> DrawResult<'s, 'a, W> {
+        let place = self.state_rel_base(place.into());
+        let region_node = self.rsc.widgets().is_region_node(id.id());
+        let declared = self.declared_lens(id);
+        let align = self.rsc.widgets().alignment(id.id());
+        let (rel_base, region) =
+            place.rel_base_and_region(self.region, self.rel_base, declared, align);
+        #[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.
-        if !self.children.contains(&id.id()) {
+        let re_asked = self.children.contains(&id.id());
+        if !re_asked {
             self.children.push(id.id());
         }
-        let size = self.state.draw_inner(
-            self.layer,
+        let px = rel_base.to_px(self.window);
+        let (size, answer_holds, holds) = 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,
-            Some(self.id),
-            self.mask,
+                placed: place,
+                asked: place,
+                re_asked,
+                px,
+            },
             None,
             self.rsc,
         );
+        let holds = self.in_parent(holds, region, place, declared);
+        let answer_holds = self.in_parent(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,
+            answer_holds,
         }
     }
 
-    /// What a child says its length is without being drawn, if it can say.
-    /// Asking counts as reading its size.
-    pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<Len> {
-        let hint = self.rsc.widgets().get_dyn(id.id())?.size_hint(axis)?;
-        self.depend_on_size(id);
-        Some(hint)
+    /// Takes back a child that was drawn only to find out how long it is.
+    /// Its drawing is dropped and it is not one of this widget's children
+    /// 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);
     }
 
-    fn depend_on_size<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
+    /// Puts a child in `place` of this widget's box, where that box is the
+    /// answer the child already gave: the drawing is re-expressed there
+    /// 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.state_rel_base(place.into());
+        let states_rel_base = AXES
+            .iter()
+            .any(|&axis| place[axis].stated_rel_base().is_some());
+        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,
+        }
+    }
+
+    /// This widget as the thing its children are placed within.
+    fn placing(&self) -> Placing {
+        Placing {
+            id: self.id,
+            region: self.region,
+            rel_base: self.rel_base,
+            window: self.window,
+            depth: self.depth,
+            move_idx: self.move_idx,
+            mask: self.mask,
+        }
+    }
+
+    /// What a widget's rules declare its lengths to be, which whoever draws
+    /// it resolves into its rel base. Reading them depends on nothing -- the box
+    /// that comes of them is kept on the child, and `redraw` compares it
+    /// there.
+    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> Declared {
+        self.rsc.widgets().declared_lens(id.id())
+    }
+
+    /// 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 widgets = self.rsc.widgets();
+        // 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.
+        let hint = widgets.size_rules(id.id())[axis].exact().or_else(|| {
+            widgets
+                .get_dyn(id.id())
+                .and_then(|widget| widget.size_hint(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(
+    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)
     }
 
+    /// 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));
-            region.x.end = region.x.start + UiScalar::abs(glyph.entry.width as f32);
-            region.y.end = region.y.start + UiScalar::abs(glyph.entry.height as f32);
-            self.write(
+                let mut region = region.offset(UiVec2::from_px(glyph.offset));
+                let size = PxVec2::new(
+                    Px::from_int(glyph.entry.width as i32),
+                    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,
@@ -154,27 +379,119 @@ impl<'a> Painter<'a> {
                     color: text.color,
                     flags: glyph.entry.flags(),
                 },
-                region,
+                place(origin),
+                place(resolved),
             );
         }
     }
 
-    pub fn region(&self) -> UiRegion {
-        self.region
+    /// The symbolic length of this widget's own box along one axis, in the
+    /// 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
     }
 
-    /// The output's size in pixels. A widget that reads it draws again when
-    /// the output changes, since nothing else can put that right.
-    pub fn output_size(&mut self) -> Vec2 {
-        self.reads_output = true;
-        self.state.output_size
+    /// This widget's rel base along one axis: what a fraction it or anything
+    /// 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
     }
 
-    /// This widget's box in pixels. Resolved against the output's size, so a
-    /// widget that reads it draws again when the output changes.
-    pub fn px_size(&mut self) -> Vec2 {
-        self.reads_output = true;
-        self.region.size().to_abs(self.state.output_size)
+    /// Where this widget sits in a box longer than the length it takes. A
+    /// widget that positions its own content reads it to place that content
+    /// the way the box around it would have placed the widget.
+    pub fn alignment(&self) -> RegionAlign {
+        self.rsc.widgets().alignment(self.id)
+    }
+
+    /// Whether a rule beside this widget gives its length on `axis` outright,
+    /// 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()
+    }
+
+    /// This widget's own box in pixels. Reading it makes the drawing one
+    /// 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))
+    }
+
+    /// One axis of this widget's own box in pixels. Prefer this to
+    /// [`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)
+    }
+
+    /// 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 text_data(&mut self) -> &mut TextData {
@@ -185,6 +502,18 @@ impl<'a> Painter<'a> {
         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);
     }
@@ -205,16 +534,23 @@ 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 {
-        self.painter.depend_on_size(self.child);
+        #[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) -> Len {
-        self.size().axis(axis)
+    pub fn len(self, axis: Axis) -> LayoutLen {
+        self.size()[axis]
     }
 }
 
@@ -242,3 +578,165 @@ impl PrimitiveLike for &TextureHandle {
         self.into()
     }
 }
+
+/// Moves what a child depends on into this widget's own terms: this
+/// method's `impl` block is where a `Painter`'s own boxes are, so it takes
+/// only what the child was asked with.
+impl Painter<'_> {
+    /// 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 AXES {
+            let declared = declared[axis];
+            let holds = holds[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;
+            let at = place[axis];
+            let reaches = at.stated_rel_base().is_none()
+                && !at.is_sized()
+                && declared.is_none_or(|len| len.rel != Rel::ZERO);
+            result.rel_base = holds.rel_base.and(reaches.then(|| self.rel_base[axis]));
+            match (at.within_span(), declared.is_some()) {
+                // 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.
+                (Some(span), false) => {
+                    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 a widget's box is where a rule or its own hint says so outright.
+    pub(crate) fn declared_lens(&self, id: WidgetId) -> Declared {
+        let rules = self.size_rules(id);
+        let widget = self.get_dyn(id);
+        Declared::per_axis(|axis| {
+            rules[axis].declared().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.
+                widget
+                    .and_then(|widget| widget.size_hint(axis))
+                    .and_then(LayoutLen::declared)
+            })
+        })
+    }
+}
+
+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(crate) fn fills(self, declared: Option<Len>, decided: bool) -> bool {
+        self.leftover != Weight::ZERO || declared.is_some() || decided
+    }
+}
+
+/// 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.
+impl PlaceDesc {
+    pub(crate) fn placement(
+        self,
+        region: UiRegion,
+        size: Size,
+        declared: Declared,
+        align: RegionAlign,
+    ) -> UiRegion {
+        let mut placed = region;
+        for axis in AXES {
+            let reported = size[axis];
+            if reported.fills(declared[axis], self[axis].does_fill()) {
+                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(crate) 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 AXES {
+            let base = self[axis]
+                .stated_rel_base()
+                .unwrap_or_else(|| parent_rel_base[axis]);
+            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 +1,252 @@
+use crate::util::impl_axis_index;
+use crate::{Axis, AxisAlign, Len, PrimitiveHandle, RegionAlign, UiRegion, UiSpan};
+
+/// 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 {
+    span: PlaceSpan,
+    fills: bool,
+    rel_base: RelBase,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum PlaceSpan {
+    Within(UiSpan),
+    Shifted(UiSpan),
+    Sized(Len),
+}
+
+/// What a child's fractions are of, where the caller has not named a length.
+#[derive(Clone, Copy, Debug, PartialEq)]
+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 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
+    }
+
+    /// Whether the region is the placement outright, rather than a box the
+    /// answer is placed inside.
+    pub(crate) const fn does_fill(self) -> bool {
+        self.fills
+    }
+
+    /// Where it lands in the coordinates `own` is in.
+    pub(crate) 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),
+        }
+    }
+
+    /// The child's rel base, where this says one outright. `None` forwards
+    /// the caller's own, and [`RelBase::WithRegion`] is resolved by whoever
+    /// can read that rel base, so it does not reach here.
+    pub(crate) const fn stated_rel_base(self) -> Option<Len> {
+        match self.rel_base {
+            RelBase::Len(len) => Some(len),
+            _ => None,
+        }
+    }
+
+    /// The length this narrows the caller's rel base by, where it does.
+    /// `None` leaves that rel base alone, and reading it is then a
+    /// dependency the caller does not take.
+    pub(crate) const fn narrows_rel_base(self) -> Option<UiSpan> {
+        match (self.rel_base, self.span) {
+            (RelBase::WithRegion, PlaceSpan::Within(span)) => Some(span),
+            _ => None,
+        }
+    }
+
+    /// The span it composes into the caller's box, where that is what it
+    /// does: the one case whose validity maps back through the part.
+    pub(crate) const fn within_span(self) -> Option<UiSpan> {
+        match self.span {
+            PlaceSpan::Within(span) => Some(span),
+            _ => None,
+        }
+    }
+
+    /// Whether the caller decided this length rather than a place along its
+    /// own box, which is what stops its length reaching the child at all.
+    pub(crate) const fn is_sized(self) -> bool {
+        matches!(self.span, PlaceSpan::Sized(_))
+    }
+
+    /// The same, with its rel base stated outright.
+    pub(crate) const fn with_rel_base(mut self, len: Option<Len>) -> Self {
+        self.rel_base = match len {
+            Some(len) => RelBase::Len(len),
+            None => RelBase::Inherit,
+        };
+        self
+    }
+}
+
+/// 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 per_axis(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
+    }
+
+    /// The box each axis names, in the coordinates `own` is in.
+    pub(crate) 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/util/arena.rs

@@ -34,6 +34,10 @@ impl<T, I: IdNum> Arena<T, I> {
         self.tracker.free(id);
         self.data[i]
     }
+
+    pub(crate) 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

@@ -1,6 +1,5 @@
-pub const trait LerpUtil: Sized {
+pub const trait LerpUtil {
     fn lerp(self, from: Self, to: Self) -> Self;
-    fn lerp_inv(self, from: Self, to: Self) -> Option<Self>;
 }
 
 const impl LerpUtil for f32 {
@@ -9,14 +8,6 @@ const impl LerpUtil for f32 {
     fn lerp(self, from: Self, to: Self) -> Self {
         from + (to - from) * self
     }
-    /// inverse of lerp, and `None` where `from` and `to` are the same point:
-    /// every input lerps to it, so there is no one answer to come back to.
-    fn lerp_inv(self, from: Self, to: Self) -> Option<Self> {
-        match to == from {
-            true => None,
-            false => Some((self - from) / (to - from)),
-        }
-    }
 }
 
 macro_rules! impl_op {
@@ -65,6 +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)*);
     };
@@ -74,3 +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::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,

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, Len, Painter, Size};
+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;
@@ -11,21 +12,11 @@ mod widgets;
 pub use data::*;
 pub use handle::*;
 pub use like::*;
+pub use size_rule::*;
 pub use tag::*;
 pub use view::*;
 pub use widgets::*;
 
-/// What may be done to a widget's drawing when the box it was given changes
-/// on this axis, instead of drawing it again. Asked per axis, because wrapped
-/// text reads the width it is offered and not the height.
-#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
-pub enum OnResize {
-    Scale,
-    Translate,
-    #[default]
-    Redraw,
-}
-
 pub trait Widget: Any {
     /// Draws the widget, and returns what it used of the box it was given.
     fn draw(&mut self, painter: &mut Painter) -> Size;
@@ -33,13 +24,9 @@ pub trait Widget: Any {
     /// An exact length the widget can give without a painter or its children.
     /// 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<Len> {
+    fn size_hint(&self, _axis: Axis) -> Option<LayoutLen> {
         None
     }
-
-    fn on_resize(&self, _axis: Axis) -> OnResize {
-        OnResize::default()
-    }
 }
 
 impl Widget for () {
@@ -48,12 +35,8 @@ impl Widget for () {
         Size::default()
     }
 
-    fn size_hint(&self, _axis: Axis) -> Option<Len> {
-        Some(Len::default())
-    }
-
-    fn on_resize(&self, _axis: Axis) -> OnResize {
-        OnResize::Scale
+    fn size_hint(&self, _axis: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::default())
     }
 }
 

core/src/widget/size_rule.rs

@@ -0,0 +1,94 @@
+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(LayoutLen::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),
+        }
+    }
+
+    /// The length a widget reporting `reported` ends up with.
+    pub fn apply(&self, reported: LayoutLen) -> LayoutLen {
+        match self {
+            Self::Free => reported,
+            Self::Exact(len) => *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 per_axis(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},
 };
 
@@ -100,6 +101,71 @@ 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, for a caller holding a pair.
+    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())
     }

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

@@ -20,22 +20,26 @@ impl DefaultAppState for Client {
         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);
@@ -121,11 +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);
@@ -143,7 +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| {

examples/text.rs

@@ -28,10 +28,14 @@ impl DefaultAppState for State {
             .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 = (
-            wtext("left").size(24).text_align(Align::LEFT),
-            wtext("centred").size(24).text_align(Align::CENTER),
-            wtext("right").size(24).text_align(Align::RIGHT),
+            label("left", Align::LEFT),
+            label("centred", Align::H_CENTER),
+            label("right", Align::RIGHT),
         )
             .span(Dir::DOWN)
             .gap(8)

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 +1,182 @@
+#!/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
+
+swaymsg output HEADLESS-1 mode "$mode" >/dev/null
+# The extent `replay-touch` positions against, so a script's coordinates
+# are the output's own pixels.
+out_w=${mode%x*}
+out_h=${mode#*x}; out_h=${out_h%@*}
+
+# 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
+    swaymsg output HEADLESS-1 mode "$resize" >/dev/null
+    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 +15,10 @@ 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;
-            let size = region.size();
+            // The pointer arrives from the platform in floats; everything
+            // it is compared against is on the grid.
+            let pos = (PxVec2::from_f32(ctx.data.pos) + container_pos - id_pos).to_f32();
+            let size = region.size().to_f32();
             select(
                 rsc,
                 ctx.data.render,
@@ -70,8 +72,8 @@ 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()),
-            LogicalSize::<f32>::from(region.size().tuple()),
+            LogicalPosition::<f32>::from(region.top_left.to_f32().tuple()),
+            LogicalSize::<f32>::from(region.size().to_f32().tuple()),
         );
     }
     state.focus = Some(id);

src/default/mod.rs

@@ -251,7 +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, .. } => {

src/default/sense.rs

@@ -198,7 +198,7 @@ impl SensorUi for UiRenderState {
                 let Some(region) = region_of(id) else {
                     continue;
                 };
-                if !cursor.exists || !region.contains(cursor.pos) {
+                if !cursor.exists || !region.contains(PxVec2::from_f32(cursor.pos)) {
                     continue;
                 }
                 hovered.now.push(id);
@@ -249,8 +249,8 @@ fn deliver<Rsc: HasEvents>(
     region: PixelRegion,
 ) -> bool {
     let data = CursorData {
-        pos: cursor.pos - region.top_left,
-        size: region.bot_right - region.top_left,
+        pos: cursor.pos - region.top_left.to_f32(),
+        size: region.size().to_f32(),
         scroll_delta: cursor.scroll_delta,
         hover,
         cursor: cursor.clone(),

src/harness.rs

@@ -29,14 +29,95 @@ macro_rules! assert_corners {
         assert_eq!(
             $harness.region(&$id).expect("widget drew nothing"),
             $crate::core::PixelRegion {
-                top_left: $crate::core::util::Vec2::new($x0 as f32, $y0 as f32),
-                bot_right: $crate::core::util::Vec2::new($x1 as f32, $y1 as f32),
+                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>,
@@ -63,9 +144,9 @@ impl Harness {
         // bound that comes with `SyncSender` is far past anything a test
         // leaves unread.
         let (send, updates) = sync_channel(1024);
-        let rsc = DefaultRsc::init(Arc::new(Queue(send)));
+        let mut rsc = DefaultRsc::init(Arc::new(Queue(send)));
         let mut render = UiRenderState::new();
-        render.resize(size);
+        render.resize(size, rsc.widgets_mut());
         Self {
             rsc,
             render,
@@ -76,11 +157,18 @@ impl Harness {
     }
 
     pub fn size(&self) -> Vec2 {
-        self.render.output_size()
+        self.render.output_size().to_f32()
     }
 
     pub fn resize(&mut self, size: impl Into<Vec2>) {
-        self.render.resize(size);
+        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.
@@ -160,6 +248,23 @@ impl Harness {
         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 {
@@ -178,3 +283,51 @@ impl Harness {
         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

@@ -8,6 +8,7 @@
 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 +1,931 @@
+//! 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;
+
+/// The declared lengths of one widget carrying a size rule, by axis.
+pub type Lens = [Option<LayoutLen>; 2];
+
+/// Where one widget carrying an alignment sits, by axis. `None` uses the
+/// centered default.
+pub type Aligns = [Option<AxisAlign>; 2];
+
+/// 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, Lens>,
+    /// 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, Aligns>,
+    /// 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.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.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<Lens>,
+    /// The alignment it carries, under the same one-offer rule.
+    pub align: Option<Aligns>,
+    /// 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,
+    },
+    /// 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.
+            Kind::Wrapped => out.push(Kind::OneLine),
+            Kind::OneLine => 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(4) {
+            0 => Kind::Wrapped,
+            1 => Kind::OneLine,
+            _ => {
+                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) -> Aligns {
+        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 => [Some(AxisAlign::CENTER), y],
+            false => [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 = [self.len(), self.len()];
+        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);
+        // A row takes the height it is given rather than its tallest child,
+        // which is a rule beside it. Derived from an existing choice and
+        // consuming no randomness: a seed must keep growing the same tree
+        // when the generator gains another configuration.
+        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(),
+    };
+    let root = build.node(plan);
+    (root, build.tree)
+}
+
+struct Build<'a, Rsc> {
+    rsc: &'a mut Rsc,
+    tree: Tree,
+}
+
+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[0], lens[1]);
+            self.tree.sized.push(id);
+        }
+        if let Some(align) = plan.align {
+            let widgets = &mut self.rsc.ui_mut().widgets;
+            for (axis, align) in [Axis::X, Axis::Y].into_iter().zip(align) {
+                widgets.set_alignment(id, axis, align.unwrap_or_default());
+            }
+            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
+    }
+
+    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::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);
+                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

