Hold a bound's length where it is decided, and say each thing once

A quality sweep over the deferred request system, which no earlier round
has reviewed.

`Bound::outside` said which end a length fell outside and left the caller
to look that end up through `Bound::at`, which `expect`s an end the value
it is given does not promise: only the pairing of the two calls kept
`at(Shorter)` off a bound with no floor. It already had the length in
hand, so it returns that, and `Outside` and `at` go with the state that
could panic.

`measured_request` pinned the rel base for any bound at all, so a measured
share under a cap in pixels was invalidated by a change to a base its
answer cannot depend on. That question is `Bound::has_fraction` now, which
is also the one `Placing::ask` and `SizeRule::has_fraction` were each
writing out over a bare array.

The rest is one name where there were several spellings: `Span::gaps`,
`Padding::along`, `Plan::drop_bounds` behind one `IRIS_UNBOUNDED` in both
rigs that had grown their own, and `Stack::size_request` resolving its
sizing child the way its draw already does. `Span`'s placement loop asked
three times whether the row was allocated, twice to decide one child's
length; one match answers all three, so the allocated and plain rules are
read side by side.

The buffers `draw_at` now reuses for their capacity are empty only because
every path to it drains them in `remove`; a `debug_assert` says so, since
a drawing over primitives left in one would record them twice.

Comments: `with_requests` named discovery as the hazard where it is a
child drawn mid-row, `Painter::allocate` documented the window it holds
for rather than what it does, `minimum_request` had none, and the note
saying a span carries its children's weight whole -- which is still what
the unallocated path does, and still the surprising part -- had been
replaced by one about the other path.

Cargo.toml

@@ -25,6 +25,12 @@ tokio = { workspace = true, features = ["sync", "rt", "rt-multi-thread", "time"]
 [workspace]
 members = ["core", "macro", "rig-input"]
 
+# Full debug info was the bulk of what the linker wrote here and almost none of
+# what anything read. `dev` keeps line tables and scopes, which is what stepping
+# through an example wants; the tests keep the line tables alone, which is what
+# a backtrace reads. Measured when the tests became one target: relinking them
+# went from 9.8 s to 7.7 s with these, and target/ from 45 GB to 13 GB with the
+# two changes together.
 [profile.dev]
 debug = 1
 

TODO

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

core/src/fixed.rs

@@ -131,14 +131,6 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
         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))
     }
@@ -246,16 +238,6 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
     }
 }
 
-/// 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() {
@@ -531,15 +513,6 @@ mod tests {
         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));

core/src/layout_diagnostics.rs

@@ -15,7 +15,7 @@
 //! 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 crate::{Axis, LayoutHolds, LayoutLen, PxVec2, Size, UiRegion, UiVec2, WidgetId};
 use std::{
     cell::RefCell,
     collections::{HashMap, HashSet},
@@ -23,104 +23,71 @@ use std::{
     time::Instant,
 };
 
-#[derive(Clone, Copy)]
-pub(crate) enum Counter {
-    Updates,
-    DrawRequests,
-    WidgetDraws,
-    RegionNodeDraws,
-    SizeReads,
-    HintHits,
-    HintMisses,
-    RetainedSizeHits,
-    ReuseAttempts,
-    ReuseExact,
-    ReuseMoved,
-    ReuseDirty,
-    ReuseWrongParent,
-    ReuseRemapped,
-    ReuseOutside,
-    ReuseWrongLayer,
-    ReuseWrongNode,
-    PlaceRedraws,
-    QueuePops,
-    DepthReads,
-    LocalRedraws,
-    SizeChanges,
-    ReaderEdges,
-    PrimitiveWrites,
-    TextRenders,
-    TextShapeHits,
-    TextShapes,
-    TextBreaks,
-    GlyphPlacements,
-    OutsidePinnedLen,
-    OutsideFrame,
-    OutsideExtent,
+/// Declares a counter or timer kind beside the name its report prints. Two
+/// lists in the same order was one list too many: a variant inserted without
+/// its label moving with it renames every total after it, and nothing says
+/// so.
+macro_rules! labelled {
+    ($(#[$meta:meta])* $vis:vis enum $Name:ident { $($variant:ident = $label:literal,)* }) => {
+        $(#[$meta])*
+        #[derive(Clone, Copy)]
+        $vis enum $Name { $($variant,)* }
+
+        impl $Name {
+            const COUNT: usize = [$($label,)*].len();
+            const NAMES: [&'static str; Self::COUNT] = [$($label,)*];
+        }
+    };
 }
 
-impl Counter {
-    const COUNT: usize = Self::OutsideExtent 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",
-        "retained size hits",
-        "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",
-        "placed by redrawing",
-        "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 frame length",
-        "reuse outside: an extent length",
-    ];
+labelled! {
+    pub(crate) enum Counter {
+        Updates = "updates",
+        DrawRequests = "draw requests",
+        WidgetDraws = "widget draws",
+        RegionNodeDraws = "region-node draws",
+        SizeReads = "draw-result size reads",
+        HintHits = "hint hits",
+        HintMisses = "hint misses",
+        ReuseAttempts = "reuse attempts",
+        ReuseExact = "reuse exact",
+        ReuseMoved = "reuse moved",
+        ReuseDirty = "reuse: dirty",
+        ReuseUndrawn = "reuse: nothing drawn to keep",
+        ReuseWrongParent = "reuse: wrong parent",
+        ReuseRemapped = "reuse remapped",
+        ReuseOutside = "reuse: outside what it holds for",
+        ReuseWrongLayer = "reuse: another layer",
+        ReuseWrongNode = "reuse: region-node choice changed",
+        ReuseWrongMask = "reuse: a different inherited mask",
+        QueuePops = "redraw queue pops",
+        DepthReads = "depth reads",
+        LocalRedraws = "local redraws",
+        SizeChanges = "size changes",
+        ReaderEdges = "reader edges",
+        PrimitiveWrites = "primitive writes",
+        TextRenders = "text renders",
+        TextShapeHits = "text shape hits",
+        TextShapes = "text shapes",
+        TextBreaks = "text line breaks",
+        GlyphPlacements = "glyph placements",
+        OutsidePinnedLen = "reuse outside: the length it was pinned to",
+        OutsideWindow = "reuse outside: this window",
+        OutsideRelBase = "reuse outside: a rel base",
+        OutsideRegion = "reuse outside: a region length",
+    }
 }
 
-#[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",
-    ];
+labelled! {
+    pub(crate) enum TimerKind {
+        Update = "update total",
+        FullLayout = "full layout",
+        IncrementalLayout = "incremental layout",
+        TextRender = "text render",
+        TextShape = "text shape",
+        TextBreak = "text line break",
+        GlyphPlacement = "glyph placement",
+    }
 }
 
 #[derive(Clone)]
@@ -255,6 +222,8 @@ pub enum ReuseOutcome {
     Dirty,
     WrongParent,
     WrongLayer,
+    WrongMask,
+    WrongNode,
     Remapped,
     Outside,
     Undrawn,
@@ -268,7 +237,7 @@ pub enum TraceEvent {
         id: WidgetId,
         parent: Option<WidgetId>,
         region: UiRegion,
-        pixel_size: PxVec2,
+        region_px: PxVec2,
         region_node: bool,
     },
     Reuse {
@@ -364,7 +333,7 @@ pub(crate) fn draw_request(
     id: WidgetId,
     parent: Option<WidgetId>,
     region: UiRegion,
-    pixel_size: PxVec2,
+    region_px: PxVec2,
     region_node: bool,
 ) {
     trace(
@@ -373,7 +342,7 @@ pub(crate) fn draw_request(
             id,
             parent,
             region,
-            pixel_size,
+            region_px,
             region_node,
         },
     );
@@ -383,6 +352,41 @@ pub(crate) fn reuse(id: WidgetId, outcome: ReuseOutcome) {
     trace(id, TraceEvent::Reuse { id, outcome });
 }
 
+/// A drawing that cannot be reused because the box on offer is outside what
+/// it holds for, and which of the four contracts said so. They overlap: a
+/// drawing can be outside two of them at once, and counting each is what
+/// says where a rel base redrawing more than it should is coming from.
+pub(crate) fn outside(
+    id: WidgetId,
+    holds: LayoutHolds,
+    region: UiRegion,
+    rel_base: UiVec2,
+    window: PxVec2,
+) {
+    for axis in Axis::BOTH {
+        let holds = holds[axis];
+        let len = region[axis].len();
+        let window = window[axis];
+        if holds.region_len.is_some_and(|pinned| pinned != len) {
+            bump(Counter::OutsidePinnedLen);
+        }
+        if !holds.window.contains(window) {
+            bump(Counter::OutsideWindow);
+        }
+        if holds
+            .rel_base
+            .is_some_and(|pinned| pinned != rel_base[axis])
+        {
+            bump(Counter::OutsideRelBase);
+        }
+        if !holds.region.contains(len.to_px(window)) {
+            bump(Counter::OutsideRegion);
+        }
+    }
+    bump(Counter::ReuseOutside);
+    reuse(id, ReuseOutcome::Outside);
+}
+
 pub(crate) fn size_reported(id: WidgetId, size: Size) {
     trace(id, TraceEvent::SizeReported { id, size });
 }

core/src/lib.rs

@@ -9,6 +9,7 @@
 #![feature(unsize)]
 #![feature(coerce_unsized)]
 #![feature(option_into_flat_iter)]
+#![feature(const_index)]
 