@@ -8,15 +8,11 @@ pub struct Image {
 impl Widget for Image {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         painter.primitive(&self.handle);
-        Size::abs(self.handle.size())
+        Size::px(self.handle.size())
     }
 
-    fn size_hint(&self, axis: Axis) -> Option<Len> {
-        Some(Len::abs(self.handle.size().axis(axis)))
-    }
-
-    fn on_resize(&self, _: Axis) -> OnResize {
-        OnResize::Scale
+    fn size_hint(&self, axis: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::px(self.handle.size()[axis]))
     }
 }
 

src/widget/mask.rs

@@ -6,12 +6,17 @@ pub struct Masked {
 
 impl Widget for Masked {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        painter.set_mask(painter.region());
-        painter.widget(&self.inner).size()
+        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
     }
 
-    /// It clips to the box it was given, not to the part its child used.
-    fn on_resize(&self, _: Axis) -> OnResize {
-        OnResize::Redraw
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
     }
 }

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

@@ -1,22 +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) -> Size {
-        // Drawn where it may be too big, then given its aligned box once its
-        // size is known.
-        let size = painter.widget(&self.inner).size();
-        let region = match self.align.tuple() {
-            (Some(x), Some(y)) => size.to_uivec2().align(RegionAlign { x, y }),
-            (Some(x), None) => UiRegion::new(size.x.apply_rest().align(x), UiSpan::FULL),
-            (None, Some(y)) => UiRegion::new(UiSpan::FULL, size.y.apply_rest().align(y)),
-            (None, None) => UiRegion::FULL,
-        };
-        painter.widget_within(&self.inner, region);
-        size
-    }
-}

src/widget/position/max_size.rs

@@ -1,25 +0,0 @@
-use crate::prelude::*;
-
-pub struct MaxSize {
-    pub inner: StrongWidget,
-    pub x: Option<Len>,
-    pub y: Option<Len>,
-}
-
-impl Widget for MaxSize {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        let child = painter.widget(&self.inner).size();
-        let output = painter.output_size();
-        Size {
-            x: capped(child.x, self.x, output.x),
-            y: capped(child.y, self.y, output.y),
-        }
-    }
-}
-
-fn capped(len: Len, max: Option<Len>, output: f32) -> Len {
-    match max {
-        Some(max) if len.apply_rest().to_abs(output) > max.apply_rest().to_abs(output) => max,
-        _ => len,
-    }
-}

src/widget/position/mod.rs

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

src/widget/position/offset.rs

@@ -7,7 +7,8 @@ pub struct Offset {
 
 impl Widget for Offset {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let region = UiRegion::FULL.offset(self.amt);
-        painter.widget_within(&self.inner, region).size()
+        painter
+            .widget_at(&self.inner, UiRegion::FULL.offset(self.amt))
+            .size()
     }
 }

src/widget/position/pad.rs

@@ -7,16 +7,26 @@ pub struct Pad {
 
 impl Widget for Pad {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let inner = painter
-            .widget_within(&self.inner, self.padding.region())
-            .size();
+        // 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: Len {
-                abs: inner.x.abs + self.padding.left + self.padding.right,
+            x: LayoutLen {
+                px: inner.x.px + self.padding.left + self.padding.right,
                 ..inner.x
             },
-            y: Len {
-                abs: inner.y.abs + self.padding.top + self.padding.bottom,
+            y: LayoutLen {
+                px: inner.y.px + self.padding.top + self.padding.bottom,
                 ..inner.y
             },
         }
@@ -24,22 +34,22 @@ impl Widget for Pad {
 }
 
 pub struct Padding {
-    pub left: f32,
-    pub right: f32,
-    pub top: f32,
-    pub bottom: f32,
+    pub left: Px,
+    pub right: Px,
+    pub top: Px,
+    pub bottom: Px,
 }
 
 impl Padding {
     pub const ZERO: Self = Self {
-        left: 0.0,
-        right: 0.0,
-        top: 0.0,
-        bottom: 0.0,
+        left: Px::ZERO,
+        right: Px::ZERO,
+        top: Px::ZERO,
+        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,
@@ -47,80 +57,82 @@ impl Padding {
             bottom: amt,
         }
     }
-    pub fn region(&self) -> UiRegion {
-        let mut region = UiRegion::FULL;
-        region.x.start.abs += self.left;
-        region.y.start.abs += self.top;
-        region.x.end.abs -= self.right;
-        region.y.end.abs -= self.bottom;
+    /// `region` less this padding on each side.
+    pub fn region_of(&self, mut region: UiRegion) -> UiRegion {
+        region.x.start.px += self.left;
+        region.y.start.px += self.top;
+        region.x.end.px -= self.right;
+        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,
-            bottom: 0.0,
+            ..Self::ZERO
         }
     }
     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

@@ -3,36 +3,75 @@ use crate::prelude::*;
 pub struct Scroll {
     inner: StrongWidget,
     axis: Axis,
-    amt: f32,
+    amt: Px,
     snap_end: bool,
-    container_len: f32,
-    content_len: f32,
+    container_len: Px,
+    content_len: Px,
 }
 
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let output_len = painter.output_size().axis(self.axis);
-        let container_len = painter.region().axis(self.axis).len();
-        // Drawn in the whole container to learn its length, then placed at
-        // the scrolled offset.
-        let child = painter.widget(&self.inner).size();
-        let content_len = child
-            .axis(self.axis)
-            .apply_rest()
-            .within_len(container_len)
-            .to_abs(output_len);
-        self.container_len = container_len.to_abs(output_len);
-        self.content_len = content_len;
+        let container_len = painter.px_len(self.axis);
+        // Asked in the whole viewport, then put at the scrolled offset.
+        let answer_len = painter
+            .widget_at(&self.inner, PlaceDesc::WHOLE.fills())
+            .len(self.axis);
+        let fixed = painter.to_px(answer_len.without_leftover(), self.axis);
+        self.container_len = container_len;
+        self.content_len = fixed.max(container_len);
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
         }
         self.update_amt();
+        let align = painter.alignment()[self.axis];
+        // Content of a fixed length that fits sits at the start of any box it
+        // fits in -- but only anchored there. Anywhere else it is a part of
+        // the room left over, so it moves with every length the box takes and
+        // the drawing holds for that length alone. 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 fixed_len = answer_len.is_px();
+        if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
+            painter.holds(self.axis, fixed..=Px::MAX);
+        } else if fixed_len && !self.snap_end {
+            let left = self.content_len - self.amt;
+            painter.holds(self.axis, Px::MIN..=left);
+        }
 
-        let mut region = UiRegion::FULL.offset(Vec2::from_axis(self.axis, -self.amt, 0.0));
-        region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
-        painter.widget_within(&self.inner, region);
-        child
+        // Content shorter than the viewport has room to sit in, and where it
+        // sits is this widget's own alignment -- the same property that would
+        // have placed the whole scroll in a box longer than it.
+        let slack = (self.container_len - self.content_len).max(Px::ZERO);
+        let anchor = slack.mul(align.rel());
+        // Content that fills the viewport and has not been scrolled is the
+        // viewport, and is handed back as it came. 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 moved = anchor != Px::ZERO || self.amt != Px::ZERO;
+        let content = match moved || self.content_len != self.container_len {
+            true => {
+                let start = Len::from_parts(Rel::ZERO, anchor - 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.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 size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
     }
 }
 
@@ -41,22 +80,24 @@ impl Scroll {
         Self {
             inner,
             axis,
-            amt: 0.0,
+            amt: Px::ZERO,
             snap_end: true,
-            container_len: 0.0,
-            content_len: 0.0,
+            container_len: Px::ZERO,
+            content_len: Px::ZERO,
         }
     }
 
     pub fn update_amt(&mut self) {
-        self.amt = self.amt.max(0.0);
-        let len = (self.content_len - self.container_len).max(0.0);
+        self.amt = self.amt.max(Px::ZERO);
+        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/set_size.rs

@@ -1,26 +0,0 @@
-use crate::prelude::*;
-
-pub struct SetSize {
-    pub inner: StrongWidget,
-    pub x: Option<Len>,
-    pub y: Option<Len>,
-}
-
-impl Widget for SetSize {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        let child = painter.widget(&self.inner).size();
-        Size {
-            x: self.x.unwrap_or(child.x),
-            y: self.y.unwrap_or(child.y),
-        }
-    }
-
-    /// A declared axis is known without looking at the child, which is what
-    /// lets a span lay out around `.height(rest(1))` without drawing it.
-    fn size_hint(&self, axis: Axis) -> Option<Len> {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-}

src/widget/position/span.rs

@@ -4,57 +4,152 @@ 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) -> Size {
         let axis = self.dir.axis;
-        // A length for every child before any is placed: from its own hint
-        // where it has one, and from drawing it where it does not.
-        let lens: Vec<Len> = self
-            .children
-            .iter()
-            .map(|child| match painter.size_hint(child, axis) {
+        // The row: this span's own box, as a length of the rel base its children
+        // are laid out against. Its start is nothing's business -- a slot is
+        // a length from it -- so what this reads is the length alone.
+        let far = painter.region_len(axis);
+        let along = |from: Len, to: Len| match self.dir.sign {
+            Sign::Pos => UiSpan::new(from, to),
+            Sign::Neg => UiSpan::new(far - to, far - from),
+        };
+        // 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::rel_min();
+        let mut lens = Vec::with_capacity(self.children.len());
+        for child in &self.children {
+            let len = match painter.size_hint(child, axis) {
                 Some(len) => len,
-                None => painter.widget(child).len(axis),
-            })
-            .collect();
-
-        let gap = self.gap * self.children.len().saturating_sub(1) as f32;
-        let total = lens.iter().fold(Len::abs(gap), |sum, len| sum + *len);
-
-        let mut start = UiScalar::rel_min();
-        let mut ortho = Len::ZERO;
-        for (child, len) in self.children.iter().zip(&lens) {
-            let mut span = UiSpan::FULL;
-            span.start = start;
-            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));
+                None => {
+                    // Across itself the child sits where its own alignment
+                    // says, in the whole of the row: a span is what contains
+                    // its children there, and nothing divides that axis.
+                    let room = along(cursor, far).shifted_desc().axis(axis);
+                    painter.widget_at(child, room).len(axis)
                 }
-            start.abs += len.abs;
-            start.rel += len.rel;
-            span.end = start;
-            let mut region = UiRegion::from_axis(axis, span, UiSpan::FULL);
-            if self.dir.sign == Sign::Neg {
-                region.flip(axis);
-            }
-            let used = painter.widget_within(child, region).size().axis(!axis);
-            // TODO: rel shouldn't do this, but no easy way before actually calculating pixels
-            if used.rel > 0.0 || used.rest > 0.0 {
-                ortho = Len::REST;
-            } else if ortho.rest == 0.0 {
-                ortho.abs = ortho.abs.max(used.abs);
-            }
-            start.abs += self.gap;
+            };
+            cursor.px += len.px + self.gap;
+            cursor.rel += len.rel;
+            lens.push(len);
         }
 
-        let along = match total.rest == 0.0 && total.rel == 0.0 {
-            true => total,
-            false => Len::default(),
+        let gaps = self
+            .gap
+            .mul_int(self.children.len().saturating_sub(1) as i32);
+        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 room = far - total.without_leftover();
+        // Whether anything is left over is a question in pixels: `rel(0.5)`
+        // beside 300 px is full at 600 and overfull at 400. Asked of `room`
+        // itself, and answered back through the same expression, so the
+        // boundary is the drawing's own and not a second way of finding it:
+        // 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 `through` already reads. What the
+        // generated oracle checks is the consequence, since which children
+        // exist at all turns on this.
+        let mut shares = false;
+        if total.leftover > Weight::ZERO {
+            shares = painter.to_px(room, axis) > Px::ZERO;
+            let holds = match shares {
+                true => Holds::from(Px::STEP..=Px::MAX),
+                false => Holds::from(Px::MIN..=Px::ZERO),
+            };
+            painter.window_holds(axis, holds.through(room));
+        }
+
+        // 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. A position is one from the other rather than a step
+        // from the last child: the share of the room is rounded, and taking
+        // each from the one before it would carry every rounding along the
+        // row.
+        let mut fixed = Len::rel_min();
+        let mut taken = Weight::ZERO;
+        let mut start = Len::rel_min();
+        let mut ortho = LayoutLen::ZERO;
+        let shared = |fixed: Len, taken: Weight| match taken == Weight::ZERO {
+            true => fixed,
+            false => 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() && !shares {
+                painter.undraw(child);
+                fixed.px += self.gap;
+                start = shared(fixed, taken);
+                continue;
+            }
+            let from = start;
+            if len.leftover > Weight::ZERO && shares {
+                taken += len.leftover;
+            }
+            fixed += len.without_leftover();
+            start = shared(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 = along(from, start);
+            let mut place = slot.shifted_desc().fills().axis(axis);
+            if len.leftover > Weight::ZERO && shares {
+                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 Children 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;
+            start = shared(fixed, taken);
+        }
+
+        // Carried whole rather than collapsed to one share: a span that sizes
+        // from its children does not resolve `leftover`, it passes the weight up,
+        // so nesting spans divides the same space rather than re-dividing a
+        // 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 along = total;
+        let ortho = match shrinks {
+            true => ortho,
+            false => LayoutLen::rel(1.0),
         };
         Size::from_axis(axis, along, ortho)
     }
@@ -65,12 +160,12 @@ impl Span {
         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
     }
 
@@ -86,7 +181,7 @@ impl Span {
 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)>,
 }
 
@@ -112,13 +207,13 @@ 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

@@ -13,21 +13,49 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
-        let mut size = Size::default();
+        // 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()
+            }
+            None => Size::LEFTOVER,
+        };
+        // 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::per_axis(|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() {
-            match i {
-                0 => painter.child_layer(),
-                _ => painter.next_layer(),
-            }
-            let drawn = painter.widget(child);
-            // Only the child that sizes the stack is read, so the others
-            // changing size does not redraw it.
             if sizing == Some(i) {
-                size = drawn.size();
+                continue;
             }
+            painter.child_layer_at(i);
+            painter.widget_at(child, place);
         }
         size
     }
+
+    /// 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 => Some(LayoutLen::LEFTOVER),
+            StackSize::Child(_) => None,
+        }
+    }
 }
 
 #[derive(Default, Debug)]

src/widget/rect.rs

@@ -35,16 +35,11 @@ impl Widget for Rect {
             thickness: self.thickness,
             inner_radius: self.inner_radius,
         });
-        Size::REST
+        Size::LEFTOVER
     }
 
-    fn size_hint(&self, _: Axis) -> Option<Len> {
-        Some(Len::REST)
-    }
-
-    /// Its box is its primitive's own region, so a new one is written there.
-    fn on_resize(&self, _: Axis) -> OnResize {
-        OnResize::Scale
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
     }
 }
 

src/widget/text/edit.rs

@@ -93,10 +93,6 @@ impl Widget for TextEdit {
         );
         size
     }
-
-    fn on_resize(&self, axis: Axis) -> OnResize {
-        self.view.on_resize(axis)
-    }
 }
 
 const CARET_WIDTH: f32 = 1.0;
@@ -130,7 +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;
     }
 
@@ -177,7 +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());
     }
 
@@ -190,7 +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
     }
@@ -268,7 +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);
     }
 
@@ -284,7 +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;
 

src/widget/text/mod.rs

@@ -14,11 +14,8 @@ pub struct Text {
 }
 
 pub struct TextView {
-    pub attrs: MutDetect<TextAttrs>,
-    pub buf: MutDetect<TextBuffer>,
-    // cache
-    tex: Option<RenderedText>,
-    width: Option<f32>,
+    pub attrs: TextAttrs,
+    pub buf: TextBuffer,
     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 {
-            attrs: attrs.into(),
-            buf: buf.into(),
-            tex: None,
-            width: None,
-            hint,
-        }
+        Self { attrs, buf, hint }
     }
 
     /// region where the text should be draw
@@ -52,22 +43,22 @@ impl TextView {
             .align(self.align)
     }
 
+    /// The text shaped for the width it is drawn in. The buffer keeps its
+    /// answers under the attrs too, so changing those asks a new question
+    /// rather than invalidating anything.
     fn render(&mut self, painter: &mut Painter) -> &RenderedText {
-        let width = if self.attrs.wrap {
-            Some(painter.px_size().x)
-        } else {
-            None
-        };
-        if width != self.width || self.tex.is_none() || self.attrs.changed || self.buf.changed {
-            self.width = width;
-            self.tex = Some(painter.render_text(&mut self.buf, &self.attrs, width));
-            self.attrs.changed = false;
-            self.buf.changed = false;
+        let width = self.attrs.wrap.then(|| painter.px_len(Axis::X));
+        // The shaper measures in floats, which is where a glyph advance comes
+        // from; what it answers goes back on the grid.
+        painter.render_text(&mut self.buf, &self.attrs, width.map(Px::to_f32));
+        if width.is_some() {
+            painter.holds(Axis::X, self.buf.width_holds());
         }
-        self.tex.as_ref().unwrap()
+        self.buf.rendered().expect("render_text placed the glyphs")
     }
+
     pub fn tex(&self) -> Option<&RenderedText> {
-        self.tex.as_ref()
+        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) {
@@ -83,28 +74,16 @@ impl TextView {
 
         let tex = self.render(painter);
         let region = tex.size.align(align);
-        let size = Size::abs(tex.size);
-        let within = region.within(&painter.region());
-        painter.glyphs(tex, within);
+        // The step at or above what the shaper measured, so a parent that
+        // hands back the length this reports hands back a box the longest
+        // 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)
     }
 
-    /// Wrapping reads the width it is offered, so a wider box reshapes it and
-    /// a taller one does not. Alignment matters too, and separately: glyphs
-    /// anchored to the start of an axis stay put when that extent changes,
-    /// but centred or end-aligned ones move even though the shaping stands.
-    pub fn on_resize(&self, axis: Axis) -> OnResize {
-        let reshapes = axis == Axis::X && self.attrs.wrap;
-        let anchored = match axis {
-            Axis::X => self.align.x,
-            Axis::Y => self.align.y,
-        } == AxisAlign::Neg;
-        match reshapes || !anchored {
-            true => OnResize::Redraw,
-            false => OnResize::Translate,
-        }
-    }
-
     pub fn content(&self) -> String {
         self.buf.text().to_string()
     }
@@ -131,10 +110,6 @@ impl Widget for Text {
         self.update_buf();
         self.view.draw(painter).1
     }
-
-    fn on_resize(&self, axis: Axis) -> OnResize {
-        self.view.on_resize(axis)
-    }
 }
 
 impl Deref for Text {

src/widget/trait_fns.rs

@@ -12,14 +12,23 @@ widget_trait! {
         }
     }
 
-    fn align(self, align: impl Into<Align>) -> impl WidgetFn<Rsc, Aligned> {
-        move |state| Aligned {
-            inner: self.add_strong(state),
-            align: align.into(),
+    fn align(self, align: impl Into<Align>) -> impl WidgetIdFn<Rsc, WL::Widget> {
+        // An axis left out keeps whatever it had, which is centered unless
+        // something else set it.
+        let align = align.into();
+        move |state| {
+            let id = self.add(state);
+            let widgets = &mut state.ui_mut().widgets;
+            for (axis, align) in [(Axis::X, align.x), (Axis::Y, align.y)] {
+                if let Some(align) = align {
+                    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, SetSize> {
+    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| SetSize {
-            inner: self.add_strong(state),
-            x: Some(size.x),
-            y: Some(size.y),
+        move |state| {
+            let id = self.add(state);
+            let widgets = &mut state.ui_mut().widgets;
+            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 {
-            inner: self.add_strong(state),
-            x: Some(len),
-            y: None,
+        move |state| {
+            let id = self.add(state);
+            state
+                .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 {
-            inner: self.add_strong(state),
-            x: None,
-            y: Some(len),
-        }
-    }
-
-    fn width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, SetSize> {
-        let len = len.into();
-        move |state| SetSize {
-            inner: self.add_strong(state),
-            x: Some(len),
-            y: None,
-        }
-    }
-
-    fn height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, SetSize> {
-        let len = len.into();
-        move |state| SetSize {
-            inner: self.add_strong(state),
-            x: None,
-            y: Some(len),
+        move |state| {
+            let id = self.add(state);
+            state
+                .ui_mut()
+                .widgets
+                .set_size_rule(id, Axis::Y, SizeRule::Exact(len));
+            id
         }
     }
 
@@ -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) {
-        let id = self.add_strong(state);
-        state.ui_mut().widgets[ptr].inner = Some(id);
+    // Named for the type it makes rather than as `wrapped`, which would read
+    // as the text setting. `widget_trait!` takes no attributes, so what it is
+    // for is on `Wrapper` itself.
+    fn wrapper(self) -> impl WidgetFn<Rsc, Wrapper> {
+        |state| Wrapper {
+            inner: Some(self.add_strong(state)),
+        }
     }
 }
 

src/widget/wrapper.rs

@@ -1,11 +1,19 @@
 use crate::prelude::*;
 use std::marker::Unsize;
 
-pub struct WidgetPtr {
+/// 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.
+pub struct Wrapper {
     pub inner: Option<StrongWidget>,
 }
 
-impl Widget for WidgetPtr {
+impl Widget for Wrapper {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         match &self.inner {
             Some(id) => painter.widget(id).size(),
@@ -14,7 +22,7 @@ impl Widget for WidgetPtr {
     }
 }
 
-impl WidgetPtr {
+impl Wrapper {
     pub fn new() -> Self {
         Self::default()
     }
@@ -35,7 +43,7 @@ impl WidgetPtr {
     }
 }
 
-impl Default for WidgetPtr {
+impl Default for Wrapper {
     fn default() -> Self {
         Self::empty()
     }

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.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.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().get_dyn_mut(wide);
+            h.rsc.widgets_mut().get_dyn_mut(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().get_dyn_mut(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().get_dyn_mut(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/layout.rs