 #[cfg(feature = "layout-diagnostics")]
 pub mod layout_diagnostics;

core/src/orientation/align.rs

@@ -1,8 +1,9 @@
+use crate::util::impl_axis_index;
 use crate::{Px, Rel};
 
 use super::*;
 
-#[derive(Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Align {
     pub x: Option<AxisAlign>,
     pub y: Option<AxisAlign>,
@@ -83,28 +84,6 @@ pub struct RegionAlign {
     pub y: AxisAlign,
 }
 
-impl RegionAlign {
-    /// 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,
-    };
-
-    pub fn axis(&self, axis: Axis) -> AxisAlign {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut AxisAlign {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-}
-
 impl RegionAlign {
     pub const TOP_LEFT: Self = Self::new(AxisAlign::NEG, AxisAlign::NEG);
     pub const TOP_CENTER: Self = Self::new(AxisAlign::CENTER, AxisAlign::NEG);
@@ -172,13 +151,15 @@ impl Vec2 {
 }
 
 impl Len {
+    /// This length placed in the box it is measured in: the alignment names a
+    /// point along that box, and the two ends are that point less the part of
+    /// the length falling before it and plus the part falling after.
     pub const fn align(&self, align: AxisAlign) -> UiSpan {
         let rel = align.rel();
-        let 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))),
+            start: at - self.scale(rel),
+            end: at + self.scale(Rel::ONE.sub(rel)),
         }
     }
 }
@@ -231,3 +212,6 @@ impl RegionAlign {
         UiVec2::from(self)
     }
 }
+
+impl_axis_index!(RegionAlign => AxisAlign);
+impl_axis_index!(Align => Option<AxisAlign>);

core/src/orientation/axis.rs

@@ -1,4 +1,5 @@
 use super::*;
+use crate::util::impl_axis_index;
 use crate::{Fixed, FixedVec2};
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
@@ -8,14 +9,8 @@ pub enum Axis {
 }
 
 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],
-        }
-    }
+    /// Both of them, for the layout code that asks the same question of each.
+    pub const BOTH: [Self; 2] = [Self::X, Self::Y];
 }
 
 impl std::ops::Not for Axis {
@@ -53,20 +48,6 @@ pub enum Sign {
 }
 
 impl<const SHIFT: u32> FixedVec2<SHIFT> {
-    pub const fn axis(&self, axis: Axis) -> Fixed<SHIFT> {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-
-    pub const fn axis_mut(&mut self, axis: Axis) -> &mut Fixed<SHIFT> {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-
     pub const fn from_axis(axis: Axis, aligned: Fixed<SHIFT>, ortho: Fixed<SHIFT>) -> Self {
         match axis {
             Axis::X => Self::new(aligned, ortho),
@@ -76,20 +57,6 @@ impl<const SHIFT: u32> FixedVec2<SHIFT> {
 }
 
 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 {
@@ -104,47 +71,5 @@ impl Vec2 {
     }
 }
 
-pub const trait AxisT {
-    fn get() -> Axis;
-}
-
-pub struct XAxis;
-const impl AxisT for XAxis {
-    fn get() -> Axis {
-        Axis::X
-    }
-}
-
-pub struct YAxis;
-const impl AxisT for YAxis {
-    fn get() -> Axis {
-        Axis::Y
-    }
-}
-
-#[derive(Clone, Copy, Debug, Default)]
-pub struct BothAxis<T> {
-    pub x: T,
-    pub y: T,
-}
-
-impl<T> BothAxis<T> {
-    pub const fn axis<A: const AxisT>(&mut self) -> &mut T {
-        match A::get() {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-    pub fn take_axis<A: const AxisT>(self) -> T {
-        match A::get() {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-    pub fn axis_dyn(&mut self, axis: Axis) -> &mut T {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-}
+impl_axis_index!({const SHIFT: u32} FixedVec2<SHIFT> => Fixed<SHIFT>);
+impl_axis_index!(Vec2 => f32);

core/src/orientation/len.rs

@@ -1,4 +1,5 @@
 use super::*;
+use crate::util::impl_axis_index;
 use crate::{Px, PxVec2, Rel, UiNum, Weight, util::impl_op};
 
 #[derive(Debug, Default, Clone, Copy, PartialEq)]
@@ -22,6 +23,14 @@ pub struct LayoutLen {
     pub leftover: Weight,
 }
 
+/// A bare number is pixels, which is the one length that needs no box to be
+/// read in.
+impl<N: UiNum> From<N> for Len {
+    fn from(value: N) -> Self {
+        Len::px(value.to_f32())
+    }
+}
+
 impl<N: UiNum> From<N> for LayoutLen {
     fn from(value: N) -> Self {
         LayoutLen::px(value.to_f32())
@@ -118,20 +127,6 @@ impl Size {
             },
         }
     }
-
-    pub fn axis(&self, axis: Axis) -> LayoutLen {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut LayoutLen {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
 }
 
 impl LayoutLen {
@@ -158,11 +153,39 @@ impl LayoutLen {
         Len::from_parts(self.rel.add(share), self.px)
     }
 
+    /// 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 = Len::from_parts(self.rel, self.px).within_len(len);
+        let part = self.without_leftover().within_len(len);
         Self {
             px: part.px,
             rel: part.rel,
@@ -236,3 +259,5 @@ impl std::fmt::Display for LayoutLen {
         Ok(())
     }
 }
+
+impl_axis_index!(Size => LayoutLen);

core/src/orientation/pos.rs

@@ -1,3 +1,4 @@
+use crate::util::impl_axis_index;
 use std::{fmt::Display, marker::Destruct};
 
 use super::*;
@@ -61,20 +62,6 @@ impl UiVec2 {
         }
     }
 
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut Len {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
-        }
-    }
-
-    pub fn axis(&self, axis: Axis) -> Len {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.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 {
@@ -176,14 +163,6 @@ impl Len {
         Self::from_parts(Rel::ZERO, Px::from_f32(px))
     }
 
-    pub const fn rel_min() -> Self {
-        Self::ZERO
-    }
-
-    pub const fn rel_max() -> Self {
-        Self::FULL
-    }
-
     pub const fn max(&self, other: Self) -> Self {
         Self {
             rel: self.rel.max(other.rel),
@@ -226,10 +205,6 @@ impl Len {
         })
     }
 
-    pub fn select_len(&self, len: Len) -> Self {
-        len.within_len(*self)
-    }
-
     pub const fn flip(&mut self) {
         self.rel = Rel::ONE.sub(self.rel);
         self.px = self.px.neg();
@@ -294,17 +269,17 @@ impl UiSpan {
         }
     }
 
-    pub const fn len(&self) -> Len {
-        self.end - self.start
+    /// 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)
     }
 
-    /// 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,
-        }
+    pub const fn len(&self) -> Len {
+        self.end - self.start
     }
 }
 