@@ -0,0 +1,860 @@
+//! Where a frame puts things, with no window to put them in.
+
+use iris::harness::{Harness, assert_corners};
+use iris::prelude::*;
+
+/// A fixed 100 wide, and the rest of the 400 to its neighbour.
+fn two_rects(h: &mut Harness) -> (WidgetId, WidgetId) {
+    let left = rect(Color::RED).width(100).add(&mut h.rsc);
+    let right = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+    (left.id(), right.id())
+}
+
+#[test]
+fn a_span_gives_each_child_the_width_it_asked_for() {
+    let mut h = Harness::new((400, 200));
+    let (left, right) = two_rects(&mut h);
+
+    assert_corners!(h, left, (0, 0), (100, 200));
+    assert_corners!(h, right, (100, 0), (400, 200));
+}
+
+/// A span places each child in the room left after the one before, because a
+/// text has to wrap at the width actually there, but the child's region is
+/// the whole row. So two children asking for half each take the whole row
+/// between them, however much of it was left when each was asked, and a third
+/// overflows -- and a span passes its own region on unchanged, so a child of
+/// a nested span asking for half asks for half of the same row.
+#[test]
+fn a_span_reads_a_child_report_as_a_fraction_of_the_row() {
+    let mut h = Harness::new((400, 100));
+    let half = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut h.rsc);
+    let nested = (inner,).span(Dir::RIGHT).add(&mut h.rsc);
+    let tail = rect(Color::BLUE).width(100).add(&mut h.rsc);
+    h.set_root((half, nested, tail).span(Dir::RIGHT).width(rel(1.0)));
+
+    // The nested span is placed at the length it reported, and its own child
+    // asks for half of the row rather than half of that placement.
+    assert_corners!(h, nested, (200, 0), (400, 100));
+    assert_corners!(h, inner, (200, 0), (400, 100));
+    assert_corners!(h, tail, (400, 0), (500, 100));
+}
+
+/// The same fraction either way round: after a 100 px child in a 400 px row,
+/// `rel(0.5)` is 100 to 300 whether the child's own rule says so or the child
+/// drew half of what it was offered and reported that. Half the row, not half
+/// of the 300 px left of it.
+#[test]
+fn a_reported_fraction_is_of_the_row_like_a_declared_one() {
+    let mut declaring = Harness::new((400, 100));
+    let head = rect(Color::RED).width(100).add(&mut declaring.rsc);
+    let declared = rect(Color::GREEN).width(rel(0.5)).add(&mut declaring.rsc);
+    declaring.set_root((head, declared).span(Dir::RIGHT).width(rel(1.0)));
+    assert_corners!(declaring, declared, (100, 0), (300, 100));
+
+    let mut reporting = Harness::new((400, 100));
+    let head = rect(Color::RED).width(100).add(&mut reporting.rsc);
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut reporting.rsc);
+    let reported = (inner,).span(Dir::RIGHT).add(&mut reporting.rsc);
+    reporting.set_root((head, reported).span(Dir::RIGHT).width(rel(1.0)));
+    assert_corners!(reporting, reported, (100, 0), (300, 100));
+}
+
+/// What the fraction a child reports is of and what box it is offered are
+/// two different lengths, and only the first is the whole row: a text still
+/// wraps at the room actually left after its neighbour, so the same
+/// paragraph is taller where less of the row is left for it.
+#[test]
+fn a_text_in_a_span_wraps_at_the_room_left_rather_than_the_whole_row() {
+    let paragraph = "Wrapping shapes one source into as many lines as the box \
+                     leaves room for, so a paragraph's height is an answer.";
+    let height_after = |head_width: i32| {
+        let mut h = Harness::new((400, 400));
+        let head = rect(Color::RED).width(head_width).add(&mut h.rsc);
+        let text = wtext(paragraph).size(16).wrap(true).add(&mut h.rsc);
+        h.set_root((head, text).span(Dir::RIGHT).width(rel(1.0)));
+        let region = h.region(&text).unwrap();
+        (region.bot_right.y - region.top_left.y).to_f32()
+    };
+
+    let (crowded, whole_row) = (height_after(300), height_after(0));
+    assert!(crowded > whole_row, "{crowded} against {whole_row}");
+}
+
+/// Padding is an inset: it narrows the frame a fraction resolves against and
+/// adds itself back to the padded widget's reported length.
+#[test]
+fn a_pad_puts_its_padding_around_a_fraction_of_the_whole_box() {
+    let mut h = Harness::new((400, 100));
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut h.rsc);
+    let padded = (inner,).span(Dir::RIGHT).pad(10).add(&mut h.rsc);
+    let tail = rect(Color::BLUE).width(100).add(&mut h.rsc);
+    // Ruled to the window: a root reporting a fraction of it is otherwise
+    // placed inside it by its own alignment, which is not what is under test.
+    h.set_root((padded, tail).span(Dir::RIGHT).width(rel(1.0)));
+
+    assert_corners!(h, inner, (10, 10), (200, 90));
+    assert_corners!(h, padded, (0, 0), (210, 100));
+    assert_corners!(h, tail, (210, 0), (310, 100));
+}
+
+const PARAGRAPH: &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.";
+
+/// The worked example of what padding insets: in a 900 px row after a 24 px
+/// icon, a `rel(1.0)` inside `pad(16)` is 900 - 32 and overflows the row by
+/// the icon's width, while a wrapping text beside it is asked in the room
+/// left, 900 - 24 - 32, and wraps there.
+#[test]
+fn padding_keeps_the_rel_base_distinct_from_the_room_left_in_a_row() {
+    let mut h = Harness::new((900, 200));
+    let icon = rect(Color::RED).width(24).add(&mut h.rsc);
+    let fill = rect(Color::GREEN).width(rel(1.0)).add(&mut h.rsc);
+    let padded = fill.pad(16).add(&mut h.rsc);
+    h.set_root((icon, padded).span(Dir::RIGHT).width(rel(1.0)));
+    let fill_width = h.region(&fill).unwrap().size().x;
+    assert_eq!(fill_width, Px::from_int(868));
+
+    let mut h = Harness::new((900, 200));
+    let icon = rect(Color::RED).width(24).add(&mut h.rsc);
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let padded = text.pad(16).add(&mut h.rsc);
+    h.set_root((icon, padded).span(Dir::RIGHT).width(rel(1.0)));
+    let active = &h.render.active[&text.id()];
+    let window = h.render.output_size().x;
+    let asked = active.region.x.len().to_px(window);
+    assert_eq!(active.rel_base.x.to_px(window), Px::from_int(868));
+    assert_eq!(asked, Px::from_int(844));
+}
+
+/// The other way round: a share inside padding. A slot is a length of the
+/// row, which is already the padded width, so what the span decided reaches
+/// the child as it stands -- taking the padding off a second time would make
+/// `rel(1.0)` in the slot shorter than the slot.
+#[test]
+fn a_share_inside_padding_fills_the_slot_it_was_given() {
+    let mut h = Harness::new((900, 200));
+    let fill = rect(Color::GREEN).width(rel(1.0)).add(&mut h.rsc);
+    let first = Span {
+        children: vec![fill.add_strong(&mut h.rsc)],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .width(leftover(1))
+    .add(&mut h.rsc);
+    let second = rect(Color::BLUE).width(leftover(1)).add(&mut h.rsc);
+    let row = (first, second).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root(row.pad(16));
+
+    assert_eq!(h.region(&first).unwrap().size().x, Px::from_int(434));
+    assert_eq!(h.region(&fill).unwrap().size().x, Px::from_int(434));
+}
+
+/// The same padding in a share instead: the slot is 450, so both the
+/// fraction and the wrap are the slot less the padding, and the two agree.
+#[test]
+fn padding_narrows_both_rel_base_and_box_inside_a_share() {
+    let mut h = Harness::new((900, 200));
+    let fill = rect(Color::GREEN).width(rel(1.0)).add(&mut h.rsc);
+    let padded = fill.pad(16).width(leftover(1)).add(&mut h.rsc);
+    let other = rect(Color::BLUE).width(leftover(1)).add(&mut h.rsc);
+    h.set_root((padded, other).span(Dir::RIGHT).width(rel(1.0)));
+    assert_eq!(h.region(&fill).unwrap().size().x, Px::from_int(418));
+
+    let mut h = Harness::new((900, 200));
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let padded = text.pad(16).width(leftover(1)).add(&mut h.rsc);
+    let other = rect(Color::BLUE).width(leftover(1)).add(&mut h.rsc);
+    h.set_root((padded, other).span(Dir::RIGHT).width(rel(1.0)));
+    let active = &h.render.active[&text.id()];
+    let window = h.render.output_size().x;
+    assert_eq!(active.rel_base.x.to_px(window), Px::from_int(418));
+    assert_eq!(active.region.x.len().to_px(window), Px::from_int(418));
+}
+
+#[test]
+fn a_span_ruled_across_itself_does_not_measure_its_children_there() {
+    let mut h = Harness::new((400, 200));
+    let child = rect(Color::RED).height(40).add(&mut h.rsc);
+    let span = (child,).span(Dir::RIGHT).height(rel(1.0)).add(&mut h.rsc);
+    h.set_root(span);
+
+    assert_eq!(h.render.active[&span.id()].size.y, LayoutLen::rel(1.0));
+}
+
+#[test]
+fn a_span_reports_its_tallest_fixed_child() {
+    let mut h = Harness::new((400, 200));
+    let short = rect(Color::RED).height(40).add(&mut h.rsc);
+    let tall = rect(Color::BLUE).height(70).add(&mut h.rsc);
+    let span = (short, tall).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root(span);
+
+    assert_eq!(h.render.active[&span.id()].size.y, LayoutLen::px(70.0));
+}
+
+#[test]
+fn resizing_relays_out_against_the_new_output() {
+    let mut h = Harness::new((400, 200));
+    let (left, right) = two_rects(&mut h);
+
+    h.resize((800, 100));
+    assert!(h.needs_redraw());
+    h.frame();
+
+    assert_corners!(h, left, (0, 0), (100, 100));
+    assert_corners!(h, right, (100, 0), (800, 100));
+}
+
+#[test]
+fn an_empty_widget_takes_a_share_of_a_span() {
+    let mut h = Harness::new((400, 200));
+    let gap = ().add(&mut h.rsc);
+    let right = rect(Color::BLUE).width(100).add(&mut h.rsc);
+    h.set_root((gap, right).span(Dir::RIGHT));
+
+    assert_corners!(h, gap, (0, 0), (300, 200));
+    assert_corners!(h, right, (300, 0), (400, 200));
+}
+
+#[test]
+fn a_child_drawn_twice_moves_once() {
+    let mut h = Harness::new((400, 200));
+    // The span measures a child and then places it; listing it twice would
+    // move it twice. The span's own fixed total is shorter than the window,
+    // so the span is centred in it and everything under it carries that.
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let centered = inner.center().width(200).add(&mut h.rsc);
+    let left = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((left, centered).span(Dir::RIGHT));
+    assert_corners!(h, inner, (150, 0), (350, 200));
+
+    h.set_len(left, Axis::X, 150);
+    h.frame();
+
+    assert_corners!(h, inner, (175, 0), (375, 200));
+}
+
+#[test]
+fn alignment_accepts_an_arbitrary_fraction_and_changes_at_runtime() {
+    let mut h = Harness::new((400, 200));
+    let fixed = rect(Color::BLUE).sized((100, 100)).add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_alignment(fixed, Axis::X, AxisAlign::new(0.25));
+    h.rsc
+        .widgets_mut()
+        .set_alignment(fixed, Axis::Y, AxisAlign::NEG);
+    h.set_root(fixed);
+    assert_corners!(h, fixed, (75, 0), (175, 100));
+
+    h.rsc
+        .widgets_mut()
+        .set_alignment(fixed, Axis::X, AxisAlign::new(0.75));
+    h.frame();
+    assert_corners!(h, fixed, (225, 0), (325, 100));
+}
+
+#[test]
+fn a_resize_lands_where_a_cold_start_would() {
+    let build = |h: &mut Harness| {
+        let para = wtext(
+            "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.",
+        )
+        .size(20)
+        .wrap(true)
+        .pad(16)
+        .add(&mut h.rsc);
+        let below = rect(Color::RED).add(&mut h.rsc);
+        let root = (para, below).span(Dir::DOWN).pad(12);
+        h.set_root(root);
+        (para, below)
+    };
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_para, cold_below) = build(&mut cold);
+
+    let mut resized = Harness::new((1920, 1200));
+    let (para, below) = build(&mut resized);
+    resized.resize((900, 1200));
+    resized.frame();
+
+    assert_eq!(resized.region(&para), cold.region(&cold_para), "paragraph");
+    assert_eq!(resized.region(&below), cold.region(&cold_below), "below");
+}
+
+#[test]
+fn a_fixed_box_is_drawn_again_rather_than_stretched() {
+    let mut h = Harness::new((400, 400));
+    // The panel fills a stack sized by its sibling, so it is first asked in
+    // the whole box and then given the shorter one. Reusing it in that fixed
+    // box afterwards would leave it whatever height it happened to have.
+    let panel = rect(Color::BLUE).add(&mut h.rsc);
+    let leaf = rect(Color::RED).height(100).add(&mut h.rsc);
+    let stack = (panel, leaf)
+        .stack()
+        .size(StackSize::Child(1))
+        .add(&mut h.rsc);
+    h.set_root(stack.align(Align::TOP));
+    assert_corners!(h, panel, (0, 0), (400, 100));
+
+    h.set_len(leaf, Axis::Y, 250);
+    h.frame();
+
+    assert_corners!(h, panel, (0, 0), (400, 250));
+}
+
+#[test]
+fn a_moved_subtree_takes_its_children_with_it() {
+    let mut h = Harness::new((400, 400));
+    let first = rect(Color::RED).height(40).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let row = inner.pad(10).height(40).region_node().add(&mut h.rsc);
+    // 80 of fixed rows in a 400 window, so the span takes 80 and sits in the
+    // middle of what it was given.
+    h.set_root((first, row).span(Dir::DOWN));
+    assert_corners!(h, inner, (10, 210), (390, 230));
+
+    h.set_len(first, Axis::Y, 80);
+    h.frame();
+
+    // The row opted into one movable region, so its descendants follow one
+    // entry rather than having their primitive regions rewritten.
+    assert_corners!(h, inner, (10, 230), (390, 250));
+}
+
+#[test]
+fn a_fixed_length_child_keeps_it_when_the_box_around_it_grows() {
+    let mut h = Harness::new((400, 200));
+    let fixed = rect(Color::BLUE).width(50).add(&mut h.rsc);
+    let leftover = rect(Color::GREEN).add(&mut h.rsc);
+    let panel = (fixed, leftover).span(Dir::RIGHT).add(&mut h.rsc);
+    // Changing the bar's width is the only thing that changes the box the
+    // panel and everything under it was drawn for.
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, panel).span(Dir::RIGHT));
+    assert_corners!(h, fixed, (100, 0), (150, 200));
+    assert_corners!(h, leftover, (150, 0), (400, 200));
+
+    h.set_len(bar, Axis::X, 200);
+    h.frame();
+
+    // The panel's box is 100 shorter, so the fixed child is the same 50 wide
+    // against its new start and the one taking what is left absorbs the change.
+    assert_corners!(h, fixed, (200, 0), (250, 200));
+    assert_corners!(h, leftover, (250, 0), (400, 200));
+}
+
+#[test]
+fn a_box_with_a_fixed_length_can_be_stretched_on_its_other_axis() {
+    let mut h = Harness::new((400, 200));
+    // The row is 40 tall whatever happens, which used to make its drawing
+    // impossible to take out of: recovering a fraction of a box needs a
+    // relative extent, and it has none on that axis.
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let row = inner.pad(10).height(40).add(&mut h.rsc);
+    let filler = rect(Color::GREEN).add(&mut h.rsc);
+    // This column is an item in a row, so it takes the width left for it
+    // rather than asking for a full row-width in addition to the bar.
+    let column = (row, filler).span(Dir::DOWN).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, column).span(Dir::RIGHT));
+    assert_corners!(h, inner, (110, 10), (390, 30));
+
+    h.set_len(bar, Axis::X, 200);
+    h.frame();
+
+    assert_corners!(h, inner, (210, 10), (390, 30));
+}
+
+#[test]
+fn only_a_region_node_lengthens_the_chain_and_it_can_be_removed() {
+    let mut h = Harness::new((400, 200));
+    let leaf = rect(Color::BLUE).add(&mut h.rsc);
+    let buried = leaf.pad(4).pad(4).pad(4).pad(4).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, buried).span(Dir::RIGHT));
+
+    let move_idx = h.render.active[&leaf.id()].parent_move;
+    assert_eq!(h.render.moves.depth(move_idx), 0, "the window is no entry");
+
+    h.rsc.widgets_mut().set_region_node(buried, true);
+    h.frame();
+    let move_idx = h.render.active[&leaf.id()].parent_move;
+    assert_eq!(
+        h.render.moves.depth(move_idx),
+        1,
+        "the opted-in widget's region alone"
+    );
+
+    h.rsc.widgets_mut().set_region_node(buried, false);
+    h.frame();
+    let move_idx = h.render.active[&leaf.id()].parent_move;
+    assert_eq!(h.render.moves.depth(move_idx), 0);
+}
+
+/// A span that sizes from its children passes their `leftover` weight up
+/// than collapsing it to one share, so nesting divides the same space instead
+/// of re-dividing a share of it.
+#[test]
+fn nested_spans_divide_the_space_once_however_deep_the_nesting_is() {
+    let mut h = Harness::new((400, 200));
+    let (a, b, c, d) = (
+        rect(Color::RED).add(&mut h.rsc),
+        rect(Color::BLUE).add(&mut h.rsc),
+        rect(Color::GREEN).add(&mut h.rsc),
+        rect(Color::WHITE).add(&mut h.rsc),
+    );
+    let left = (a, b).span(Dir::RIGHT).add(&mut h.rsc);
+    let right = (c, d).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+
+    for (i, id) in [a, b, c, d].into_iter().enumerate() {
+        let x = i as f32 * 100.0;
+        assert_corners!(h, id, (x, 0), (x + 100.0, 200));
+    }
+}
+
+/// The same space, unevenly nested: weights carried up mean a share is a
+/// share of the whole, not of whatever branch a widget happens to sit in.
+///
+/// Each edge lands on the even division or one step below it, since a share
+/// is a fraction of the room and a truncating multiply gives up what that
+/// fraction does not divide. What stays exact is that each share starts
+/// where the last one ended and the row ends at its own edge.
+#[test]
+fn an_uneven_nesting_still_gives_every_share_the_same_length() {
+    let mut h = Harness::new((400, 200));
+    let (a, b, c, d) = (
+        rect(Color::RED).add(&mut h.rsc),
+        rect(Color::BLUE).add(&mut h.rsc),
+        rect(Color::GREEN).add(&mut h.rsc),
+        rect(Color::WHITE).add(&mut h.rsc),
+    );
+    let one = (a,).span(Dir::RIGHT).add(&mut h.rsc);
+    let three = (b, c, d).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((one, three).span(Dir::RIGHT));
+
+    let mut start = Px::ZERO;
+    for (i, id) in [a, b, c, d].into_iter().enumerate() {
+        let got = h.region(&id).expect("widget drew nothing");
+        let even = Px::from_int((i as i32 + 1) * 100);
+        assert_eq!(got.top_left, PxVec2::new(start, Px::ZERO), "share {i}");
+        assert_eq!(got.bot_right.y, Px::from_int(200), "share {i}");
+        assert!(
+            got.bot_right.x == even || got.bot_right.x == even.next_down(),
+            "share {i} ends at {:?}, not {even:?}",
+            got.bot_right.x
+        );
+        start = got.bot_right.x;
+    }
+    assert_eq!(
+        start,
+        Px::from_int(400),
+        "the row stopped short of its edge"
+    );
+}
+
+/// However many ways a row is divided, the shares add up to the row: each
+/// one is the fixed parts before it plus a share of the room, rather than a
+/// step from where the last one ended, so the roundings do not accumulate
+/// along it. Chained, two hundred of them ended a step short of the edge.
+#[test]
+fn a_row_of_equal_shares_fills_it_exactly() {
+    for n in [2usize, 3, 7, 64, 200] {
+        let mut h = Harness::new((1000, 100));
+        let mut ids = Vec::new();
+        let mut kids: Vec<StrongWidget> = Vec::new();
+        for _ in 0..n {
+            let kid = rect(Color::RED).add(&mut h.rsc);
+            ids.push(kid.id());
+            kids.push(kid.add_strong(&mut h.rsc));
+        }
+        let span = Span {
+            children: kids,
+            dir: Dir::RIGHT,
+            gap: Px::ZERO,
+        }
+        .add(&mut h.rsc);
+        h.set_root(span);
+        h.frame();
+
+        for (i, id) in ids.iter().enumerate() {
+            let at = h.region(id).expect("a share drew nothing").top_left.x;
+            let want = Px::from_f32(1000.0 * (i as f32) / (n as f32));
+            assert!(
+                (at - want).abs() <= Px::STEP,
+                "{n} shares: the {i}th starts at {at:?}, not {want:?}"
+            );
+        }
+        let end = h.region(ids.last().unwrap()).unwrap().bot_right.x;
+        assert_eq!(end, Px::from_int(1000), "{n} shares do not reach the edge");
+    }
+}
+
+/// Where the shader puts an edge: the fraction resolved against the window
+/// plus the pixel offset, taken to the boundary it composes to within half
+/// a step of. Kept in step with `snap_floor` in `prelude.wgsl`.
+fn drawn_edges(h: &Harness, id: WidgetId, axis: Axis) -> (f32, f32) {
+    let active = &h.render.active[&id];
+    let region = h.render.moves.resolve(active.move_idx, active.placement);
+    let dim = h.size()[axis];
+    let snap = |v: f32| (v + Px::STEP.to_f32() * 0.5).floor();
+    let edge = |s: Len| snap(s.rel.to_f32() * dim + s.px.to_f32());
+    let span = region[axis];
+    (edge(span.start), edge(span.end))
+}
+
+fn hairline(h: &mut Harness, marks: &mut Vec<WidgetId>) -> StrongWidget {
+    let mark = rect(Color::RED).width(1).add_strong(&mut h.rsc);
+    marks.push(mark.id());
+    mark
+}
+
+fn share(h: &mut Harness, inner: StrongWidget, ratio: f32) -> StrongWidget {
+    h.set_len(&inner, Axis::X, LayoutLen::leftover(ratio));
+    inner
+}
+
+/// Shares in weights no binary fraction lands on, a padding on one branch
+/// and not the other, so an edge falls near an integer as often as it can.
+fn hairlines(h: &mut Harness, depth: usize, marks: &mut Vec<WidgetId>) -> StrongWidget {
+    let mut span = Span::empty(Dir::RIGHT);
+    if depth == 0 {
+        let left = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let left = share(h, left, 3.0);
+        span.push(left);
+        let mark = hairline(h, marks);
+        span.push(mark);
+        let right = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let right = share(h, right, 7.0);
+        span.push(right);
+        return span.add_strong(&mut h.rsc);
+    }
+    let first = hairlines(h, depth - 1, marks);
+    let first = share(h, first, 3.0);
+    span.push(first);
+    let second = hairlines(h, depth - 1, marks);
+    let second = Pad {
+        padding: Padding {
+            left: Px::from_int(3),
+            right: Px::from_int(7),
+            top: Px::ZERO,
+            bottom: Px::ZERO,
+        },
+        inner: second,
+    }
+    .add_strong(&mut h.rsc);
+    let second = share(h, second, 5.0);
+    span.push(second);
+    span.add_strong(&mut h.rsc)
+}
+
+/// A one-pixel line is a pixel wherever it is drawn. Both edges of a fixed
+/// length share their box's fraction, so composing the chain moves them
+/// together and the shader's `floor` cannot round the pixel between them
+/// away -- only shift it. A separator that disappeared at one window size
+/// would be a defect no size comparison catches.
+#[test]
+fn a_one_pixel_line_keeps_its_pixel_through_a_chain() {
+    let mut h = Harness::new((1920, 1200));
+    let mut marks = Vec::new();
+    let root = hairlines(&mut h, 4, &mut marks);
+    h.state.set_root(root);
+    h.frame();
+    assert_eq!(marks.len(), 16);
+
+    for size in [(1920, 1200), (1919, 1201), (997, 1003), (1367, 733)] {
+        h.resize(size);
+        h.frame();
+        for mark in &marks {
+            let (start, end) = drawn_edges(&h, *mark, Axis::X);
+            assert_eq!(end - start, 1.0, "at {size:?}, mark {mark:?}");
+        }
+    }
+}
+
+/// A span short of room takes it from its shares, which go to nothing and
+/// then to nothing wider; the fixed lengths between them keep their pixels.
+/// Collapsing those to make room would delete a separator the caller asked
+/// for, which is worse than overflowing.
+#[test]
+fn a_span_out_of_room_shrinks_its_shares_and_not_its_fixed_lengths() {
+    let mut h = Harness::new((400, 20));
+    let mut marks = Vec::new();
+    let mut span = Span::empty(Dir::RIGHT);
+    for _ in 0..3 {
+        let share_of = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let share_of = share(&mut h, share_of, 1.0);
+        span.push(share_of);
+        let mark = hairline(&mut h, &mut marks);
+        span.push(mark);
+    }
+    let root = span.add_strong(&mut h.rsc);
+    h.state.set_root(root);
+    h.frame();
+
+    for width in [400, 10, 3, 1] {
+        h.resize((width, 20));
+        h.frame();
+        for mark in &marks {
+            let (start, end) = drawn_edges(&h, *mark, Axis::X);
+            assert_eq!(end - start, 1.0, "at {width} wide, mark {mark:?}");
+        }
+    }
+}
+
+#[test]
+fn only_a_pure_leftover_child_disappears_when_nothing_is_left() {
+    let mut h = Harness::new((100, 20));
+    let fixed = rect(Color::RED).width(100).add(&mut h.rsc);
+    let leftover = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((fixed, leftover).span(Dir::RIGHT));
+
+    assert_corners!(h, fixed, (0, 0), (100, 20));
+    assert_eq!(h.region(&leftover), None);
+
+    // An undrawn child remains a dependency of the span, so making room for
+    // it draws it without rebuilding the tree.
+    h.set_len(fixed, Axis::X, 60);
+    h.frame();
+    assert_corners!(h, leftover, (60, 0), (100, 20));
+
+    let mut h = Harness::new((100, 20));
+    let fixed = rect(Color::RED).width(100).add(&mut h.rsc);
+    let mixed = rect(Color::BLUE)
+        .width(LayoutLen::px(20) + LayoutLen::LEFTOVER)
+        .add(&mut h.rsc);
+    h.set_root((fixed, mixed).span(Dir::RIGHT));
+
+    // Pixels and fractions still overflow; only a child whose entire length
+    // is leftover is omitted.
+    assert_corners!(h, mixed, (100, 0), (120, 20));
+}
+
+#[test]
+fn leftover_children_disappear_at_the_exact_fixed_content_boundary() {
+    let mut h = Harness::new((100, 100));
+    let first = rect(Color::RED).height(90).add(&mut h.rsc);
+    let a = rect(Color::GREEN).add(&mut h.rsc);
+    let b = rect(Color::BLUE).add(&mut h.rsc);
+    let inner = (a, b).span(Dir::DOWN).gap(4).add(&mut h.rsc);
+    h.set_root((first, inner).span(Dir::DOWN));
+    assert!(h.region(&a).is_some());
+    assert!(h.region(&b).is_some());
+
+    h.set_len(first, Axis::Y, 96.0);
+    h.frame();
+
+    assert!(h.region(&a).is_none());
+    assert!(h.region(&b).is_none());
+}
+
+/// **A stack child smaller than the stack sits where its own alignment
+/// says.** `Stack` gives every child the box its sizing child defines and
+/// used to force the near edge on all of them; that override is owed only to
+/// the sizing child, which has already placed its own content in the box the
+/// stack derived from its answer. Every other child is handed a box that owes
+/// nothing to it, so where it sits in one bigger than itself is its own
+/// business -- and with the override it could not be aligned at all, which is
+/// what moved the `tabs` example's counters to the wrong corner.
+#[test]
+fn a_stack_child_smaller_than_the_stack_keeps_its_own_alignment() {
+    let mut h = Harness::new((400, 200));
+    let big = rect(Color::BLUE).add(&mut h.rsc);
+    let small = rect(Color::RED).sized((50, 50)).add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_alignment(small.id(), Axis::X, AxisAlign::POS);
+    let (a, b) = (big.add_strong(&mut h.rsc), small.add_strong(&mut h.rsc));
+    let children: Vec<StrongWidget> = vec![a, b];
+    h.set_root(Stack {
+        children,
+        size: StackSize::Default,
+    });
+
+    assert_corners!(h, big, (0, 0), (400, 200));
+    // The far edge on X because it asked for it, the middle on Y because
+    // that is the default.
+    assert_corners!(h, small, (350, 75), (400, 125));
+}
+/// Five children of one span, buried under three containers that are each a
+/// fraction of their parent so no length reaches the window without being
+/// composed and rounded on the way. Returns each child's drawn width and
+/// each gap between them, in pixels.
+fn row_under_fractions(kid: Option<LayoutLen>, gap: f32, box_w: f32) -> (Vec<Px>, Vec<Px>) {
+    let mut h = Harness::new((box_w, 400.0));
+    let mut ids = Vec::new();
+    let mut kids: Vec<StrongWidget> = Vec::new();
+    for _ in 0..5 {
+        let r = rect(Color::RED).add(&mut h.rsc);
+        if let Some(len) = kid {
+            h.rsc
+                .widgets_mut()
+                .set_size_rule(r.id(), Axis::X, SizeRule::Exact(len));
+        }
+        ids.push(r.id());
+        kids.push(r.add_strong(&mut h.rsc));
+    }
+    let span = Span {
+        children: kids,
+        dir: Dir::RIGHT,
+        gap: Px::from_f32(gap),
+    }
+    .add(&mut h.rsc);
+    let a = (span.width(rel(0.9)),).span(Dir::RIGHT).add(&mut h.rsc);
+    let b = (a.width(rel(0.8)),).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((b.width(rel(0.7)),).span(Dir::RIGHT));
+    let boxes: Vec<_> = ids
+        .iter()
+        .map(|id| h.region(id).expect("a child drew nothing"))
+        .collect();
+    (
+        boxes.iter().map(|b| b.bot_right.x - b.top_left.x).collect(),
+        boxes
+            .windows(2)
+            .map(|p| p[1].top_left.x - p[0].bot_right.x)
+            .collect(),
+    )
+}
+
+/// **A length given in pixels is that many pixels, wherever it ends up.** A
+/// gap and a declared width compose additively -- `Len::within` adds a part's
+/// own pixels rather than scaling them, and both ends of a gap carry the same
+/// fraction, so the multiply that rounds is the same on each -- which is why
+/// nesting the row inside fractions of fractions cannot move them. Swept over
+/// 2,100 box widths when this was written and exact at every one; five here,
+/// including widths that divide badly by five.
+#[test]
+fn a_length_in_pixels_is_that_many_pixels_however_it_is_nested() {
+    for box_w in [300.0, 1000.0, 1001.0, 1003.0, 1920.0] {
+        let want = Px::from_int(7);
+        let (_, gaps) = row_under_fractions(None, 7.0, box_w);
+        assert!(
+            gaps.iter().all(|g| *g == want),
+            "box {box_w}: gaps between leftover children are {gaps:?}"
+        );
+        let (widths, gaps) = row_under_fractions(Some(LayoutLen::px(100.0)), 7.0, box_w);
+        assert!(
+            gaps.iter().all(|g| *g == want),
+            "box {box_w}: gaps between fixed children are {gaps:?}"
+        );
+        assert!(
+            widths.iter().all(|w| *w == Px::from_int(100)),
+            "box {box_w}: declared widths came out {widths:?}"
+        );
+    }
+}
+
+/// **Children asking for the same share of a row are not the same length**,
+/// and this pins by how much rather than claiming they are equal. A position
+/// is the quantity that gets rounded, so the row fills exactly and no two
+/// children leave a seam; what that costs is a step or two between lengths
+/// that were asked for identically. Exact composition would shrink the
+/// spread, not remove it: five equal lengths cannot fill a row whose step
+/// count is not a multiple of five.
+#[test]
+fn equal_shares_differ_by_at_most_two_steps_and_fill_the_row() {
+    for kid in [None, Some(LayoutLen::rel(0.2))] {
+        for box_w in [300.0, 1000.0, 1001.0, 1003.0, 1920.0] {
+            let (widths, gaps) = row_under_fractions(kid, 0.0, box_w);
+            let spread = *widths.iter().max().unwrap() - *widths.iter().min().unwrap();
+            assert!(
+                spread <= Px::from_raw(2),
+                "box {box_w}, {kid:?}: widths {widths:?} spread {spread:?}"
+            );
+            assert!(
+                gaps.iter().all(|g| *g == Px::ZERO),
+                "box {box_w}, {kid:?}: children left seams {gaps:?}"
+            );
+        }
+    }
+}
+
+#[test]
+fn a_stack_sized_by_a_child_does_not_take_that_childs_fraction_twice() {
+    let mut h = Harness::new((400, 200));
+    let half = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
+    let behind = rect(Color::BLUE).add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![behind.add_strong(&mut h.rsc), half.add_strong(&mut h.rsc)],
+        size: StackSize::Child(1),
+    }
+    .add(&mut h.rsc);
+    h.set_root((stack,).span(Dir::RIGHT).width(rel(1.0)));
+
+    assert_corners!(h, stack, (0, 0), (200, 200));
+    assert_corners!(h, half, (0, 0), (200, 200));
+    assert_corners!(h, behind, (0, 0), (200, 200));
+}
+
+#[test]
+fn a_fixed_child_is_centered_in_its_wrappers_share() {
+    let mut h = Harness::new((600, 300));
+    let leaf = rect(Color::RED).sized((100, 100)).center().add(&mut h.rsc);
+    let wrapper = leaf
+        .wrapper()
+        .width(leftover(2))
+        .height(rel(1.0))
+        .add(&mut h.rsc);
+    let other = rect(Color::BLUE).width(200).add(&mut h.rsc);
+    h.set_root((other, wrapper).span(Dir::RIGHT));
+
+    assert_corners!(h, wrapper, (200, 0), (600, 300));
+    assert_corners!(h, leaf, (350, 100), (450, 200));
+
+    h.resize((900, 400));
+    h.frame();
+    assert_corners!(h, wrapper, (200, 0), (900, 400));
+    assert_corners!(h, leaf, (500, 150), (600, 250));
+}
+
+/// The root's frame is the window and its rule is a fraction of that, which
+/// is one resolution and not two: nothing above it narrowed anything.
+#[test]
+fn a_root_with_a_fraction_rule_is_that_fraction_of_the_window() {
+    let mut h = Harness::new((900, 200));
+    let root = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
+    h.set_root(root);
+    assert_eq!(h.region(&root).unwrap().size().x, Px::from_int(450));
+}
+
+#[test]
+fn a_collapsed_share_keeps_the_gaps_before_the_next_slot() {
+    for dir in [Dir::RIGHT, Dir::LEFT, Dir::DOWN, Dir::UP] {
+        for collapsed in [1, 2] {
+            let mut h = Harness::new((400, 400));
+            let head = rect(Color::RED).add(&mut h.rsc);
+            h.set_len(head, dir.axis, 200);
+            let tail = rect(Color::BLUE).add(&mut h.rsc);
+            let tail_len = 200 - 10 * (collapsed + 1);
+            h.set_len(tail, dir.axis, tail_len);
+            let mut children: Vec<StrongWidget> = vec![head.add_strong(&mut h.rsc)];
+            let mut shares = Vec::new();
+            for _ in 0..collapsed {
+                let share = rect(Color::GREEN).add(&mut h.rsc);
+                shares.push(share);
+                children.push(share.add_strong(&mut h.rsc));
+            }
+            children.push(tail.add_strong(&mut h.rsc));
+            h.set_root(Span {
+                children,
+                dir,
+                gap: Px::from_int(10),
+            });
+            for share in shares {
+                assert!(h.region(&share).is_none());
+            }
+            let region = h.region(&tail).unwrap();
+            let (from, to) = match dir.sign {
+                Sign::Pos => (400 - tail_len, 400),
+                Sign::Neg => (0, tail_len),
+            };
+            assert_eq!(region.top_left[dir.axis], Px::from_int(from));
+            assert_eq!(region.bot_right[dir.axis], Px::from_int(to));
+        }
+    }
+}