@@ -316,17 +291,6 @@ 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,
@@ -348,20 +312,6 @@ 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(),
@@ -462,3 +412,6 @@ impl Display for PixelRegion {
         write!(f, "{} -> {}", self.top_left, self.bot_right)
     }
 }
+
+impl_axis_index!(UiVec2 => Len);
+impl_axis_index!(UiRegion => UiSpan);

core/src/primitive/text.rs

@@ -133,10 +133,6 @@ impl TextBuffer {
         }
     }
 
-    pub fn new_empty() -> Self {
-        Self::new("")
-    }
-
     pub fn text(&self) -> &str {
         &self.text
     }

core/src/render/atlas.rs

@@ -167,10 +167,6 @@ impl GlyphAtlas {
     pub fn page_count(&self) -> u32 {
         self.pages.len() as u32
     }
-
-    pub fn glyph_count(&self) -> usize {
-        self.entries.len()
-    }
 }
 
 impl Page {

core/src/render/mod.rs

@@ -23,13 +23,22 @@ 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.
+    // Every number both sides count in, written once here rather than a
+    // second time in the shader: a grid the two disagree about puts every
+    // coordinate somewhere else, and a sentinel they disagree about makes one
+    // of them walk a chain from a slot the other says is not there.
     format!(
-        "const PX_STEP: f32 = 1.0 / {}.0;\nconst REL_STEP: f32 = 1.0 / {}.0;\n{PRELUDE}\n{wgsl}",
+        "const PX_STEP: f32 = 1.0 / {}.0;\n\
+         const REL_STEP: f32 = 1.0 / {}.0;\n\
+         const MASK_NONE: u32 = {}u;\n\
+         const MOVE_NONE: u32 = {}u;\n\
+         const CHAIN_LIMIT: u32 = {}u;\n\
+         {PRELUDE}\n{wgsl}",
         1u32 << crate::PX_SHIFT,
         1u32 << crate::REL_SHIFT,
+        MaskIdx::NONE.idx(),
+        MoveIdx::NONE.idx(),
+        crate::CHAIN_LIMIT,
     )
 }
 

core/src/render/shader/prelude.wgsl

@@ -26,9 +26,11 @@ struct MoveOffset {
     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.
+// `PX_STEP`, `REL_STEP`, `MASK_NONE`, `MOVE_NONE` and `CHAIN_LIMIT` are
+// prepended from `iris_core`'s own constants, so none of them is written
+// twice. What the CPU stores is a whole count of each step, and both steps
+// are powers of two, so decoding is exact and the number here is the number
+// the CPU decided.
 
 // Every coordinate the CPU decided is a whole count of `PX_STEP`, so one that
 // composes to within half a step of a pixel boundary is on that boundary and
@@ -70,12 +72,6 @@ struct Region {
     y: UiSpan,
 }
 
-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
@@ -171,7 +167,7 @@ fn vs_main(
 }
 
 fn masked(in: VertexOutput, color: vec4<f32>) -> vec4<f32> {
-    if in.mask_idx == 4294967295u {
+    if in.mask_idx == MASK_NONE {
         return color;
     }
     let mask = masks[in.mask_idx];

core/src/ui/active.rs

@@ -1,6 +1,6 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive, Size,
-    TextureHandle, UiRegion, WidgetId,
+    Bounds, Declared, LayerId, LayoutHolds, MaskIdx, MoveIdx, PlaceDesc, RegionAlign,
+    RetainedPrimitive, Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -9,35 +9,34 @@ use crate::{
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
-    /// Its frame in `parent_move`'s coordinates: what a fraction it declares
-    /// or reports is a fraction of, composed. Everything it draws sits inside
-    /// this by way of `extent`.
-    pub frame_abs: UiRegion,
-    /// Where its drawing goes, in the frame's own coordinates.
-    pub extent: UiRegion,
-    /// That frame in its parent's frame coordinates, before composition:
-    /// forwarded whole by a transparent container, narrowed by a declared
-    /// length. Its length is the same on every ask, which is what
-    /// a local redraw relies on to ask its parent's own question again.
-    pub frame: UiRegion,
-    /// What of its parent's extent the drawing was given, and what it was
-    /// given at the parent's first ask of it -- the question a cold layout
-    /// asks. A part is a length from the extent's start, so an extent that
-    /// moved re-places every child by re-adding that start.
-    pub place: [Place; 2],
-    pub offer_place: [Place; 2],
-    /// The box that ask gave it, in its frame's coordinates. Kept rather
-    /// than worked out again from where its parent's own box is now: a
-    /// parent drawn again in the box its own answer chose gives its children
-    /// boxes it never measured anything in, and the measurement this widget
-    /// answered is the one its parent's layout was built on.
-    pub offer_part: UiRegion,
+    /// Where its drawing goes, in its region node's coordinates.
+    pub placement: UiRegion,
+    /// What a fraction declared or reported under this widget is a fraction
+    /// of, as a length of the window.
+    pub rel_base: UiVec2,
+    /// Where its drawing was put, and where it was asked. The two differ
+    /// where a container asks in one place and puts the answer in another --
+    /// a row measures from its cursor and puts the child in its slot. Each
+    /// carries the rel base that ask stated, so asking again from either is
+    /// the same question it was.
+    pub placed: PlaceDesc,
+    pub asked: PlaceDesc,
+    /// The box it was asked in, in the parent's region-node coordinates: the
+    /// box its drawing was made in and the one its contract is about. Its
+    /// drawing is placed elsewhere by re-expression, never by asking again.
+    pub region: UiRegion,
     /// The measured answer and its dependencies. A hint-only dependency or
     /// a widget first encountered during placement has no measurement yet.
-    pub answer: Option<(Size, LayoutHolds)>,
-    /// What the widget said it used of its frame, the last time it drew.
+    pub answer: Option<Answer>,
+    /// Asked more than once in its parent's last draw -- measured in one box
+    /// and then asked in the one the parent decided. The parent's layout
+    /// rests on the first answer and its drawing on the last, so only the
+    /// parent can ask either again.
+    pub re_asked: bool,
+    /// What the widget reported, in window-unit lengths.
     pub size: Size,
-    /// The frame and extent reads that this drawing holds for.
+    /// 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>,
@@ -47,23 +46,29 @@ pub struct ActiveData {
     pub depth: usize,
     pub textures: Vec<TextureHandle>,
     /// Its primitives, each keeping the box it was written in -- in this
-    /// widget's extent coordinates, which is what a move recomposes from.
+    /// 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>,
+    pub request_deps: Vec<WidgetId>,
+    pub(crate) scratch: DrawScratch,
     /// 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 frame.
+    /// 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: [Option<LayoutLen>; 2],
+    pub declared: Declared,
+    /// Its bounds, resolved the same way. The answer is held to these where
+    /// the box was not, so a change to one changes what it answers even
+    /// where its declared lengths stand.
+    pub bounds: Bounds,
     /// 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 `frame_abs` uses.
+    /// 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.
@@ -77,20 +82,33 @@ pub struct ActiveData {
 }
 
 impl ActiveData {
-    /// What it answered when its parent measured it, where it has been
-    /// measured at all. Not `size`, which is what its last drawing reported:
-    /// a drawing made in the box that answer chose is answering a different
-    /// question.
+    /// 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)
+        self.answer.map(|answer| answer.size)
     }
 
-    /// Whether what it answered still stands for a frame of these pixel
-    /// lengths. The answer was given in the box its parent first asked
-    /// about, which is what it is checked against -- `holds` on the record
-    /// is about the box the answer then chose.
-    pub fn answers_at(&self, px: crate::PxVec2, part: UiRegion) -> bool {
-        self.answer
-            .is_some_and(|(_, holds)| holds.contains(px, part))
+    /// Whether it owns a region node rather than sharing the one it was drawn
+    /// under, which is what its two move indices being different says.
+    pub fn is_region_node(&self) -> bool {
+        self.move_idx != self.parent_move
     }
 }
+
+/// What a widget answered when it was asked: the size it reported, and the
+/// boxes and windows that answer holds for.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Answer {
+    pub size: Size,
+    pub holds: LayoutHolds,
+}
+
+#[derive(Debug, Default)]
+pub(crate) struct DrawScratch {
+    pub children: Vec<WidgetId>,
+    pub size_deps: Vec<WidgetId>,
+    pub under: Vec<(WidgetId, LayoutHolds)>,
+    pub requests: Vec<crate::RequestedLen>,
+    pub lengths: Vec<crate::Px>,
+}

core/src/ui/holds.rs

@@ -1,4 +1,4 @@
-use crate::{Len, Px, REL_SHIFT, fixed::div_toward, fixed::narrow};
+use crate::{Bound, 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:
@@ -19,6 +19,63 @@ pub struct Holds {
     pub hi: Px,
 }
 
+impl Len {
+    /// Whether this is longer than `than` in a window this wide, and the
+    /// windows that answer holds for.
+    ///
+    /// Which is longer is a question in pixels -- `rel(0.5)` is longer than
+    /// 300 px at a box of 600 and shorter at 400 -- and it is asked of the
+    /// difference and answered back through that same difference, so the
+    /// boundary is the comparison's own rather than a second way of finding
+    /// it.
+    pub fn longer_than(&self, than: Len, window: Px) -> (bool, Holds) {
+        let over = *self - than;
+        let longer = over.to_px(window) > Px::ZERO;
+        let side = match longer {
+            true => Px::STEP..=Px::MAX,
+            false => Px::MIN..=Px::ZERO,
+        };
+        (longer, Holds::from(side).through(over))
+    }
+}
+
+impl Bound {
+    /// The end of this bound `len` falls outside, which is the length it
+    /// gets instead of its own, and the windows that answer holds for.
+    /// Nothing where it is inside, which is the answer wherever there is no
+    /// bound at all.
+    ///
+    /// `len` and this bound are lengths of the same thing, whichever that
+    /// is: a box in window lengths wants the bound resolved, and a length a
+    /// widget declares of its rel base wants it as the rule wrote it. Both
+    /// comparisons are in pixels, so each is a question about this window,
+    /// and the box is decided again on the other side of a crossing.
+    pub fn outside(&self, len: Len, window: Px) -> (Option<Len>, Holds) {
+        let mut held = None;
+        let mut holds = Holds::ANY;
+        if let Some(min) = self.min {
+            let (shorter, kept) = min.longer_than(len, window);
+            holds = holds.and(kept);
+            if shorter {
+                held = Some(min);
+            }
+        }
+        if let Some(max) = self.max {
+            let (longer, kept) = held.unwrap_or(len).longer_than(max, window);
+            holds = holds.and(kept);
+            if longer {
+                debug_assert!(
+                    held.is_none(),
+                    "a floor of {:?} over a cap of {max:?} bounds nothing",
+                    self.min,
+                );
+                held = Some(max);
+            }
+        }
+        (held, holds)
+    }
+}
+
 impl Holds {
     pub const ANY: Self = Self {
         lo: Px::MIN,
@@ -33,7 +90,13 @@ impl Holds {
         len.raw() >= self.lo.raw() && len.raw() <= self.hi.raw()
     }
 
-    pub const fn and(self, other: Self) -> Self {
+    /// 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),
@@ -51,7 +114,7 @@ impl Holds {
     /// 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 {
+    pub const fn through(&self, len: Len) -> Self {
         if self.lo.raw() == Px::MIN.raw() && self.hi.raw() == Px::MAX.raw() {
             return Self::ANY;
         }

core/src/ui/layout_holds.rs

@@ -1,63 +1,108 @@
-use crate::{Axis, Holds, Len, PxVec2, UiRegion};
+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: the pixel lengths of its
-/// frame and of its own box that its drawing and its answer hold for, and
-/// the symbolic length of its own box where it read one.
+/// 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 length is a pin rather than a range: a container places its
-/// children as lengths of its frame measured from where its own box starts,
+/// 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. It 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.
+/// 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 frame: [Holds; 2],
-    pub extent: [Holds; 2],
-    pub extent_len: [Option<Len>; 2],
+    pub x: AxisHolds,
+    pub y: AxisHolds,
 }
 
 impl LayoutHolds {
     pub const ANY: Self = Self {
-        frame: [Holds::ANY; 2],
-        extent: [Holds::ANY; 2],
-        extent_len: [None; 2],
+        x: AxisHolds::ANY,
+        y: AxisHolds::ANY,
     };
 
-    pub fn and(self, other: Self) -> Self {
-        let mut result = Self::ANY;
-        for n in 0..2 {
-            result.frame[n] = self.frame[n].and(other.frame[n]);
-            result.extent[n] = self.extent[n].and(other.extent[n]);
-            debug_assert!(
-                self.extent_len[n].is_none()
-                    || other.extent_len[n].is_none()
-                    || self.extent_len[n] == other.extent_len[n]
-            );
-            result.extent_len[n] = self.extent_len[n].or(other.extent_len[n]);
+    pub fn and(&self, other: Self) -> Self {
+        Self {
+            x: self.x.and(other.x),
+            y: self.y.and(other.y),
         }
-        result
     }
 
-    pub fn covers(self, other: Self) -> bool {
-        (0..2).all(|n| {
-            self.frame[n].lo <= other.frame[n].lo
-                && self.frame[n].hi >= other.frame[n].hi
-                && self.extent[n].lo <= other.extent[n].lo
-                && self.extent[n].hi >= other.extent[n].hi
-                && self.extent_len[n].is_none_or(|len| other.extent_len[n] == Some(len))
-        })
+    pub fn covers(&self, other: Self) -> bool {
+        self.x.covers(other.x) && self.y.covers(other.y)
     }
 
-    pub fn contains(self, px: PxVec2, extent: UiRegion) -> bool {
-        AXES.into_iter().all(|axis| {
-            let n = axis as usize;
-            let len = extent.axis(axis).len();
-            self.frame[n].contains(px.axis(axis))
-                && self.extent[n].contains(len.to_px(px.axis(axis)))
-                && self.extent_len[n].is_none_or(|pinned| pinned == len)
-        })
+    pub fn contains(&self, window: PxVec2, rel_base: UiVec2, region: UiRegion) -> bool {
+        Axis::BOTH
+            .into_iter()
+            .all(|axis| self[axis].contains(window[axis], rel_base[axis], region[axis].len()))
     }
 }
+
+impl_axis_index!(LayoutHolds => AxisHolds);

core/src/ui/mod.rs

@@ -20,7 +20,7 @@ pub use active::*;
 pub use holds::*;
 pub use layout_holds::*;
 pub use painter::{Painter, PrimitiveLike};
-pub use place::*;
+pub use place::{PlaceDesc, PlaceDescAxis, PlaceFit, RetainedPrimitive};
 pub use render_state::*;
 
 #[derive(Default)]

core/src/ui/place.rs

@@ -1,54 +1,225 @@
-use crate::{PrimitiveHandle, UiRegion, UiSpan};
+use crate::util::impl_axis_index;
+use crate::{Axis, AxisAlign, Len, PrimitiveHandle, RegionAlign, UiRegion, UiSpan, UiVec2};
 
-/// What of a widget's own box a child is given, along one axis.
+/// 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 enum Part {
-    /// The whole of it.
-    All,
-    /// Frame lengths from where the box starts, which is what a container
-    /// dividing room among its children speaks: a child's report is a length
-    /// of the frame, so the cursor that sums those reports is one too. A
-    /// moved box re-places every child by re-adding its start, exactly.
-    From(UiSpan),
-    /// A part of the box in its own coordinates, which is what a container
-    /// that insets one speaks: taking eleven pixels off the end needs no
-    /// length, where saying the same thing in frame lengths would make the
-    /// container read its own box -- and a box chosen from its own answer
-    /// then feeds back into the answer.
-    Of(UiSpan),
+pub struct PlaceDescAxis {
+    pub span: PlaceSpan,
+    pub fit: PlaceFit,
+    pub rel_base: RelBase,
 }
 
-impl Part {
-    /// Where it lands in the coordinates `extent` is in.
-    pub(crate) fn of(self, extent: UiSpan) -> UiSpan {
-        match self {
-            Self::All => extent,
-            Self::From(span) => UiSpan::new(extent.start + span.start, extent.start + span.end),
-            Self::Of(span) => span.within(&extent),
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum PlaceFit {
+    Align,
+    Fill,
+    /// The parent has already evaluated the child's size request.
+    Allocated,
+}
+
+impl PlaceFit {
+    pub fn fills(&self) -> bool {
+        !matches!(self, Self::Align)
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum PlaceSpan {
+    Within(UiSpan),
+    Shifted(UiSpan),
+    Sized(Len),
+}
+
+/// What a child's fractions are of. [`PlaceSpan::Sized`] is a length the
+/// caller named, which is always its own base, so nothing here constructs one
+/// beside anything but [`Self::Len`].
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum RelBase {
+    /// The caller's own, unchanged.
+    Inherit,
+    /// The caller's own, narrowed the way the region is.
+    WithRegion,
+    /// This length of the window.
+    Len(Len),
+}
+
+impl PlaceDescAxis {
+    /// The whole of the caller's box.
+    pub const WHOLE: Self = UiSpan::FULL.within_desc();
+
+    /// This region is the child's placement: its answer is not placed inside
+    /// it again. A container uses it where it hands back exactly what the
+    /// child asked for -- a row placing a child at the length it reported.
+    pub const fn fills(mut self) -> Self {
+        self.fit = PlaceFit::Fill;
+        self
+    }
+
+    /// A final allocation, including any comparisons in the child's request.
+    pub const fn allocated(mut self) -> Self {
+        self.fit = PlaceFit::Allocated;
+        self
+    }
+
+    /// This along `axis`, and the whole of the caller's box across it: what
+    /// a container dividing one axis says, since nothing divides the other.
+    /// [`PlaceDesc::from_axis`] says the across one where it is not the
+    /// whole.
+    pub const fn on_axis(self, axis: Axis) -> PlaceDesc {
+        PlaceDesc::from_axis(axis, self, Self::WHOLE)
+    }
+
+    /// What the child's fractions are of, as a length of the window: a
+    /// resolved share, or a box a sibling's answer decided.
+    pub const fn rel_base(mut self, len: Len) -> Self {
+        self.rel_base = RelBase::Len(len);
+        self
+    }
+
+    /// Where it lands in the coordinates `own` is in.
+    pub fn of(self, own: UiSpan, align: AxisAlign) -> UiSpan {
+        match self.span {
+            PlaceSpan::Within(span) => span.within(&own),
+            PlaceSpan::Shifted(mut span) => {
+                span.shift(own.start);
+                span
+            }
+            PlaceSpan::Sized(len) => own.place(len, align),
         }
     }
 }
 