tests/cases/plan.rs

@@ -0,0 +1,121 @@
+//! 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::random::{Edits, Kind, Plan, Rng, SpanEdit, 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, [Some(LayoutLen::LEFTOVER), None]))
+            .collect(),
+        aligns: pick(aligned, &mut rng)
+            .into_iter()
+            .map(|i| (i, [Some(AxisAlign::POS), 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,
+    }
+}
+
+use iris::prelude::*;
+
+/// 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"
+        );
+    }
+}
+
+/// Every simplification is strictly smaller, so taking them in turn reaches a
+/// fixed point instead of circling. A shrinker that can return to a tree it
+/// has already tried does not stop.
+#[test]
+fn every_simplification_of_a_plan_is_smaller_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()
+        );
+    }
+}

tests/cases/pointer.rs

@@ -2,7 +2,7 @@
 
 use std::{cell::RefCell, rc::Rc};
 
-use iris::harness::Harness;
+use iris::harness::{Harness, TouchScript};
 use iris::prelude::*;
 
 #[test]
@@ -58,3 +58,28 @@ fn hover_ends_when_the_cursor_leaves_the_window() {
     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/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().get_dyn_mut(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);
+    h.set_root(clipper);
+    h.frame();
+}
+
+/// 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/chain_cost.rs

@@ -0,0 +1,223 @@
+//! 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.
+//!
+//! The instance is leaked deliberately, for the reason `draw_cost.rs` gives.
+
+use iris::prelude::*;
+use iris_core::{
+    Len, MaskIdx, MoveIdx, PrimitiveInst, RectPrimitive, UiData, UiRegion, UiRenderNode,
+    UiRenderState, UiSpan,
+};
+use wgpu::{Color as GpuColor, *};
+
+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 all = Instance::new(InstanceDescriptor::new_without_display_handle());
+    let instance = match pollster::block_on(all.request_adapter(&RequestAdapterOptions::default()))
+    {
+        Ok(_) => all,
+        Err(_) => Instance::new(InstanceDescriptor {
+            backends: Backends::GL,
+            ..InstanceDescriptor::new_without_display_handle()
+        }),
+    };
+    let instance: &'static Instance = Box::leak(Box::new(instance));
+    let adapter =
+        pollster::block_on(instance.request_adapter(&RequestAdapterOptions::default())).ok()?;
+    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))
+}
+
+fn config(format: TextureFormat) -> SurfaceConfiguration {
+    SurfaceConfiguration {
+        usage: TextureUsages::RENDER_ATTACHMENT,
+        format,
+        color_space: SurfaceColorSpace::Auto,
+        width: SIZE,
+        height: SIZE,
+        present_mode: PresentMode::Fifo,
+        desired_maximum_frame_latency: 2,
+        alpha_mode: CompositeAlphaMode::Auto,
+        view_formats: vec![],
+    }
+}
+
+/// 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, &config(format));
+    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

@@ -22,8 +22,8 @@ use std::time::Instant;
 
 use iris::prelude::*;
 use iris_core::{
-    GlyphPrimitive, MaskIdx, PrimitiveInst, RectPrimitive, TextureHandle, TexturePrimitive, UiData,
-    UiRegion, UiRenderNode, UiRenderState,
+    GlyphPrimitive, MaskIdx, MoveIdx, PrimitiveInst, RectPrimitive, TextureHandle,
+    TexturePrimitive, UiData, UiRegion, UiRenderNode, UiRenderState,
 };
 use wgpu::{Color as GpuColor, *};
 
@@ -95,6 +95,7 @@ fn fill(
                     primitive: RectPrimitive::color(UiColor::WHITE),
                     region: UiRegion::FULL,
                     mask_idx: MaskIdx::NONE,
+                    move_idx: MoveIdx::NONE,
                 },
             );
             render.layers.write(
@@ -111,6 +112,7 @@ fn fill(
                     },
                     region: UiRegion::FULL,
                     mask_idx: MaskIdx::NONE,
+                    move_idx: MoveIdx::NONE,
                 },
             );
         }
@@ -123,6 +125,7 @@ fn fill(
                     primitive: TexturePrimitive::from(h),
                     region: UiRegion::FULL,
                     mask_idx: MaskIdx::NONE,
+                    move_idx: MoveIdx::NONE,
                 },
             );
         }

tests/generated.rs

@@ -0,0 +1,130 @@
+//! Laying a tree out again has to land where growing it that way would.
+//!
+//! Every case is one of `scenario`'s, over the trees `iris::random` grows
+//! from a seed. The fast test takes a handful of seeds and the ignored one
+//! takes as many as it is asked for; both run the same cases the shrinker
+//! does over the same trees, so a seed that fails here is reduced by
+//!
+//!     SHRINK_SEED=<seed> SHRINK_DEPTH=<depth> SHRINK_CASE=<case> \
+//!       cargo test --release --test shrink -- --ignored --nocapture
+//!
+//! `IRIS_GENERATED_SEED`, `IRIS_GENERATED_SEEDS` and `IRIS_GENERATED_DEPTH`
+//! select what the long run covers.
+
+#[path = "scenario/mod.rs"]
+mod scenario;
+
+use iris::random::{Edits, Plan, plan};
+use scenario::{ALL, Case, diverges, env, over_seeds};
+
+/// How deep the generator branches. The generator widens two to four ways per
+/// level, so depth is exponential in width and a deep narrow tree is not
+/// reachable by raising this -- it buys more overlap between dependency
+/// paths, not more ancestry.
+fn depth() -> usize {
+    env("IRIS_GENERATED_DEPTH", 4)
+}
+
+/// The seeds the ordinary tests take. Seven that have never failed; 86,
+/// which a `Scroll` fixed point once settled differently on; and 20, which
+/// caught a locally redrawn widget being placed twice in the box its parent
+/// had already placed it in.
+const SEEDS: [u64; 10] = [1, 2, 3, 5, 8, 10, 13, 20, 86, 98];
+
+fn check(seed: u64, depth: usize, case: Case) {
+    check_plan(&plan(seed, depth, &Edits::default()), seed, depth, case);
+}
+
+fn check_plan(grown: &Plan, seed: u64, depth: usize, case: Case) {
+    if let Some(how) = diverges(grown, case, seed) {
+        panic!(
+            "seed {seed} at depth {depth} differs after {}: {how}\n\
+             reduce it with SHRINK_SEED={seed} SHRINK_DEPTH={depth} \
+             SHRINK_CASE={} cargo test --release --test shrink -- --ignored --nocapture",
+            case.name(),
+            case.name(),
+        );
+    }
+}
+
+macro_rules! case {
+    ($name:ident, $case:expr) => {
+        #[test]
+        fn $name() {
+            for seed in SEEDS {
+                check(seed, depth(), $case);
+            }
+        }
+    };
+}
+
+case!(
+    many_widgets_redrawing_at_once_leaves_every_box_where_it_was,
+    Case::RepaintSome
+);
+case!(
+    everything_redrawing_at_once_leaves_every_box_where_it_was,
+    Case::Repaint
+);
+case!(
+    a_resize_lands_where_starting_at_that_size_would,
+    Case::Resize
+);
+case!(
+    a_resize_and_a_repaint_land_where_starting_that_way_would,
+    Case::ResizeRepaint
+);
+case!(
+    a_size_change_after_a_resize_lands_the_same_way,
+    Case::ResizeSize
+);
+case!(
+    a_size_change_lands_where_growing_it_that_way_would,
+    Case::Size
+);
+case!(
+    every_size_changing_at_once_lands_where_growing_it_that_way_would,
+    Case::EverySize
+);
+case!(
+    an_alignment_change_lands_where_growing_it_that_way_would,
+    Case::Align
+);
+case!(
+    giving_and_taking_a_movable_region_rebuilds_what_resolves_it,
+    Case::RegionNode
+);
+case!(
+    reordering_a_span_lands_where_growing_it_that_way_would,
+    Case::Reorder
+);
+
+#[test]
+fn adding_and_removing_span_children_lands_where_growing_it_that_way_would() {
+    for case in ALL {
+        if matches!(case, Case::Shuffle(_)) {
+            for seed in SEEDS {
+                check(seed, depth(), case);
+            }
+        }
+    }
+}
+
+#[test]
+#[ignore = "as many seeds as it is asked for, rather than the nine the others check"]
+fn a_long_run_of_seeds_agrees() {
+    let depth = depth();
+    let seeds: Vec<u64> = match std::env::var("IRIS_GENERATED_SEED")
+        .ok()
+        .and_then(|v| v.parse().ok())
+    {
+        Some(seed) => vec![seed],
+        None => (1..=env("IRIS_GENERATED_SEEDS", 100_u64)).collect(),
+    };
+    over_seeds(seeds, |seed| {
+        let grown = plan(seed, depth, &Edits::default());
+        for case in ALL {
+            check_plan(&grown, seed, depth, case);
+        }
+    });
+}

tests/layout.rs

@@ -1,111 +0,0 @@
-//! Where a frame puts things, with no window to put them in.
-
-use iris::harness::{Harness, assert_corners};
-use iris::prelude::*;
-
-/// A fixed 100 wide, and the rest of the 400 to its neighbour.
-fn two_rects(h: &mut Harness) -> (WidgetId, WidgetId) {
-    let left = rect(Color::RED).width(100).add(&mut h.rsc);
-    let right = rect(Color::BLUE).add(&mut h.rsc);
-    h.set_root((left, right).span(Dir::RIGHT));
-    (left.id(), right.id())
-}
-
-#[test]
-fn a_span_gives_each_child_the_width_it_asked_for() {
-    let mut h = Harness::new((400, 200));
-    let (left, right) = two_rects(&mut h);
-
-    assert_corners!(h, left, (0, 0), (100, 200));
-    assert_corners!(h, right, (100, 0), (400, 200));
-}
-
-#[test]
-fn resizing_relays_out_against_the_new_output() {
-    let mut h = Harness::new((400, 200));
-    let (left, right) = two_rects(&mut h);
-
-    h.resize((800, 100));
-    assert!(h.needs_redraw());
-    h.frame();
-
-    assert_corners!(h, left, (0, 0), (100, 100));
-    assert_corners!(h, right, (100, 0), (800, 100));
-}
-
-#[test]
-fn an_empty_widget_takes_a_share_of_a_span() {
-    let mut h = Harness::new((400, 200));
-    let gap = ().add(&mut h.rsc);
-    let right = rect(Color::BLUE).width(100).add(&mut h.rsc);
-    h.set_root((gap, right).span(Dir::RIGHT));
-
-    assert_corners!(h, gap, (0, 0), (300, 200));
-    assert_corners!(h, right, (300, 0), (400, 200));
-}
-
-#[test]
-fn a_child_drawn_twice_moves_once() {
-    let mut h = Harness::new((400, 200));
-    // `Aligned` draws its child twice; listing it twice would move it twice.
-    let inner = rect(Color::BLUE).add(&mut h.rsc);
-    let centered = inner.center().width(200).add(&mut h.rsc);
-    let left = rect(Color::RED).width(100).add(&mut h.rsc);
-    h.set_root((left, centered).span(Dir::RIGHT));
-    assert_corners!(h, inner, (100, 0), (300, 200));
-
-    h.rsc[left].x = Some(Len::abs(150));
-    h.frame();
-
-    assert_corners!(h, inner, (150, 0), (350, 200));
-}
-
-#[test]
-fn a_resize_lands_where_a_cold_start_would() {
-    let build = |h: &mut Harness| {
-        let para = wtext(
-            "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.",
-        )
-        .size(20)
-        .wrap(true)
-        .pad(16)
-        .add(&mut h.rsc);
-        let below = rect(Color::RED).add(&mut h.rsc);
-        let root = (para, below).span(Dir::DOWN).pad(12);
-        h.set_root(root);
-        (para, below)
-    };
-
-    let mut cold = Harness::new((900, 1200));
-    let (cold_para, cold_below) = build(&mut cold);
-
-    let mut resized = Harness::new((1920, 1200));
-    let (para, below) = build(&mut resized);
-    resized.resize((900, 1200));
-    resized.frame();
-
-    assert_eq!(resized.region(&para), cold.region(&cold_para), "paragraph");
-    assert_eq!(resized.region(&below), cold.region(&cold_below), "below");
-}
-
-#[test]
-fn a_fixed_box_is_drawn_again_rather_than_stretched() {
-    let mut h = Harness::new((400, 400));
-    // The panel fills a stack sized by its sibling, so it is drawn in the
-    // whole box and then placed in the shorter one. Reusing it in that fixed
-    // box afterwards would leave it whatever height it happened to have.
-    let panel = rect(Color::BLUE).add(&mut h.rsc);
-    let leaf = rect(Color::RED).height(100).add(&mut h.rsc);
-    let stack = (panel, leaf)
-        .stack()
-        .size(StackSize::Child(1))
-        .add(&mut h.rsc);
-    h.set_root(stack.align(Align::TOP));
-    assert_corners!(h, panel, (0, 0), (400, 100));
-
-    h.rsc[leaf].y = Some(Len::abs(250));
-    h.frame();
-
-    assert_corners!(h, panel, (0, 0), (400, 250));
-}