-/// Where a child goes along one axis, as a part of this widget's box.
+/// 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 enum Place {
-    /// The child's answer, aligned inside the part by the child's alignment.
-    Within(Part),
-    /// Exactly the part; the answer is not placed inside it again.
-    Fill(Part),
+pub struct PlaceDesc {
+    pub x: PlaceDescAxis,
+    pub y: PlaceDescAxis,
 }
 
-impl Place {
-    pub(crate) fn part(self) -> Part {
-        match self {
-            Self::Within(part) | Self::Fill(part) => part,
+impl PlaceDesc {
+    /// The whole of the caller's box, on both axes.
+    pub const WHOLE: Self = Self::splat(PlaceDescAxis::WHOLE);
+
+    pub const fn new(x: PlaceDescAxis, y: PlaceDescAxis) -> Self {
+        Self { x, y }
+    }
+
+    /// The same on both axes.
+    pub const fn splat(place: PlaceDescAxis) -> Self {
+        Self { x: place, y: place }
+    }
+
+    /// A description per axis, where the two differ and neither is the
+    /// axis a container divides.
+    pub fn from_axes(f: impl Fn(Axis) -> PlaceDescAxis) -> Self {
+        Self::new(f(Axis::X), f(Axis::Y))
+    }
+
+    /// `aligned` on `axis` and `ortho` on the other, which is how a
+    /// container that divides one axis says what it is doing.
+    pub const fn from_axis(axis: Axis, aligned: PlaceDescAxis, ortho: PlaceDescAxis) -> Self {
+        match axis {
+            Axis::X => Self::new(aligned, ortho),
+            Axis::Y => Self::new(ortho, aligned),
         }
     }
 
-    /// Whether the part is the drawing's box outright, rather than the box
-    /// the answer is placed inside.
-    pub(crate) fn fills(self) -> bool {
-        matches!(self, Self::Fill(_))
+    /// Both regions are the child's placement. See [`PlaceDescAxis::fills`].
+    pub const fn fills(self) -> Self {
+        Self::new(self.x.fills(), self.y.fills())
+    }
+
+    /// The child's rel base on one axis. See [`PlaceDescAxis::rel_base`].
+    pub const fn rel_base(mut self, axis: Axis, len: Len) -> Self {
+        self[axis] = self[axis].rel_base(len);
+        self
+    }
+
+    /// What a child's fractions on one axis are of, as a length of the
+    /// window: a length this place names, or the rel base of the widget
+    /// giving it, which is `parent_rel_base`.
+    pub(super) fn base(&self, axis: Axis, parent_rel_base: UiVec2) -> Len {
+        match self[axis].rel_base {
+            RelBase::Len(len) => len,
+            RelBase::Inherit | RelBase::WithRegion => parent_rel_base[axis],
+        }
+    }
+
+    /// The box each axis names, in the coordinates `own` is in.
+    pub fn of(self, own: UiRegion, align: RegionAlign) -> UiRegion {
+        UiRegion::new(self.x.of(own.x, align.x), self.y.of(own.y, align.y))
+    }
+}
+
+impl UiSpan {
+    /// This span composed into the caller's own box, so it moves and scales
+    /// with it: [`UiSpan::within`], which is what a container that insets
+    /// one speaks. Taking eleven pixels off the end needs no length, where
+    /// saying the same thing in window lengths would make the container read
+    /// its own box -- and a box chosen from its own answer then feeds back
+    /// into the answer.
+    ///
+    /// The child's rel base is narrowed the same way, so padding takes its
+    /// pixels off both and `rel(1)` under it fills the caller rather than
+    /// overflowing it.
+    pub const fn within_desc(self) -> PlaceDescAxis {
+        PlaceDescAxis {
+            span: PlaceSpan::Within(self),
+            fit: PlaceFit::Align,
+            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),
+            fit: PlaceFit::Align,
+            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),
+            fit: PlaceFit::Align,
+            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)
     }
 }
 
@@ -59,3 +230,5 @@ pub struct RetainedPrimitive {
     pub handle: PrimitiveHandle,
     pub region: UiRegion,
 }
+
+impl_axis_index!(PlaceDesc => PlaceDescAxis);

core/src/util/arena.rs

@@ -35,7 +35,7 @@ impl<T, I: IdNum> Arena<T, I> {
         self.data[i]
     }
 
-    pub(crate) fn get_mut(&mut self, id: Id<I>) -> &mut T {
+    pub fn get_mut(&mut self, id: Id<I>) -> &mut T {
         &mut self.data[id.idx()]
     }
 }

core/src/util/math.rs

@@ -93,3 +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/widget/mod.rs

@@ -4,6 +4,7 @@ use std::any::Any;
 mod data;
 mod handle;
 mod like;
+mod request;
 mod size_rule;
 mod tag;
 mod view;
@@ -12,6 +13,7 @@ mod widgets;
 pub use data::*;
 pub use handle::*;
 pub use like::*;
+pub use request::*;
 pub use size_rule::*;
 pub use tag::*;
 pub use view::*;
@@ -21,6 +23,12 @@ 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;
 
+    /// Describes an axis before painting. Return `None` when discovering it
+    /// needs a concrete box or work performed by `draw`.
+    fn size_request(&self, requests: &mut SizeRequests, axis: Axis) -> Option<RequestedLen> {
+        self.size_hint(axis).map(|len| requests.length(len))
+    }
+
     /// 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.

core/src/widget/request.rs

@@ -0,0 +1,449 @@
+use crate::{
+    ActiveData, Axis, Bound, LayoutLen, Len, Px, Rel, SizeRule, StrongWidget, UiNum, Weight,
+    WidgetId, Widgets, util::HashMap,
+};
+use std::{cmp::Ordering, sync::Arc};
+
+impl<N: UiNum> From<N> for SizeRequest {
+    fn from(value: N) -> Self {
+        LayoutLen::px(value).into()
+    }
+}
+
+impl LayoutLen {
+    pub fn min(self, other: impl Into<SizeRequest>) -> SizeRequest {
+        SizeRequest::from(self).min(other)
+    }
+
+    pub fn max(self, other: impl Into<SizeRequest>) -> SizeRequest {
+        SizeRequest::from(self).max(other)
+    }
+
+    pub fn clamp(self, min: impl Into<SizeRequest>, max: impl Into<SizeRequest>) -> SizeRequest {
+        SizeRequest::from(self).clamp(min, max)
+    }
+}
+
+/// A size request before a container has divided its leftover space.
+/// Comparisons keep both operands until the share is known.
+#[derive(Clone, Debug, PartialEq)]
+pub enum SizeRequest {
+    Linear(LayoutLen),
+    Sum(Arc<(Self, Self)>),
+    Min(Arc<(Self, Self)>),
+    Max(Arc<(Self, Self)>),
+}
+
+impl From<LayoutLen> for SizeRequest {
+    fn from(len: LayoutLen) -> Self {
+        Self::Linear(len)
+    }
+}
+
+impl From<Len> for SizeRequest {
+    fn from(len: Len) -> Self {
+        LayoutLen::from(len).into()
+    }
+}
+
+impl SizeRequest {
+    pub fn min(self, other: impl Into<Self>) -> Self {
+        let other = other.into();
+        if let (Self::Linear(a), Self::Linear(b)) = (&self, &other)
+            && let Some(order) = independent_order(*a, *b)
+        {
+            return if !order.is_gt() { self } else { other };
+        }
+        if self == other {
+            self
+        } else {
+            Self::Min(Arc::new((self, other)))
+        }
+    }
+
+    pub fn max(self, other: impl Into<Self>) -> Self {
+        let other = other.into();
+        if let (Self::Linear(a), Self::Linear(b)) = (&self, &other)
+            && let Some(order) = independent_order(*a, *b)
+        {
+            return if !order.is_lt() { self } else { other };
+        }
+        if self == other {
+            self
+        } else {
+            Self::Max(Arc::new((self, other)))
+        }
+    }
+
+    pub fn clamp(self, min: impl Into<Self>, max: impl Into<Self>) -> Self {
+        self.max(min).min(max)
+    }
+}
+
+impl std::ops::Add for SizeRequest {
+    type Output = Self;
+
+    fn add(self, other: Self) -> Self {
+        match (self, other) {
+            (Self::Linear(a), Self::Linear(b)) => Self::Linear(a + b),
+            (a, b) => Self::Sum(Arc::new((a, b))),
+        }
+    }
+}
+
+/// A discovered length. Deferred values belong to the current layout pass;
+/// widgets must not retain them. Ordinary requests remain inline lengths.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct RequestedLen(RequestValue);
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum RequestValue {
+    Linear(LayoutLen),
+    Deferred {
+        index: usize,
+        epoch: u64,
+        leftover: bool,
+    },
+}
+
+impl From<LayoutLen> for RequestedLen {
+    fn from(len: LayoutLen) -> Self {
+        Self(RequestValue::Linear(len))
+    }
+}
+impl From<Len> for RequestedLen {
+    fn from(len: Len) -> Self {
+        LayoutLen::from(len).into()
+    }
+}
+impl RequestedLen {
+    /// The length itself, where no comparison is waiting on an allocation.
+    pub fn linear(&self) -> Option<LayoutLen> {
+        match self.0 {
+            RequestValue::Linear(len) => Some(len),
+            _ => None,
+        }
+    }
+    pub fn has_leftover(&self) -> bool {
+        match self.0 {
+            RequestValue::Linear(len) => len.leftover > Weight::ZERO,
+            RequestValue::Deferred { leftover, .. } => leftover,
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+enum Op {
+    Sum,
+    Min,
+    Max,
+}
+struct Node {
+    op: Op,
+    a: RequestedLen,
+    b: RequestedLen,
+}
+
+#[derive(Default)]
+pub(crate) struct RequestArena {
+    nodes: Vec<Node>,
+    epoch: u64,
+}
+
+impl RequestArena {
+    pub(crate) fn reset(&mut self) {
+        self.nodes.clear();
+        self.epoch = self
+            .epoch
+            .checked_add(1)
+            .expect("layout generation exhausted");
+    }
+    pub(crate) fn import(&mut self, request: &SizeRequest, base: Len) -> RequestedLen {
+        let (op, pair) = match request {
+            SizeRequest::Linear(len) => return len.within_len(base).into(),
+            SizeRequest::Sum(pair) => (Op::Sum, pair),
+            SizeRequest::Min(pair) => (Op::Min, pair),
+            SizeRequest::Max(pair) => (Op::Max, pair),
+        };
+        let a = self.import(&pair.0, base);
+        let b = self.import(&pair.1, base);
+        self.combine(op, a, b)
+    }
+    fn combine(&mut self, op: Op, a: RequestedLen, b: RequestedLen) -> RequestedLen {
+        if let (Some(x), Some(y)) = (a.linear(), b.linear()) {
+            if matches!(op, Op::Sum) {
+                return (x + y).into();
+            }
+            let order = independent_order(x, y);
+            if let Some(order) = order {
+                let take_a = match op {
+                    Op::Min => !order.is_gt(),
+                    _ => !order.is_lt(),
+                };
+                return if take_a { a } else { b };
+            }
+        }
+        if a == b && !matches!(op, Op::Sum) {
+            return a;
+        }
+        let index = self.nodes.len();
+        self.nodes.push(Node { op, a, b });
+        RequestedLen(RequestValue::Deferred {
+            index,
+            epoch: self.epoch,
+            leftover: a.has_leftover() || b.has_leftover(),
+        })
+    }
+    pub(crate) fn minimum(&self, request: RequestedLen, window: Px) -> Px {
+        Px::from_raw(self.segment(request, Ratio::ZERO, window).fixed as i32)
+    }
+    fn segment(&self, request: RequestedLen, at: Ratio, window: Px) -> Segment {
+        match request.0 {
+            RequestValue::Linear(len) => {
+                debug_assert!(
+                    len.leftover >= Weight::ZERO,
+                    "a leftover weight cannot be negative"
+                );
+                Segment {
+                    fixed: i64::from(len.without_leftover().to_px(window).raw()),
+                    weight: i64::from(len.leftover.raw()),
+                    end: None,
+                }
+            }
+            RequestValue::Deferred { index, epoch, .. } => {
+                assert_eq!(epoch, self.epoch, "request retained beyond its layout pass");
+                let Node { op, a, b } = self.nodes[index];
+                let a = self.segment(a, at, window);
+                let b = self.segment(b, at, window);
+                if matches!(op, Op::Sum) {
+                    return a + b;
+                }
+                // At a crossing choose the branch to its right, so the next
+                // iteration advances rather than selecting that crossing again.
+                let order = a.value(at).cmp(&b.value(at)).then(a.weight.cmp(&b.weight));
+                let take_a = match op {
+                    Op::Min => !order.is_gt(),
+                    _ => !order.is_lt(),
+                };
+                let mut selected = if take_a { a } else { b };
+                selected.end = first(a.end, b.end);
+                if a.weight != b.weight {
+                    let crossing = Ratio::new(b.fixed - a.fixed, a.weight - b.weight);
+                    if crossing > at {
+                        selected.end = first(selected.end, Some(crossing));
+                    }
+                }
+                selected
+            }
+        }
+    }
+    /// Divides `room` between requests whose weights are nonnegative, one
+    /// length per request. A floor can overflow the room and a cap can leave
+    /// part of it unused, so the lengths need not come to `room`. Each edge
+    /// is rounded from the running total rather than from the length before
+    /// it, so two neighbouring slots meet exactly.
+    pub(crate) fn allocate<'a>(
+        &'a self,
+        requests: &'a [RequestedLen],
+        room: Px,
+        window: Px,
+    ) -> impl Iterator<Item = Px> + 'a {
+        let mut at = Ratio::ZERO;
+        loop {
+            let total = requests.iter().fold(Segment::ZERO, |total, request| {
+                total + self.segment(*request, at, window)
+            });
+            if total.value(at) >= i128::from(room.raw()) * i128::from(at.den) {
+                break;
+            }
+            if total.weight != 0 {
+                let solution = Ratio::new(i64::from(room.raw()) - total.fixed, total.weight);
+                if total.end.is_none_or(|end| solution <= end) {
+                    at = solution;
+                    break;
+                }
+            }
+            match total.end {
+                Some(end) => at = end,
+                None => break,
+            }
+        }
+        let mut prefix = 0_i128;
+        let mut previous = 0_i128;
+        requests.iter().map(move |request| {
+            prefix += self.segment(*request, at, window).value(at);
+            let den = i128::from(at.den);
+            // Half away from zero, which is what `Fixed` rounds a division
+            // to: the two decide the same edge, and a change to one of them
+            // is a change to the other.
+            let edge = prefix.signum() * ((prefix.abs() + den / 2) / den);
+            let len = Px::from_raw((edge - previous) as i32);
+            previous = edge;
+            len
+        })
+    }
+}
+
+#[derive(Clone, Copy, Debug, Eq)]
+struct Ratio {
+    num: i64,
+    den: i64,
+}
+
+impl PartialEq for Ratio {
+    fn eq(&self, other: &Self) -> bool {
+        self.cmp(other).is_eq()
+    }
+}
+
+impl Ratio {
+    const ZERO: Self = Self { num: 0, den: 1 };
+
+    fn new(num: i64, den: i64) -> Self {
+        debug_assert_ne!(den, 0, "a ratio of nothing");
+        if den < 0 {
+            Self {
+                num: -num,
+                den: -den,
+            }
+        } else {
+            Self { num, den }
+        }
+    }
+}
+
+impl Ord for Ratio {
+    fn cmp(&self, other: &Self) -> Ordering {
+        (i128::from(self.num) * i128::from(other.den))
+            .cmp(&(i128::from(other.num) * i128::from(self.den)))
+    }
+}
+
+impl PartialOrd for Ratio {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+#[derive(Clone, Copy)]
+struct Segment {
+    fixed: i64,
+    weight: i64,
+    end: Option<Ratio>,
+}
+
+impl Segment {
+    const ZERO: Self = Self {
+        fixed: 0,
+        weight: 0,
+        end: None,
+    };
+
+    fn value(self, at: Ratio) -> i128 {
+        i128::from(self.fixed) * i128::from(at.den) + i128::from(self.weight) * i128::from(at.num)
+    }
+}
+
+impl std::ops::Add for Segment {
+    type Output = Self;
+    fn add(self, other: Self) -> Self {
+        Self {
+            fixed: self.fixed + other.fixed,
+            weight: self.weight + other.weight,
+            end: first(self.end, other.end),
+        }
+    }
+}
+
+fn first(a: Option<Ratio>, b: Option<Ratio>) -> Option<Ratio> {
+    match (a, b) {
+        (Some(a), Some(b)) => Some(a.min(b)),
+        (a, b) => a.or(b),
+    }
+}
+
+/// Read-only discovery of requests through a widget's children. A request is
+/// expressed in window lengths; `rel_base` supplies the base for declarations.
+pub struct SizeRequests<'a> {
+    pub(crate) arena: &'a mut RequestArena,
+    pub(crate) measured: Option<&'a HashMap<WidgetId, ActiveData>>,
+    pub(crate) widgets: &'a Widgets,
+    pub(crate) dependencies: &'a mut Vec<WidgetId>,
+    pub(crate) rel_base: Len,
+}
+
+impl SizeRequests<'_> {
+    pub fn sum(&mut self, a: RequestedLen, b: RequestedLen) -> RequestedLen {
+        self.arena.combine(Op::Sum, a, b)
+    }
+    pub fn min(&mut self, a: RequestedLen, b: RequestedLen) -> RequestedLen {
+        self.arena.combine(Op::Min, a, b)
+    }
+    pub fn max(&mut self, a: RequestedLen, b: RequestedLen) -> RequestedLen {
+        self.arena.combine(Op::Max, a, b)
+    }
+
+    pub fn widget<W: ?Sized>(
+        &mut self,
+        child: &StrongWidget<W>,
+        axis: Axis,
+    ) -> Option<RequestedLen> {
+        self.dependencies.push(child.id());
+        let rules = self.widgets.size_rules(child.id());
+        let rule = &rules[axis];
+        if let SizeRule::Request(request) = rule {
+            return Some(self.arena.import(request, self.rel_base));
+        }
+        if let Some(exact) = rule.exact() {
+            return Some(exact.within_len(self.rel_base).into());
+        }
+        let widget = self.widgets.get_dyn(child.id())?;
+        let request = widget.size_request(self, axis).or_else(|| {
+            self.measured?
+                .get(&child.id())?
+                .measured()
+                .map(|size| size[axis].into())
+        })?;
+        Some(self.bounded(request, rule.bound()))
+    }
+
+    pub(crate) fn bounded(&mut self, request: RequestedLen, bound: Bound) -> RequestedLen {
+        let bound = bound.within_len(self.rel_base);
+        let request = match bound.min {
+            Some(min) => self.max(request, min.into()),
+            None => request,
+        };
+        match bound.max {
+            Some(max) => self.min(request, max.into()),
+            None => request,
+        }
+    }
+
+    pub fn length(&self, len: LayoutLen) -> RequestedLen {
+        len.within_len(self.rel_base).into()
+    }
+
+    pub fn inset<W: ?Sized>(
+        &mut self,
+        child: &StrongWidget<W>,
+        axis: Axis,
+        padding: Px,
+    ) -> Option<RequestedLen> {
+        let base = self.rel_base;
+        self.rel_base.px -= padding;
+        let request = self.widget(child, axis);
+        self.rel_base = base;
+        request.map(|request| self.sum(request, Len::from_parts(Rel::ZERO, padding).into()))
+    }
+}
+
+// Equal fractions keep this valid even when padding makes a rel base negative.
+fn independent_order(a: LayoutLen, b: LayoutLen) -> Option<Ordering> {
+    if a.rel == b.rel && a.leftover == b.leftover {
+        Some(a.px.cmp(&b.px))
+    } else if a.px == b.px && a.rel == b.rel {
+        Some(a.leftover.cmp(&b.leftover))
+    } else {
+        None
+    }
+}