tests/layout_diagnostics.rs

@@ -0,0 +1,256 @@
+//! Retained CPU-layout diagnostics on one reproducible random tree.
+//!
+//! Counters and phase timers:
+//!
+//!     cargo test --release --features layout-diagnostics \
+//!       --test layout_diagnostics -- --ignored --nocapture
+//!
+//! Build the uninstrumented test with `cargo test --release --test
+//! layout_diagnostics --no-run`, then run the emitted executable directly:
+//!
+//!     IRIS_PHASE=resize IRIS_FRAMES=10000 perf stat -r 7 \
+//!       -e cycles:u,instructions:u /path/to/layout_diagnostics --ignored --nocapture
+//!
+//! `IRIS_PHASE` is `cold`, `repaint`, `many`, `size`, `scroll`, `resize`, or
+//! `all`. `IRIS_SEED`, `IRIS_DEPTH`, and `IRIS_FRAMES` select the load, and
+//! `IRIS_DIRTY` how many widgets `many` marks at once.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+use iris::random::{Edits, Tree, grow};
+use std::time::Instant;
+
+const OUTPUT: (f32, f32) = (1920.0, 1200.0);
+
+#[cfg(feature = "layout-diagnostics")]
+#[test]
+fn a_selected_widget_retains_its_layout_events() {
+    use iris::core::layout_diagnostics::{self as diagnostics, TraceEvent};
+
+    diagnostics::clear_traced_widgets();
+    let _ = diagnostics::take();
+    let mut harness = Harness::new((400, 200));
+    let leaf = rect(Color::RED).region_node().add(&mut harness.rsc);
+    let other = rect(Color::BLUE).add(&mut harness.rsc);
+    let root = (leaf, other).span(Dir::RIGHT).add(&mut harness.rsc);
+    harness.set_root(root);
+    diagnostics::trace_widget(leaf.id());
+    let _ = diagnostics::take();
+
+    let _ = harness.rsc.widgets_mut().get_dyn_mut(root.id());
+    let _ = harness.rsc.widgets_mut().get_dyn_mut(leaf.id());
+    harness.frame();
+
+    let report = diagnostics::take();
+    assert!(
+        report
+            .traces()
+            .iter()
+            .any(|event| matches!(event, TraceEvent::RegionNode { id, .. } if *id == leaf.id()))
+    );
+    assert!(
+        report
+            .traces()
+            .iter()
+            .any(|event| matches!(event, TraceEvent::DrawRequest { id, .. } if *id == leaf.id()))
+    );
+    assert!(
+        report
+            .traces()
+            .iter()
+            .any(|event| matches!(event, TraceEvent::SizeRead { id, .. } if *id == leaf.id()))
+    );
+    assert!(
+        report
+            .traces()
+            .iter()
+            .any(|event| matches!(event, TraceEvent::SizeReported { id, .. } if *id == leaf.id()))
+    );
+    diagnostics::clear_traced_widgets();
+}
+
+fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|value| value.parse().ok())
+        .unwrap_or(fallback)
+}
+
+#[cfg(feature = "layout-diagnostics")]
+fn trace_selected(tree: &Tree) {
+    let Ok(value) = std::env::var("IRIS_TRACE_INDEX") else {
+        return;
+    };
+    let index = value
+        .parse::<usize>()
+        .expect("IRIS_TRACE_INDEX must be a tree.ids index");
+    let id = tree.ids[index];
+    iris::core::layout_diagnostics::trace_widget(id);
+    println!("tracing tree.ids[{index}] = {id:?}");
+}
+
+#[cfg(not(feature = "layout-diagnostics"))]
+fn trace_selected(_: &Tree) {}
+
+/// The shape a cost is measured on must not depend on what layout measured,
+/// or two commits are compared on two different trees. See `Edits`.
+fn rig_edits() -> Edits {
+    Edits {
+        fixed_branches: true,
+        ..Default::default()
+    }
+}
+
+fn warm(seed: u64, depth: usize) -> (Harness, Tree) {
+    let mut harness = Harness::new(OUTPUT);
+    let (root, tree) = grow(&mut harness.rsc, seed, depth, &rig_edits());
+    harness.state.root = Some(root);
+    harness.frame();
+    println!(
+        "fixture: seed {seed}, depth {depth}, {} widgets, {} active",
+        tree.ids.len(),
+        harness.render.active_widgets()
+    );
+    #[cfg(feature = "layout-diagnostics")]
+    let _ = iris::core::layout_diagnostics::take();
+    (harness, tree)
+}
+
+fn report(label: &str, mut elapsed: Vec<f64>, _harness: &Harness) {
+    elapsed.sort_by(|a, b| a.partial_cmp(b).unwrap());
+    let frames = elapsed.len();
+    // The worst frame is the stutter somebody sees, so it goes beside the
+    // median; p99 says whether it is the load or a single interruption.
+    println!(
+        "{label}: {frames} frame(s), min {:.3} ms, median {:.3} ms, p99 {:.3} ms, \
+         max {:.3} ms, total {:.1} ms",
+        elapsed[0],
+        elapsed[frames / 2],
+        elapsed[frames * 99 / 100],
+        elapsed[frames - 1],
+        elapsed.iter().sum::<f64>(),
+    );
+    #[cfg(feature = "layout-diagnostics")]
+    {
+        let diagnostics = iris::core::layout_diagnostics::take();
+        print!("{}", diagnostics.per_frame(frames));
+        for event in diagnostics.traces() {
+            println!("    {event:?}");
+        }
+        for callsite in diagnostics.hot_text().iter().take(3) {
+            let mut ancestry = Vec::new();
+            let mut id = Some(callsite.id);
+            while let Some(widget) = id {
+                ancestry.push(_harness.rsc.widgets().label(widget).as_str());
+                id = _harness
+                    .render
+                    .active
+                    .get(&widget)
+                    .and_then(|active| active.parent);
+            }
+            println!("    text ancestry: {}", ancestry.join(" < "));
+        }
+    }
+}
+
+fn run(
+    label: &str,
+    frames: usize,
+    harness: &mut Harness,
+    mut change: impl FnMut(&mut Harness, usize),
+) {
+    let mut elapsed = Vec::with_capacity(frames);
+    for frame in 0..frames {
+        change(harness, frame);
+        let start = Instant::now();
+        harness.frame();
+        elapsed.push(start.elapsed().as_secs_f64() * 1_000.0);
+    }
+    report(label, elapsed, harness);
+}
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn layout_cost() {
+    let seed = env("IRIS_SEED", 1_u64);
+    let depth = env("IRIS_DEPTH", 7_usize);
+    let frames = env("IRIS_FRAMES", 100_usize);
+    assert!(frames > 0, "IRIS_FRAMES must be greater than zero");
+    let phase = env("IRIS_PHASE", String::from("all"));
+    assert!(
+        ["all", "cold", "repaint", "many", "size", "scroll", "resize"].contains(&phase.as_str()),
+        "unknown IRIS_PHASE {phase:?}"
+    );
+    let selected = |name| phase == "all" || phase == name;
+
+    if selected("cold") {
+        let mut harness = Harness::new(OUTPUT);
+        let (root, tree) = grow(&mut harness.rsc, seed, depth, &rig_edits());
+        harness.state.root = Some(root);
+        println!(
+            "fixture: seed {seed}, depth {depth}, {} widgets",
+            tree.ids.len()
+        );
+        trace_selected(&tree);
+        #[cfg(feature = "layout-diagnostics")]
+        let _ = iris::core::layout_diagnostics::take();
+        run("cold", 1, &mut harness, |_, _| {});
+        drop(tree);
+    }
+
+    if selected("repaint") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        let leaf = tree.ids[0];
+        run("repaint", frames, &mut harness, move |harness, _| {
+            let _ = harness.rsc.widgets_mut().get_dyn_mut(leaf);
+        });
+    }
+
+    if selected("many") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        // Spread through the tree rather than taken from one subtree, so the
+        // dependency paths the frame settles overlap.
+        let wanted = env("IRIS_DIRTY", 32_usize).max(1);
+        let step = (tree.ids.len() / wanted).max(1);
+        let dirty: Vec<_> = tree.ids.iter().copied().step_by(step).collect();
+        println!("marking {} of {} widgets", dirty.len(), tree.ids.len());
+        run("many", frames, &mut harness, move |harness, _| {
+            for &id in &dirty {
+                harness.rsc.widgets_mut().get_dyn_mut(id);
+            }
+        });
+    }
+
+    if selected("size") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        let sized = tree.sized[0];
+        run("size", frames, &mut harness, move |harness, frame| {
+            let len = LayoutLen::px(100.0 + (frame % 2) as f32 * 40.0);
+            harness
+                .rsc
+                .widgets_mut()
+                .set_size_rule(sized, Axis::X, SizeRule::Exact(len));
+        });
+    }
+
+    if selected("scroll") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        let scroll = tree.scrolls[0];
+        run("scroll", frames, &mut harness, move |harness, frame| {
+            harness.rsc[scroll].scroll(if frame % 2 == 0 { 12.0 } else { -12.0 });
+        });
+    }
+
+    if selected("resize") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        run("resize", frames, &mut harness, |harness, frame| {
+            harness.resize((OUTPUT.0 - ((frame + 1) % 2) as f32 * 8.0, OUTPUT.1));
+        });
+        drop(tree);
+    }
+}

tests/layout_dump.rs

@@ -0,0 +1,42 @@
+//! Prints where a cold layout puts every widget of many grown trees, so two
+//! commits can be compared on cold layout alone. The warm/cold oracle cannot
+//! see a change that moves cold layout, since both of its sides move; this
+//! can, by diffing its output across the change:
+//!
+//!     IRIS_DUMP_SEEDS=400 IRIS_DUMP_DEPTH=5 cargo test --release \
+//!       --test layout_dump -- --ignored --nocapture > /tmp/before.txt
+//!
+//! then the same after, and `diff` the two. A line is one widget: the seed,
+//! its index in creation order, and its box in window pixels, or `-` where
+//! it is not drawn.
+
+use iris::harness::Harness;
+use iris::random::{Edits, grow};
+
+fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|value| value.parse().ok())
+        .unwrap_or(fallback)
+}
+
+#[test]
+#[ignore = "a dump to diff across commits, not a check"]
+fn every_cold_layout_is_printed() {
+    let seeds = env("IRIS_DUMP_SEEDS", 400_u64);
+    let depth = env("IRIS_DUMP_DEPTH", 5_usize);
+    let mut out = String::new();
+    for seed in 1..=seeds {
+        let mut harness = Harness::new((1920.0, 1200.0));
+        let (root, tree) = grow(&mut harness.rsc, seed, depth, &Edits::default());
+        harness.state.root = Some(root);
+        harness.frame();
+        for (index, id) in tree.ids.iter().enumerate() {
+            match harness.region(id) {
+                Some(region) => out.push_str(&format!("{seed} {index} {region:?}\n")),
+                None => out.push_str(&format!("{seed} {index} -\n")),
+            }
+        }
+    }
+    print!("{out}");
+}

tests/remap.rs

@@ -1,27 +0,0 @@
-//! What a drawing can be taken out of, and what it cannot.
-
-use iris::core::{Remap, UiRegion, UiScalar, UiSpan};
-
-/// A box `size` tall whose top is `rel` of the way down the window.
-fn fixed(rel: f32, size: f32) -> UiRegion {
-    UiRegion::new(
-        UiSpan::FULL,
-        UiSpan::new(UiScalar { rel, abs: 0.0 }, UiScalar { rel, abs: size }),
-    )
-}
-
-#[test]
-fn a_fixed_box_can_be_carried_but_not_stretched() {
-    let from = fixed(0.0, 164.0);
-    assert!(Remap::new(from, UiRegion::FULL).is_none());
-    assert!(Remap::new(from, fixed(0.5, 164.0)).is_some());
-    assert!(Remap::new(from, fixed(0.0, 98.0)).is_none());
-}
-
-#[test]
-fn a_relative_box_can_be_stretched_to_any_other() {
-    let remap = Remap::new(UiRegion::FULL, fixed(0.0, 98.0)).expect("relative boxes remap");
-    // A part that filled the window keeps filling what replaced it, which is
-    // exactly what `outside` could not say for a box of a fixed length.
-    assert_eq!(remap.apply(UiRegion::FULL), fixed(0.0, 98.0));
-}

tests/replace_cost.rs

@@ -0,0 +1,41 @@
+//! What remapping a subtree costs per frame, as a load for a counter rather
+//! than a check. A span of 200 fixed-height rows, five primitives each, with
+//! the row above them changing height every frame, so every row below is
+//! offered a box the same shape somewhere else.
+//!
+//!     cargo test --release --test replace_cost -- --ignored
+//!     perf stat -e instructions:u target/release/.../replace_cost-* --ignored
+//!
+//! Wall time is the wrong number here; see `draw_cost.rs`.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+const ROWS: usize = 200;
+const FRAMES: usize = 200;
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn remapping_rows_every_frame() {
+    let mut h = Harness::new((1920, 1200));
+    let first = rect(Color::RED).height(40).add(&mut h.rsc);
+    let mut span = Span::empty(Dir::DOWN);
+    span.push(first.add_strong(&mut h.rsc));
+    for i in 0..ROWS {
+        let row = (
+            rect(Color::BLUE.darker(i as f32 / (ROWS * 2) as f32)),
+            rect(Color::GREEN).pad(2),
+            wtext("row").size(16).pad(2),
+        )
+            .span(Dir::RIGHT)
+            .pad(4)
+            .height(40)
+            .add(&mut h.rsc);
+        span.push(row.add_strong(&mut h.rsc));
+    }
+    h.set_root(span);
+    for i in 0..FRAMES {
+        h.set_len(first, Axis::Y, 40.0 + (i % 2) as f32);
+        h.frame();
+    }
+}

tests/retained.rs

@@ -1,252 +0,0 @@
-//! What a second frame draws again, and what it keeps.
-
-use std::{cell::Cell, rc::Rc};
-
-use iris::harness::{Harness, assert_corners};
-use iris::prelude::*;
-
-/// A leaf that counts its draws and reports whatever size it is given, so a
-/// test can see what the retained path skipped.
-struct Counted {
-    draws: Rc<Cell<usize>>,
-    size: Size,
-    dependence: OnResize,
-}
-
-impl Widget for Counted {
-    fn draw(&mut self, _: &mut Painter) -> Size {
-        self.draws.set(self.draws.get() + 1);
-        self.size
-    }
-
-    fn on_resize(&self, _: Axis) -> OnResize {
-        self.dependence
-    }
-}
-
-struct Counts(Rc<Cell<usize>>);
-
-impl Counts {
-    fn get(&self) -> usize {
-        self.0.get()
-    }
-}
-
-fn counted(h: &mut Harness, size: Size, dependence: OnResize) -> (WeakWidget<Counted>, Counts) {
-    let draws = Rc::new(Cell::new(0));
-    let id = Counted {
-        draws: draws.clone(),
-        size,
-        dependence,
-    }
-    .add(&mut h.rsc);
-    (id, Counts(draws))
-}
-
-/// A fixed-width leaf beside one that takes the rest, so changing the first
-/// hands the second a different box without the output changing.
-fn pair(h: &mut Harness, rest: OnResize) -> (WeakWidget<Counted>, Counts, WidgetId) {
-    let (first, _) = counted(h, Size::from((100, 200)), OnResize::Translate);
-    let (second, draws) = counted(h, Size::REST, rest);
-    h.set_root((first, second).span(Dir::RIGHT));
-    (first, draws, second.id())
-}
-
-#[test]
-fn a_leaf_that_ignores_its_box_is_not_drawn_again_when_the_box_changes() {
-    let mut h = Harness::new((400, 200));
-    let (first, draws, second) = pair(&mut h, OnResize::Scale);
-    let settled = draws.get();
-    assert_corners!(h, second, (100, 0), (400, 200));
-
-    h.rsc[first].size = Size::from((150, 200));
-    h.frame();
-
-    assert_eq!(
-        draws.get(),
-        settled,
-        "its box is a field to write, not a reason to draw"
-    );
-    assert_corners!(h, second, (150, 0), (400, 200));
-}
-
-#[test]
-fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
-    let mut h = Harness::new((400, 200));
-    let (first, draws, second) = pair(&mut h, OnResize::Redraw);
-    let settled = draws.get();
-
-    h.rsc[first].size = Size::from((150, 200));
-    h.frame();
-
-    // Twice: once for the span to measure it, once for its real box. A child
-    // that can hint its length is spared the first, and a smaller number here
-    // means someone has made that cheaper rather than broken it.
-    assert_eq!(draws.get(), settled + 2);
-    assert_corners!(h, second, (150, 0), (400, 200));
-}
-
-#[test]
-fn a_span_child_that_declares_its_length_is_drawn_once() {
-    let mut h = Harness::new((400, 200));
-    let (told, told_draws) = counted(&mut h, Size::from((100, 200)), OnResize::Translate);
-    let (asked, asked_draws) = counted(&mut h, Size::from((100, 200)), OnResize::Translate);
-    // The span takes one child's length from its hint and has to draw the
-    // other to find out, so only the second is drawn before it is placed.
-    let hinted = told.width(100).add(&mut h.rsc);
-    h.set_root((hinted, asked).span(Dir::RIGHT));
-
-    assert_eq!(told_draws.get(), 1);
-    assert_eq!(
-        asked_draws.get(),
-        2,
-        "drawn to be measured, then again to be placed"
-    );
-}
-
-#[test]
-fn a_span_relays_out_when_a_child_it_measured_changes() {
-    let mut h = Harness::new((400, 200));
-    let (first, _, second) = pair(&mut h, OnResize::Translate);
-
-    h.rsc[first].size = Size::from((250, 200));
-    h.frame();
-
-    assert_corners!(h, first, (0, 0), (250, 200));
-    assert_corners!(h, second, (250, 0), (400, 200));
-}
-
-#[test]
-fn a_placed_child_survives_the_next_frame() {
-    let mut h = Harness::new((400, 200));
-    // Both children declare a length, so the span places them from their hints
-    // rather than drawing them to find out.
-    let top = rect(Color::RED).height(80).add(&mut h.rsc);
-    let bottom = rect(Color::BLUE).height(120).add(&mut h.rsc);
-    h.set_root((top, bottom).span(Dir::DOWN));
-
-    h.rsc.widgets_mut().get_dyn_mut(top.id());
-    h.frame();
-
-    assert_corners!(h, top, (0, 0), (400, 80));
-    assert_corners!(h, bottom, (0, 80), (400, 200));
-}
-
-/// Lays its child out from the hint alone, never reading what it drew.
-struct FromHint {
-    inner: StrongWidget,
-}
-
-impl Widget for FromHint {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
-        let mut region = UiRegion::FULL;
-        region.y.end = region.y.start.offset(len.abs);
-        painter.widget_within(&self.inner, region);
-        Size::REST
-    }
-}
-
-#[test]
-fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
-    let mut h = Harness::new((400, 200));
-    let inner = rect(Color::RED).height(80).add(&mut h.rsc);
-    let parent = FromHint {
-        inner: inner.add_strong(&mut h.rsc),
-    }
-    .add(&mut h.rsc);
-    h.set_root(parent);
-    assert_corners!(h, inner, (0, 0), (400, 80));
-
-    h.rsc[inner].y = Some(Len::abs(120));
-    h.frame();
-
-    assert_corners!(h, inner, (0, 0), (400, 120));
-}
-
-/// Reads the output's size, which nothing but its own draw can put right.
-struct ReadsOutput {
-    draws: Rc<Cell<usize>>,
-}
-
-impl Widget for ReadsOutput {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        self.draws.set(self.draws.get() + 1);
-        Size::abs(painter.output_size() / 4.0)
-    }
-}
-
-#[test]
-fn a_resize_does_not_redraw_what_the_shader_can_move() {
-    let mut h = Harness::new((400, 200));
-    let (leaf, draws) = counted(&mut h, Size::REST, OnResize::Redraw);
-    h.set_root(leaf);
-    let settled = draws.get();
-
-    h.resize((800, 100));
-    assert!(h.needs_redraw());
-    h.frame();
-
-    assert_eq!(
-        draws.get(),
-        settled,
-        "its box is the same fraction of a different output"
-    );
-    assert_corners!(h, leaf, (0, 0), (800, 100));
-}
-
-#[test]
-fn a_resize_redraws_what_read_the_output() {
-    let mut h = Harness::new((400, 200));
-    let draws = Rc::new(Cell::new(0));
-    let leaf = ReadsOutput {
-        draws: draws.clone(),
-    }
-    .add(&mut h.rsc);
-    h.set_root(leaf);
-    let settled = draws.get();
-
-    h.resize((800, 100));
-    h.frame();
-
-    assert_eq!(draws.get(), settled + 1);
-}
-
-#[test]
-fn narrowing_the_output_reflows_text_and_relays_out_around_it() {
-    let mut h = Harness::new((600, 400));
-    let para = wtext(
-        "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.",
-    )
-    .size(20)
-    .wrap(true)
-    .add(&mut h.rsc);
-    let below = rect(Color::RED).add(&mut h.rsc);
-    h.set_root((para, below).span(Dir::DOWN));
-    let top = h.region(&below).expect("drew nothing").top_left.y;
-
-    h.resize((300, 400));
-    h.frame();
-
-    let lower = h.region(&below).expect("drew nothing").top_left.y;
-    assert!(lower > top, "same words, half the width: {top} -> {lower}");
-}
-
-#[test]
-fn a_change_two_levels_under_its_reader_still_reaches_it() {
-    let mut h = Harness::new((400, 400));
-    // Every wrapper up to the outer pad read the size below it, so the outer
-    // pad is what draws again -- and the span it hands the box to is the same
-    // size as before, which is what lets a draw reuse its way past the leaf.
-    let (leaf, _) = counted(&mut h, Size::abs((100, 100).into()), OnResize::Redraw);
-    let padded = leaf.pad(10).add(&mut h.rsc);
-    let below = rect(Color::RED).add(&mut h.rsc);
-    h.set_root((padded, below).span(Dir::DOWN).pad(12));
-    assert_corners!(h, below, (12, 132), (388, 388));
-
-    h.rsc[leaf].size = Size::abs((100, 200).into());
-    h.frame();
-
-    assert_corners!(h, below, (12, 232), (388, 388));
-}

tests/revision_cost.rs

@@ -0,0 +1,203 @@
+//! What a resize frame costs and what it holds, on a tree the revision before
+//! #16 also builds.
+//!
+//! Deliberately written in the API subset `43ce8c7` and this branch share, so
+//! the same source can be dropped into an old worktree and measured there:
+//! that is the only like-for-like comparison with the code the retained
+//! layout replaced. The random tree cannot carry one, because the generator
+//! itself changed with the work.
+//!
+//!     ROWS=40 FRAMES=500 cargo test --release --test revision_cost \
+//!       -- --ignored --nocapture resize_cost
+//!     ROWS=2000 cargo test --release --test revision_cost \
+//!       -- --ignored --nocapture text_memory
+//!
+//! Wall time on this machine varies with CPU frequency; take the number from
+//! `perf stat -e instructions:u` on the test binary directly.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+use std::time::Instant;
+
+/// xorshift64, so one seed is one set of paragraphs on any machine.
+struct Rng(u64);
+
+impl Rng {
+    fn bits(&mut self) -> u64 {
+        self.0 ^= self.0 << 13;
+        self.0 ^= self.0 >> 7;
+        self.0 ^= self.0 << 17;
+        self.0
+    }
+
+    fn below(&mut self, n: usize) -> usize {
+        (self.bits() % n as u64) as usize
+    }
+}
+
+const WORDS: [&str; 24] = [
+    "wrapping",
+    "shapes",
+    "one",
+    "source",
+    "into",
+    "as",
+    "many",
+    "lines",
+    "as",
+    "the",
+    "box",
+    "leaves",
+    "room",
+    "for",
+    "paragraph",
+    "height",
+    "answer",
+    "setting",
+    "container",
+    "width",
+    "before",
+    "knows",
+    "measured",
+    "again",
+];
+
+/// A run of its own words, so nothing here is fast for two texts being the
+/// same string.
+fn words(rng: &mut Rng, least: usize, most: usize) -> String {
+    let words = least + rng.below(most - least);
+    let mut out = String::new();
+    for _ in 0..words {
+        if !out.is_empty() {
+            out.push(' ');
+        }
+        out.push_str(WORDS[rng.below(WORDS.len())]);
+    }
+    out
+}
+
+const OUTPUT: (f32, f32) = (900.0, 1200.0);
+
+fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|value| value.parse().ok())
+        .unwrap_or(fallback)
+}
+
+/// A row of a fixed-width rect beside a column of one wrapping and one
+/// overflowing text: the shape that makes a container measure a child in a
+/// box it will not keep.
+fn build(h: &mut Harness, rows: usize) -> Vec<WidgetId> {
+    let mut rng = Rng(1);
+    let mut paragraphs = Vec::new();
+    let mut col = Span::empty(Dir::DOWN);
+    for _ in 0..rows {
+        let mut row = Span::empty(Dir::RIGHT);
+        row.push(
+            rect(Color::RED)
+                .width(LayoutLen::px(40.0))
+                .add_strong(&mut h.rsc),
+        );
+        let mut body = Span::empty(Dir::DOWN);
+        let para = wtext(words(&mut rng, 12, 52))
+            .size(16)
+            .wrap(true)
+            .add_strong(&mut h.rsc);
+        paragraphs.push(para.id());
+        body.push(para);
+        body.push(
+            // Short, or its unwrapped width decides the row and the
+            // paragraph beside it never wraps.
+            wtext(words(&mut rng, 2, 6))
+                .size(16)
+                .wrap(false)
+                .add_strong(&mut h.rsc),
+        );
+        row.push(body.add_strong(&mut h.rsc));
+        col.push(row.add_strong(&mut h.rsc));
+    }
+    let root = col.add(&mut h.rsc);
+    h.set_root(root);
+    paragraphs
+}
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn resize_cost() {
+    let rows = env("ROWS", 40_usize);
+    let frames = env("FRAMES", 500_usize);
+    let mut h = Harness::new(OUTPUT);
+    let paragraphs = build(&mut h, rows);
+    // What it cost is only half the comparison: the old code is cheaper
+    // partly because it wraps at the container's whole width rather than the
+    // part left beside the rect, and draws past the edge of the output.
+    println!("output width {}", OUTPUT.0);
+    for (at, id) in paragraphs.iter().enumerate().take(3) {
+        println!("paragraph {at}: {:?}", h.region(id));
+    }
+
+    // Two widths in turn is the friendly case for anything that remembers an
+    // answer, so `SWEEP=1` never repeats one -- a drag rather than a toggle.
+    let sweep = env("SWEEP", 0_usize) != 0;
+    let mut elapsed = Vec::with_capacity(frames);
+    for frame in 0..frames {
+        let narrower = match sweep {
+            true => (frame % 256) as f32,
+            false => ((frame + 1) % 2) as f32 * 8.0,
+        };
+        h.resize((OUTPUT.0 - narrower, OUTPUT.1));
+        let start = Instant::now();
+        h.frame();
+        elapsed.push(start.elapsed().as_secs_f64() * 1000.0);
+    }
+    elapsed.sort_by(|a, b| a.partial_cmp(b).unwrap());
+    println!(
+        "resize: {frames} frames, min {:.3} ms, median {:.3} ms, p99 {:.3} ms, \
+         max {:.3} ms, total {:.1} ms",
+        elapsed[0],
+        elapsed[frames / 2],
+        elapsed[frames * 99 / 100],
+        elapsed[frames - 1],
+        elapsed.iter().sum::<f64>()
+    );
+}
+
+fn kb(field: &str) -> u64 {
+    std::fs::read_to_string("/proc/self/status")
+        .unwrap()
+        .lines()
+        .find(|line| line.starts_with(field))
+        .and_then(|line| line.split_whitespace().nth(1)?.parse().ok())
+        .unwrap()
+}
+
+fn report(label: &str) {
+    println!(
+        "{label:24} rss {:>7} kB   peak {:>7} kB",
+        kb("VmRSS:"),
+        kb("VmHWM:")
+    );
+}
+
+/// Run this one on its own: the figures are the whole process's.
+#[test]
+#[ignore = "measurement, not a check"]
+fn text_memory() {
+    let rows = env("ROWS", 2000_usize);
+    report("before");
+    let mut h = Harness::new(OUTPUT);
+    let paragraphs = build(&mut h, rows);
+    report("after cold frame");
+    for frame in 0..40 {
+        h.resize((OUTPUT.0 - ((frame + 1) % 2) as f32 * 8.0, OUTPUT.1));
+        h.frame();
+    }
+    report("after 40 resizes");
+    // Settled: the output holds still and one leaf repaints per frame.
+    for _ in 0..10 {
+        let _ = h.rsc.widgets_mut().get_dyn_mut(paragraphs[0]);
+        h.frame();
+    }
+    report("after settling");
+}

tests/scenario/mod.rs

@@ -0,0 +1,494 @@
+//! The scenarios both fuzzers run, over the tree a [`Plan`] describes.
+//!
+//! One implementation rather than two. The oracle grew its trees from a seed
+//! and the shrinker grew its own, with every scenario written out on each
+//! side, so a failure the oracle found could not be handed to the shrinker:
+//! there was no tree to pass it, only a seed, and a seed cannot be made
+//! smaller. Both take a plan now, so whatever finds a counterexample can also
+//! reduce it.
+//!
+//! Each target compiles this for itself, so what only one of them calls is
+//! dead code in the other.
+#![allow(dead_code)]
+
+use iris::harness::Harness;
+use iris::prelude::*;
+use iris::random::{Aligns, Edits, Kind, Lens, Plan, Rng, SpanEdit, Tree, build};
+use std::collections::HashMap;
+
+/// A seed per thread but one, since a seed grows, lays out and drops its tree
+/// alone. A failing seed still shrinks and panics on its own thread.
+pub fn over_seeds(seeds: Vec<u64>, run: impl Fn(u64) + Sync) {
+    let threads =
+        std::thread::available_parallelism().map_or(1, |n| n.get().saturating_sub(1).max(1));
+    let chunk = seeds.len().div_ceil(threads).max(1);
+    std::thread::scope(|scope| {
+        for part in seeds.chunks(chunk) {
+            let run = &run;
+            scope.spawn(move || part.iter().for_each(|&seed| run(seed)));
+        }
+    });
+}
+
+pub fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(fallback)
+}
+
+/// The window a tree is grown in, and the one a resize takes it to.
+const OUTER: (f32, f32) = (1920.0, 1200.0);
+const INNER: (f32, f32) = (640.0, 900.0);
+const STILL: (f32, f32) = (900.0, 1200.0);
+
+/// A way of changing what a span holds. Each is a shape worth its own case:
+/// taking a child out of the middle is not the same as emptying a span, and
+/// adding one is not the same as adding three.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Shuffle {
+    /// Every other child, so what is left is interleaved with what went.
+    EveryOther,
+    /// Everything but the first, which is the last step before empty.
+    AllButFirst,
+    /// Three more on the end at once.
+    AddThree,
+    /// The first out and three more on, so the count moves both ways.
+    SwapForThree,
+    /// One out of the middle and one on the end.
+    TradeOne,
+}
+
+impl Shuffle {
+    fn of(self, grown: usize) -> SpanEdit {
+        let all = |step: usize, from: usize| (from..grown).step_by(step).collect();
+        match self {
+            Self::EveryOther => SpanEdit {
+                detach: all(2, 0),
+                attach: 0,
+            },
+            Self::AllButFirst => SpanEdit {
+                detach: all(1, 1),
+                attach: 0,
+            },
+            Self::AddThree => SpanEdit {
+                detach: Vec::new(),
+                attach: 3,
+            },
+            Self::SwapForThree => SpanEdit {
+                detach: vec![0],
+                attach: 3,
+            },
+            Self::TradeOne => SpanEdit {
+                detach: vec![grown / 2],
+                attach: 1,
+            },
+        }
+    }
+}
+
+/// What a warm tree is put through before it is compared with a cold one
+/// grown the way it was left.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Case {
+    /// Nothing changes, so no box may either. What this exercises is the
+    /// order a frame settles a dirty set in.
+    Repaint,
+    /// Every fifth widget rather than all of them: marking all of them
+    /// redraws the whole tree, which is a cold start reached the long way,
+    /// where the mixed case leaves a redrawn subtree beside a retained one.
+    RepaintSome,
+    Resize,
+    ResizeRepaint,
+    /// A resize and then a size change, so a retained answer is asked to
+    /// survive two different kinds of invalidation in a row.
+    ResizeSize,
+    /// A size change and then a resize, which is the other order and not the
+    /// same test: a length answered as a fraction of one box and kept as a
+    /// fraction of another agrees at the size it was changed at and parts
+    /// from it at every other one.
+    SizeResize,
+    /// A few declared sizes.
+    Size,
+    /// Every declared size at once, so every reader of a size has a changed
+    /// descendant in the same frame and the whole dirty set settles together.
+    EverySize,
+    Align,
+    /// Giving a widget a movable region of its own, or taking it away, is a
+    /// structural change: every primitive under it changes which chain
+    /// resolves it.
+    RegionNode,
+    /// The same children in a different order, which moves every one of them
+    /// without changing what any of them is.
+    Reorder,
+    Shuffle(Shuffle),
+}
+
+pub const ALL: [Case; 16] = [
+    Case::Repaint,
+    Case::RepaintSome,
+    Case::Resize,
+    Case::ResizeRepaint,
+    Case::ResizeSize,
+    Case::SizeResize,
+    Case::Size,
+    Case::EverySize,
+    Case::Align,
+    Case::RegionNode,
+    Case::Reorder,
+    Case::Shuffle(Shuffle::EveryOther),
+    Case::Shuffle(Shuffle::AllButFirst),
+    Case::Shuffle(Shuffle::AddThree),
+    Case::Shuffle(Shuffle::SwapForThree),
+    Case::Shuffle(Shuffle::TradeOne),
+];
+
+impl Case {
+    /// The name `CASE` selects it by, and the one a failure prints.
+    pub fn name(self) -> &'static str {
+        match self {
+            Self::Repaint => "repaint",
+            Self::RepaintSome => "repaint-some",
+            Self::Resize => "resize",
+            Self::ResizeRepaint => "resize-repaint",
+            Self::ResizeSize => "resize-size",
+            Self::SizeResize => "size-resize",
+            Self::Size => "size",
+            Self::EverySize => "every-size",
+            Self::Align => "align",
+            Self::RegionNode => "region-node",
+            Self::Reorder => "reorder",
+            Self::Shuffle(Shuffle::EveryOther) => "shuffle-every-other",
+            Self::Shuffle(Shuffle::AllButFirst) => "shuffle-all-but-first",
+            Self::Shuffle(Shuffle::AddThree) => "shuffle-add-three",
+            Self::Shuffle(Shuffle::SwapForThree) => "shuffle-swap-for-three",
+            Self::Shuffle(Shuffle::TradeOne) => "shuffle-trade-one",
+        }
+    }
+
+    pub fn named(name: &str) -> Option<Self> {
+        ALL.into_iter().find(|case| case.name() == name)
+    }
+
+    /// Grown in the first, compared in the second.
+    fn window(self) -> ((f32, f32), (f32, f32)) {
+        match self {
+            Self::Resize | Self::ResizeRepaint | Self::ResizeSize => (OUTER, INNER),
+            _ => (STILL, STILL),
+        }
+    }
+
+    /// The window the warm tree is taken to after the change, where the case
+    /// is about what the change left behind rather than about the change.
+    fn then_resize(self) -> Option<(f32, f32)> {
+        match self {
+            Self::SizeResize => Some(INNER),
+            _ => None,
+        }
+    }
+}
+
+fn mark(warm: &mut Harness, tree: &Tree, step: usize) {
+    for &id in tree.ids.iter().step_by(step) {
+        warm.rsc.widgets_mut().get_dyn_mut(id);
+    }
+}
+
+fn a_len(rng: &mut Rng) -> Option<LayoutLen> {
+    Some(LayoutLen::px(20.0 + rng.below(180) as f32))
+}
+
+fn resize_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Lens {
+    let lens = [a_len(rng), a_len(rng)];
+    warm.rsc
+        .widgets_mut()
+        .set_size_rules(tree.sized[idx], lens[0], lens[1]);
+    lens
+}
+
+fn realign_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Aligns {
+    let side = |rng: &mut Rng| match rng.below(4) {
+        0 => None,
+        1 => Some(AxisAlign::NEG),
+        2 => Some(AxisAlign::CENTER),
+        _ => Some(AxisAlign::POS),
+    };
+    let align = [side(rng), side(rng)];
+    let id = tree.aligned[idx];
+    for (axis, align) in [Axis::X, Axis::Y].into_iter().zip(align) {
+        warm.rsc
+            .widgets_mut()
+            .set_alignment(id, axis, align.unwrap_or_default());
+    }
+    align
+}
+
+/// Every span's children in a different order, said both to the warm tree and
+/// to the plan the cold one is grown from.
+fn reorder(warm: &mut Harness, tree: &Tree, plan: &Plan) -> Plan {
+    for span in &tree.spans {
+        let children = &mut warm.rsc[span.id].children;
+        if !children.is_empty() {
+            children.rotate_left(1);
+        }
+    }
+    let mut out = plan.clone();
+    out.walk_mut(&mut |node| {
+        if let Kind::Span { order, .. } = &mut node.kind
+            && !order.is_empty()
+        {
+            order.rotate_left(1);
+        }
+    });
+    out
+}
+
+/// Applies `shuffle` to every third span. What it takes out is given back to
+/// the span's spares: the last share of a widget must outlive the comparison,
+/// or its id is handed to something else and the two trees stop lining up.
+fn reshuffle(warm: &mut Harness, tree: &mut Tree, shuffle: Shuffle) -> HashMap<usize, SpanEdit> {
+    let mut edits = HashMap::new();
+    for (idx, span) in tree.spans.iter_mut().enumerate().step_by(3) {
+        let edit = shuffle.of(span.grown);
+        let mut take = edit.detach.clone();
+        take.sort_unstable();
+        let children = &mut warm.rsc[span.id].children;
+        // Highest first, so an index means the same child however many of its
+        // neighbours are going too.
+        for j in take.into_iter().rev() {
+            if j < children.len() {
+                span.spares.push(children.remove(j));
+            }
+        }
+        let attach = edit.attach.min(span.spares.len());
+        let moved: Vec<_> = span.spares.drain(..attach).collect();
+        warm.rsc[span.id].children.extend(moved);
+        edits.insert(idx, edit);
+    }
+    edits
+}
+
+/// Changes the warm tree and answers with the plan a cold tree grown that way
+/// comes from. Each arm settles its own frame, so a case that changes nothing
+/// does not get a second one that could settle what the first left.
+fn change(case: Case, warm: &mut Harness, tree: &mut Tree, plan: &Plan, rng: &mut Rng) -> Plan {
+    let some_sizes = |warm: &mut Harness, tree: &Tree, rng: &mut Rng| {
+        let mut sizes = HashMap::new();
+        for _ in 0..4 {
+            if tree.sized.is_empty() {
+                break;
+            }
+            let idx = rng.below(tree.sized.len());
+            sizes.insert(idx, resize_one(warm, tree, idx, rng));
+        }
+        sizes
+    };
+    let edits = match case {
+        Case::Resize => return plan.clone(),
+        Case::Repaint | Case::ResizeRepaint => {
+            mark(warm, tree, 1);
+            warm.frame();
+            return plan.clone();
+        }
+        Case::RepaintSome => {
+            mark(warm, tree, 5);
+            warm.frame();
+            return plan.clone();
+        }
+        Case::Reorder => {
+            let out = reorder(warm, tree, plan);
+            warm.frame();
+            return out;
+        }
+        Case::Size | Case::ResizeSize | Case::SizeResize => Edits {
+            sizes: some_sizes(warm, tree, rng),
+            ..Default::default()
+        },
+        Case::EverySize => Edits {
+            sizes: (0..tree.sized.len())
+                .map(|idx| (idx, resize_one(warm, tree, idx, rng)))
+                .collect(),
+            ..Default::default()
+        },
+        Case::Align => Edits {
+            aligns: (0..tree.aligned.len())
+                .step_by(3)
+                .map(|idx| (idx, realign_one(warm, tree, idx, rng)))
+                .collect(),
+            ..Default::default()
+        },
+        Case::RegionNode => {
+            let mut nodes = HashMap::new();
+            for idx in (0..tree.nodes.len()).step_by(2) {
+                let id = tree.nodes[idx];
+                let take = !warm.rsc.widgets().is_region_node(id);
+                warm.rsc.widgets_mut().set_region_node(id, take);
+                nodes.insert(idx, take);
+            }
+            Edits {
+                nodes,
+                ..Default::default()
+            }
+        }
+        Case::Shuffle(shuffle) => Edits {
+            spans: reshuffle(warm, tree, shuffle),
+            ..Default::default()
+        },
+    };
+    warm.frame();
+    plan.edited(&edits)
+}
+
+/// What a widget was configured with, so a tree a fuzzer found can be written
+/// out by hand. A failure is a lead; the fast test that replaces it has to be
+/// buildable from what the failure printed.
+fn describe(id: WidgetId, h: &Harness) -> String {
+    let rules = h.rsc.widgets().size_rules(id);
+    let rule = |r: SizeRule| match r.exact() {
+        Some(len) => format!("{len}"),
+        None => "-".into(),
+    };
+    let align = h.rsc.widgets().alignment(id);
+    let side = |a: AxisAlign| {
+        if a == AxisAlign::NEG {
+            "neg".into()
+        } else if a == AxisAlign::CENTER {
+            "mid".into()
+        } else if a == AxisAlign::POS {
+            "pos".into()
+        } else {
+            format!("{:.2}", a.rel())
+        }
+    };
+    // A rule and an alignment are properties of whatever carries them, so
+    // they print with that widget rather than as widgets of their own.
+    let mut out = describe_widget(id, h);
+    if (rules.x, rules.y) != (SizeRule::Free, SizeRule::Free) {
+        out += &format!("[x:{},y:{}]", rule(rules.x), rule(rules.y));
+    }
+    if align != RegionAlign::default() {
+        out += &format!("@{},{}", side(align.x), side(align.y));
+    }
+    out
+}
+
+fn describe_widget(id: WidgetId, h: &Harness) -> String {
+    let label = h.rsc.widgets().label(id).to_string();
+    let Some(widget) = h.rsc.widgets().get_dyn(id) else {
+        return label;
+    };
+    let any: &dyn std::any::Any = widget;
+    if let Some(w) = any.downcast_ref::<Span>() {
+        let sign = if w.dir.sign == Sign::Neg { "-" } else { "+" };
+        return format!(
+            "Span{{dir:{:?}{sign},gap:{},n:{}}}",
+            w.dir.axis,
+            w.gap,
+            w.children.len()
+        );
+    }
+    if let Some(w) = any.downcast_ref::<Pad>() {
+        let p = &w.padding;
+        return format!(
+            "Pad{{l:{},r:{},t:{},b:{}}}",
+            p.left, p.right, p.top, p.bottom
+        );
+    }
+    if let Some(w) = any.downcast_ref::<Stack>() {
+        return format!("Stack{{n:{}}}", w.children.len());
+    }
+    label
+}
+
+/// One widget's layout as it stands: the frame its fractions resolved
+/// against, the box it was asked in, the box its drawing went in, and what
+/// it reported. In window units, which is what both trees are in.
+fn record(id: WidgetId, h: &Harness) -> String {
+    let active = &h.render.active[&id];
+    format!(
+        "rel_base {} region {} placement {} size {}",
+        active.rel_base, active.region, active.placement, active.size,
+    )
+}
+
+/// Runs `case` on the tree `plan` describes, warm and cold, and says where
+/// the two disagree. `seed` chooses only the values a case picks at random,
+/// so one plan under one case is one comparison however it was reached.
+pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
+    let (start, end) = case.window();
+    let mut warm = Harness::new(start);
+    let (root, mut tree) = build(&mut warm.rsc, plan);
+    warm.state.root = Some(root);
+    // The frame that makes it warm: without it nothing is retained and the
+    // comparison is two cold starts agreeing with each other.
+    warm.frame();
+    if start != end {
+        warm.resize(end);
+        warm.frame();
+    }
+    let cold_plan = change(case, &mut warm, &mut tree, plan, &mut Rng::new(seed));
+    // Whatever the change left, seen at another window: an answer kept as a
+    // fraction of the wrong length is the same number of pixels where it was
+    // made and a different one everywhere else.
+    let end = match case.then_resize() {
+        Some(after) => {
+            warm.resize(after);
+            warm.frame();
+            after
+        }
+        None => end,
+    };
+
+    let mut cold = Harness::new(end);
+    let (root, cold_tree) = build(&mut cold.rsc, &cold_plan);
+    cold.state.root = Some(root);
+    cold.frame();
+
+    let mut drawn = 0;
+    for (i, (&w, &c)) in tree.ids.iter().zip(&cold_tree.ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        drawn += got.is_some() as usize;
+        if got == want {
+            continue;
+        }
+        let places: HashMap<WidgetId, usize> = tree
+            .ids
+            .iter()
+            .enumerate()
+            .map(|(i, &id)| (id, i))
+            .collect();
+        // Where two trees disagree is rarely where the cause is, so the
+        // ancestry comes with it, marking the widgets that own a region.
+        let mut chain = Vec::new();
+        let mut records = Vec::new();
+        let mut at = Some(w);
+        while let Some(id) = at {
+            let active = &warm.render.active[&id];
+            let node = match active.move_idx == active.parent_move {
+                true => "",
+                false => "*",
+            };
+            chain.push(format!("{}{node}", describe(id, &warm)));
+            // What each level was asked in on both sides, since the level
+            // where the two stop agreeing is the one to look at rather than
+            // the leaf that reported the difference.
+            let cold_id = places.get(&id).and_then(|&i| cold_tree.ids.get(i));
+            records.push(format!(
+                "    {}\n      warm {}\n      cold {}",
+                describe(id, &warm),
+                record(id, &warm),
+                cold_id.map_or("-".into(), |&id| record(id, &cold)),
+            ));
+            at = active.parent;
+        }
+        return Some(format!(
+            "widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}\n{}",
+            chain.join(" < "),
+            records.join("\n"),
+        ));
+    }
+    match drawn {
+        0 => Some("nothing was drawn".into()),
+        _ => None,
+    }
+}