core/src/widget/size_rule.rs

@@ -1,4 +1,6 @@
-use crate::{Axis, LayoutLen, Weight};
+use crate::util::impl_axis_index;
+use crate::{Axis, LayoutLen, Len, Rel, SizeRequest};
+use std::sync::Arc;
 
 /// 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.
@@ -8,27 +10,81 @@ use crate::{Axis, LayoutLen, Weight};
 /// 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)]
+///
+/// Exact expressions can bound a share before allocation. Bounds on an
+/// intrinsic answer are applied after that answer becomes known.
+#[derive(Debug, Clone, PartialEq, Default)]
 pub enum SizeRule {
     /// Whatever the widget reports from drawing.
     #[default]
     Free,
     /// This length, whatever the widget reports.
     Exact(LayoutLen),
+    /// An exact request whose comparisons await the parent's allocation.
+    Request(Arc<SizeRequest>),
+    /// At least this long, and otherwise whatever the box gives it.
+    Min(Len),
+    /// At most this long.
+    Max(Len),
+    /// Between the two.
+    Clamp { min: Len, max: Len },
 }
 
 impl SizeRule {
-    /// The length this rule gives without the widget being drawn, if it can
-    /// give one. `leftover` is never among them: a share 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<LayoutLen> {
-        match self {
-            Self::Exact(len) if len.leftover == Weight::ZERO => Some(*len),
-            _ => None,
+    /// What this rule allows the length to be where it does not give one
+    /// outright.
+    pub fn bound(&self) -> Bound {
+        match *self {
+            Self::Free | Self::Exact(_) | Self::Request(_) => Bound::ANY,
+            Self::Min(min) => Bound {
+                min: Some(min),
+                max: None,
+            },
+            Self::Max(max) => Bound {
+                min: None,
+                max: Some(max),
+            },
+            Self::Clamp { min, max } => Bound {
+                min: Some(min),
+                max: Some(max),
+            },
         }
     }
 
+    /// Whether what this rule says is a fraction of the rel base, so that
+    /// the same rule against a different one is a different length.
+    pub fn has_fraction(&self) -> bool {
+        self.exact().is_some_and(|len| len.rel != Rel::ZERO) || self.bound().has_fraction()
+    }
+
+    /// This rule with a floor under it, which is the whole of it where there
+    /// was no rule.
+    pub fn at_least(&self, min: Len) -> Self {
+        match *self {
+            Self::Free | Self::Min(_) => Self::Min(min),
+            Self::Max(max) | Self::Clamp { max, .. } => Self::Clamp { min, max },
+            Self::Request(ref request) => request.as_ref().clone().max(min).into(),
+            Self::Exact(len) => len.max(min).into(),
+        }
+    }
+
+    /// This rule with a cap over it, which is the whole of it where there was
+    /// no rule.
+    pub fn at_most(&self, max: Len) -> Self {
+        match *self {
+            Self::Free | Self::Max(_) => Self::Max(max),
+            Self::Min(min) | Self::Clamp { min, .. } => Self::Clamp { min, max },
+            Self::Request(ref request) => request.as_ref().clone().min(max).into(),
+            Self::Exact(len) => len.min(max).into(),
+        }
+    }
+
+    /// The length this rule gives without the widget being drawn, if it can
+    /// give one.
+    pub fn declared(&self) -> Option<Len> {
+        self.exact().and_then(|len| len.declared())
+    }
+
     /// The length this rule gives outright, whatever the widget reports --
     /// which makes the widget's answer on that axis moot. A share counts: it
     /// is a length the widget's parent still has to divide, so it is exact
@@ -36,26 +92,89 @@ impl SizeRule {
     /// 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,
+            Self::Free | Self::Request(_) | Self::Min(_) | Self::Max(_) | Self::Clamp { .. } => {
+                None
+            }
         }
     }
 }
 
+/// What a rule allows a length to be where it does not give one outright: a
+/// floor, a cap, or both. Each is a length of the rel base the widget is
+/// asked with, which is the base a declared length is a fraction of too.
+///
+/// A bound is a [`Len`]. Comparisons involving shares are [`SizeRequest`]s.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct Bound {
+    pub min: Option<Len>,
+    pub max: Option<Len>,
+}
+
+impl Bound {
+    /// Every length.
+    pub const ANY: Self = Self {
+        min: None,
+        max: None,
+    };
+
+    /// Whether either end is a fraction of the rel base, so that the same
+    /// bound against a different one binds at a different length.
+    pub fn has_fraction(&self) -> bool {
+        [self.min, self.max]
+            .into_iter()
+            .flatten()
+            .any(|len| len.rel != Rel::ZERO)
+    }
+
+    /// This bound as lengths of the window, from lengths of a rel base that
+    /// long.
+    pub fn within_len(&self, len: Len) -> Self {
+        Self {
+            min: self.min.map(|min| min.within_len(len)),
+            max: self.max.map(|max| max.within_len(len)),
+        }
+    }
+}
+
+/// One bound per axis, as [`SizeRules`] is one rule per axis.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct Bounds {
+    pub x: Bound,
+    pub y: Bound,
+}
+
+impl Bounds {
+    pub const ANY: Self = Self {
+        x: Bound::ANY,
+        y: Bound::ANY,
+    };
+
+    pub fn from_axes(f: impl Fn(Axis) -> Bound) -> Self {
+        Self {
+            x: f(Axis::X),
+            y: f(Axis::Y),
+        }
+    }
+}
+
+impl_axis_index!(Bounds => Bound);
+
 impl From<LayoutLen> for SizeRule {
     fn from(len: LayoutLen) -> Self {
         Self::Exact(len)
     }
 }
 