tests/scroll.rs

@@ -1,26 +0,0 @@
-//! Scrolling moves content and stops at its ends.
-
-use iris::harness::{Harness, assert_corners};
-use iris::prelude::*;
-
-#[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));
-}

tests/shrink.rs

@@ -0,0 +1,103 @@
+//! A fuzzer that reduces its own counterexample.
+//!
+//! A seed is not a lead anybody can read: the tree is hundreds of widgets,
+//! and reconstructing the part that matters by hand has failed every time it
+//! has been tried. This grows the trees `iris::random` describes, takes them
+//! apart, and prints the smallest one that still fails as something to write
+//! a fast test from.
+//!
+//!     cargo test --release --test shrink -- --ignored --nocapture
+//!
+//! `SHRINK_SEEDS` how many trees to try, `SHRINK_DEPTH` how deep to grow
+//! them, `SHRINK_CASE` which scenario or `all` for every one. `SHRINK_SEED`
+//! takes a single seed, which is how a failure `generated` printed is handed
+//! straight here: the two run the same cases over the same trees, so a seed
+//! that fails there fails here and is reduced.
+//!
+//! It is a fuzzer: run it once the ordinary tests pass, and turn what it
+//! finds into a test of its own rather than leaving a seed as the record.
+
+#[path = "scenario/mod.rs"]
+mod scenario;
+
+use iris::random::{Edits, Plan, plan};
+use scenario::{ALL, Case, diverges, env, over_seeds};
+
+/// Takes the first simplification that still fails, until none does. The
+/// simplifications come biggest first, so this walks down rather than
+/// nibbling: a six-hundred-widget tree reaches single figures in a few
+/// hundred builds.
+fn shrink(mut node: Plan, case: Case, seed: u64) -> Plan {
+    loop {
+        let Some(next) = node
+            .smaller()
+            .into_iter()
+            .find(|small| diverges(small, case, seed).is_some())
+        else {
+            return node;
+        };
+        node = next;
+    }
+}
+
+fn cases() -> Vec<Case> {
+    match env("SHRINK_CASE", String::from("all")).as_str() {
+        "all" => ALL.to_vec(),
+        name => match Case::named(name) {
+            Some(case) => vec![case],
+            None => panic!(
+                "unknown SHRINK_CASE {name:?}; one of all, {}",
+                ALL.map(Case::name).join(", ")
+            ),
+        },
+    }
+}
+
+#[test]
+#[ignore = "a fuzzer; run it once the ordinary tests pass"]
+fn no_grown_tree_lays_out_differently_warm_than_cold() {
+    let depth: usize = env("SHRINK_DEPTH", 5);
+    let cases = cases();
+    let seeds: Vec<u64> = match std::env::var("SHRINK_SEED")
+        .ok()
+        .and_then(|v| v.parse().ok())
+    {
+        Some(seed) => vec![seed],
+        None => (1..=env("SHRINK_SEEDS", 400_u64)).collect(),
+    };
+    let count = seeds.len();
+
+    over_seeds(seeds, |seed| {
+        let grown = plan(seed, depth, &Edits::default());
+        for &case in &cases {
+            if diverges(&grown, case, seed).is_none() {
+                continue;
+            }
+            let small = shrink(grown.clone(), case, seed);
+            // Described from the shrunk tree: the grown tree's chain names
+            // widgets that are no longer there, and the ancestry of the
+            // failure is what a test is written from.
+            let how = diverges(&small, case, seed).unwrap_or_default();
+            println!(
+                "seed {seed} case {}: {how}\ngrown {} widgets, shrank to {}\n{small:#?}",
+                case.name(),
+                grown.size(),
+                small.size()
+            );
+            panic!(
+                "seed {seed} lays out differently warm than cold after {}",
+                case.name()
+            );
+        }
+    });
+
+    let sizes: Vec<usize> = (1..=count as u64)
+        .map(|seed| plan(seed, depth, &Edits::default()).size())
+        .collect();
+    println!(
+        "{count} trees at depth {depth} agree over {} case(s): {} widgets total, largest {}",
+        cases.len(),
+        sizes.iter().sum::<usize>(),
+        sizes.iter().max().copied().unwrap_or(0)
+    );
+}

tests/suite.rs

@@ -0,0 +1,34 @@
+//! Every ordinary correctness test, as modules of one target.
+//!
+//! One binary rather than a dozen: each `tests/*.rs` links the whole
+//! dependency graph again, which is most of what `cargo test` spends its time
+//! on here. Libtest still runs the cases in parallel, and a filter still
+//! selects them -- `cargo test --test suite layout::` for one module.
+//!
+//! The rigs stay their own targets: `shrink` and `generated` are fuzzers run
+//! on their own, and the `*_cost` and `*_diagnostics` ones are measurements.
+
+#[path = "cases/determinism.rs"]
+mod determinism;
+#[path = "cases/drift.rs"]
+mod drift;
+#[path = "cases/idempotence.rs"]
+mod idempotence;
+#[path = "cases/layout.rs"]
+mod layout;
+#[path = "cases/plan.rs"]
+mod plan;
+#[path = "cases/pointer.rs"]
+mod pointer;
+#[path = "cases/pointer_routing.rs"]
+mod pointer_routing;
+#[path = "cases/retained.rs"]
+mod retained;
+#[path = "cases/scroll.rs"]
+mod scroll;
+#[path = "cases/tasks.rs"]
+mod tasks;
+#[path = "cases/text_edit.rs"]
+mod text_edit;
+#[path = "cases/unsettled.rs"]
+mod unsettled;

tests/trace_unsettled.rs

@@ -0,0 +1,140 @@
+//! Traces the six-widget tree in `unsettled.rs`, to see what box its text is
+//! actually drawn in on a first frame against a settled one.
+
+#![cfg(feature = "layout-diagnostics")]
+
+use iris::core::layout_diagnostics::{self as diag, TraceEvent};
+use iris::harness::Harness;
+use iris::prelude::*;
+
+fn plant(h: &mut Harness) -> Vec<WidgetId> {
+    let plain = wtext("Wrapping").size(16).wrap(false).add(&mut h.rsc);
+    let wrapped = wtext("Wrapping shapes").size(16).wrap(true).add(&mut h.rsc);
+    let sized = wrapped.width(76).add(&mut h.rsc);
+    let aligned = sized;
+    h.rsc
+        .widgets_mut()
+        .set_alignment(sized, Axis::X, AxisAlign::POS);
+    h.rsc
+        .widgets_mut()
+        .set_alignment(sized, Axis::Y, AxisAlign::POS);
+    let stack = Stack {
+        children: vec![plain.add_strong(&mut h.rsc), aligned.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    let root = (stack,).span(Dir::RIGHT).add(&mut h.rsc);
+    h.state.root = Some(root.add_strong(&mut h.rsc));
+    vec![
+        plain.id(),
+        wrapped.id(),
+        sized.id(),
+        aligned.id(),
+        stack.id(),
+        root.id(),
+    ]
+}
+
+fn dump(label: &str, report: &diag::Report, text: WidgetId) {
+    println!("--- {label} ---");
+    for event in report.traces() {
+        match event {
+            TraceEvent::DrawRequest {
+                id,
+                region,
+                pixel_size,
+                ..
+            } if *id == text => {
+                println!(
+                    "  draw in {:.2}x{:.2}  region {region:?}",
+                    pixel_size.x, pixel_size.y
+                )
+            }
+            TraceEvent::SizeReported { id, size } if *id == text => {
+                println!("  reported {size}")
+            }
+            TraceEvent::SizeRead { id, reader, size } if *id == text => {
+                println!("  size read by {reader:?}: {size}")
+            }
+            TraceEvent::RegionNode { id, parent, region } if *id == text => {
+                println!("  region node under {parent:?} at {region:?}")
+            }
+            TraceEvent::Reuse { id, outcome } if *id == text => println!("  reuse: {outcome:?}"),
+            _ => {}
+        }
+    }
+}
+
+#[test]
+#[ignore = "a diagnostic, not a check"]
+fn what_box_the_text_is_drawn_in() {
+    diag::clear_traced_widgets();
+    let _ = diag::take();
+    let mut h = Harness::new((640, 900));
+    let ids = plant(&mut h);
+    let text = ids[1];
+    diag::trace_widget(text);
+    let _ = diag::take();
+
+    h.frame();
+    dump("first frame", &diag::take(), text);
+
+    for _ in 0..2 {
+        for &id in &ids {
+            h.rsc.widgets_mut().get_dyn_mut(id);
+        }
+        let _ = diag::take();
+        h.frame();
+        dump("repaint", &diag::take(), text);
+    }
+    diag::clear_traced_widgets();
+}
+
+fn plant_fixed(h: &mut Harness) -> Vec<WidgetId> {
+    let words = "Wrapping shapes one source into as many lines as the box leaves";
+    let text = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let aligned = text;
+    h.rsc
+        .widgets_mut()
+        .set_alignment(text, Axis::X, AxisAlign::NEG);
+    let inner = (aligned,).span(Dir::RIGHT).add(&mut h.rsc);
+    let sized = inner.sized((189, 176)).add(&mut h.rsc);
+    let filler = rect(Color::RED).add(&mut h.rsc);
+    let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
+    h.state.root = Some(root.add_strong(&mut h.rsc));
+    vec![
+        text.id(),
+        aligned.id(),
+        inner.id(),
+        sized.id(),
+        filler.id(),
+        root.id(),
+    ]
+}
+
+#[test]
+#[ignore = "a diagnostic, not a check"]
+fn what_box_the_fixed_text_is_drawn_in() {
+    diag::clear_traced_widgets();
+    let _ = diag::take();
+    let mut h = Harness::new((1920, 1200));
+    let ids = plant_fixed(&mut h);
+    let text = ids[0];
+    diag::trace_widget(text);
+    let _ = diag::take();
+
+    h.frame();
+    dump("first frame at 1920", &diag::take(), text);
+    h.resize((640, 900));
+    h.frame();
+    dump("after resize to 640", &diag::take(), text);
+
+    let mut cold = Harness::new((640, 900));
+    let cids = plant_fixed(&mut cold);
+    diag::clear_traced_widgets();
+    diag::trace_widget(cids[0]);
+    let _ = diag::take();
+    cold.frame();
+    dump("cold at 640", &diag::take(), cids[0]);
+    diag::clear_traced_widgets();
+}