+impl From<SizeRequest> for SizeRule {
+    fn from(request: SizeRequest) -> Self {
+        match request {
+            SizeRequest::Linear(len) => Self::Exact(len),
+            request => Self::Request(Arc::new(request)),
+        }
+    }
+}
+
 impl From<Option<LayoutLen>> for SizeRule {
     fn from(len: Option<LayoutLen>) -> Self {
         len.map_or(Self::Free, Self::Exact)
@@ -64,24 +183,35 @@ impl From<Option<LayoutLen>> for SizeRule {
 
 /// 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)]
+#[derive(Debug, Clone, PartialEq, Default)]
 pub struct SizeRules {
     pub x: SizeRule,
     pub y: SizeRule,
 }
 
-impl SizeRules {
-    pub fn axis(&self, axis: Axis) -> SizeRule {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
+impl_axis_index!(SizeRules => SizeRule);
 
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut SizeRule {
-        match axis {
-            Axis::X => &mut self.x,
-            Axis::Y => &mut self.y,
+/// What a widget's box is on each axis where something says so outright,
+/// before it is drawn: a rule beside it, or a hint it gives about itself.
+/// Whoever draws the widget resolves these against its rel base.
+///
+/// A [`Len`] rather than a [`LayoutLen`], because a share can never be one
+/// -- see [`LayoutLen::declared`].
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Declared {
+    pub x: Option<Len>,
+    pub y: Option<Len>,
+}
+
+impl Declared {
+    pub const NONE: Self = Self { x: None, y: None };
+
+    pub fn from_axes(f: impl Fn(Axis) -> Option<Len>) -> Self {
+        Self {
+            x: f(Axis::X),
+            y: f(Axis::Y),
         }
     }
 }
+
+impl_axis_index!(Declared => Option<Len>);

core/src/widget/widgets.rs

@@ -1,8 +1,8 @@
 use std::sync::mpsc::{Receiver, Sender, channel};
 
 use crate::{
-    Axis, AxisAlign, IdLike, RegionAlign, SizeRule, SizeRules, StrongWidget, WeakWidget, Widget,
-    WidgetData, WidgetId,
+    Axis, AxisAlign, IdLike, Len, RegionAlign, SizeRule, SizeRules, StrongWidget, WeakWidget,
+    Widget, WidgetData, WidgetId,
     util::{DynBorrower, HashSet, SlotVec, forget_mut, to_mut},
 };
 
@@ -30,6 +30,14 @@ impl Widgets {
         !self.needs_redraw.is_empty()
     }
 
+    /// Marks this widget for the next frame to draw again, with nothing about
+    /// it changed. Taking a widget mutably marks it too, which is the ordinary
+    /// content-change signal; this is for a change the borrow cannot express,
+    /// and for asking for the same tree over again.
+    pub fn mark_for_redraw(&mut self, id: impl IdLike) {
+        self.needs_redraw.insert(id.id());
+    }
+
     pub fn get_dyn(&self, id: WidgetId) -> Option<&dyn Widget> {
         Some(self.vec.get(id)?.widget.as_ref())
     }
@@ -41,14 +49,14 @@ impl Widgets {
 
     /// get_dyn but dynamic borrow checking of widgets
     /// lets you do recursive (tree) operations, like the painter does
-    pub(crate) fn get_dyn_dynamic<'a>(&self, id: WidgetId) -> WidgetWrapper<'a> {
+    pub(crate) fn get_dyn_dynamic<'a>(&self, id: WidgetId) -> DynBorrower<'a, dyn Widget> {
         // SAFETY: must guarantee no other mutable references to this widget exist
         // done through the borrow variable
         let data = unsafe { forget_mut(to_mut(self.vec.get(id).unwrap())) };
         if data.borrowed {
             panic!("tried to mutably borrow the same widget twice");
         }
-        WidgetWrapper::new(data.widget.as_mut(), &mut data.borrowed)
+        DynBorrower::new(data.widget.as_mut(), &mut data.borrowed)
     }
 
     pub fn get<I: IdLike>(&self, id: &I) -> Option<&I::Widget>
@@ -120,8 +128,8 @@ impl Widgets {
     }
 
     /// 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
+    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
@@ -130,13 +138,29 @@ impl Widgets {
     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_mut(axis) == rule {
+        if data.size[axis] == rule {
             return;
         }
-        *data.size.axis_mut(axis) = rule;
+        data.size[axis] = rule;
         self.needs_redraw.insert(id);
     }
 
+    /// Puts a floor under this widget's length on one axis, keeping a cap it
+    /// already had. See [`SizeRule::at_least`].
+    pub fn set_min_len(&mut self, id: impl IdLike, axis: Axis, min: Len) {
+        let id = id.id();
+        let rule = self.size_rules(id)[axis].at_least(min);
+        self.set_size_rule(id, axis, rule);
+    }
+
+    /// Puts a cap over it, keeping a floor it already had. See
+    /// [`SizeRule::at_most`].
+    pub fn set_max_len(&mut self, id: impl IdLike, axis: Axis, max: Len) {
+        let id = id.id();
+        let rule = self.size_rules(id)[axis].at_most(max);
+        self.set_size_rule(id, axis, rule);
+    }
+
     /// 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
@@ -147,14 +171,14 @@ impl Widgets {
     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_mut(axis) == align {
+        if data.align[axis] == align {
             return;
         }
-        *data.align.axis_mut(axis) = align;
+        data.align[axis] = align;
         self.needs_redraw.insert(id);
     }
 
-    /// Both axes at once, for a caller holding a pair.
+    /// Both axes at once.
     pub fn set_size_rules(
         &mut self,
         id: impl IdLike,
@@ -188,8 +212,6 @@ impl Default for Widgets {
     }
 }
 
-pub type WidgetWrapper<'a> = DynBorrower<'a, dyn Widget>;
-
 impl<I: IdLike> std::ops::Index<I> for Widgets
 where
     I::Widget: Sized + Widget,

macro/src/lib.rs

@@ -18,6 +18,7 @@ struct Input {
 }
 
 struct InputFn {
+    attrs: Vec<Attribute>,
     sig: Signature,
     body: Block,
 }
@@ -32,9 +33,10 @@ impl Parse for Input {
         input.parse::<Token![;]>()?;
         let mut fns = Vec::new();
         while !input.is_empty() {
+            let attrs = input.call(Attribute::parse_outer)?;
             let sig = input.parse()?;
             let body = input.parse()?;
-            fns.push(InputFn { sig, body })
+            fns.push(InputFn { attrs, sig, body })
         }
         if !input.is_empty() {
             input.error("function expected");
@@ -59,10 +61,15 @@ pub fn widget_trait(input: TokenStream) -> TokenStream {
         fns,
     } = parse_macro_input!(input as Input);
 
-    let sigs: Vec<_> = fns.iter().map(|f| f.sig.clone()).collect();
+    // What a method says about itself belongs on the trait, where a reader
+    // looks it up; the implementation is the same text and says it again.
+    let sigs: Vec<_> = fns
+        .iter()
+        .map(|InputFn { attrs, sig, .. }| quote! { #(#attrs)* #sig })
+        .collect();
     let impls: Vec<_> = fns
         .iter()
-        .map(|InputFn { sig, body }| quote! { #sig #body })
+        .map(|InputFn { attrs, sig, body }| quote! { #(#attrs)* #sig #body })
         .collect();
 
     let Some(GenericParam::Type(state)) = generics.params.first() else {

scripts/run-headless.sh

@@ -106,11 +106,16 @@ 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%@*}
+# The extent `replay-touch` positions against, so a script's coordinates are
+# the output's own pixels. Set beside every mode change, since a gesture
+# scaled against a mode the output no longer has lands somewhere else and
+# still looks like a run that worked.
+set_mode() {
+    swaymsg output HEADLESS-1 mode "$1" >/dev/null
+    out_w=${1%x*}
+    out_h=${1#*x}; out_h=${out_h%@*}
+}
+set_mode "$mode"
 
 # Built before the app starts, so a compile error is not reported as a
 # window that failed to move.
@@ -149,7 +154,7 @@ while [ $i -lt "$((seconds * 2))" ]; do
 done
 
 if [ -n "$resize" ] && kill -0 "$pid" 2>/dev/null; then
-    swaymsg output HEADLESS-1 mode "$resize" >/dev/null
+    set_mode "$resize"
     echo "run-headless: resized to $resize" >&2
     sleep 2
 fi

src/default/attr.rs

@@ -15,9 +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;
-            // 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();
+            // The two regions are on the grid and the pointer is not, so the
+            // step between them is taken there and the pointer keeps the
+            // precision the platform gave it.
+            let pos = ctx.data.pos + (container_pos - id_pos).to_f32();
             let size = region.size().to_f32();
             select(
                 rsc,

src/default/render.rs

@@ -22,12 +22,12 @@ impl UiRenderer {
     }
 
     pub fn draw(&mut self) {
-        let output = match self.surface.get_current_texture() {
-            CurrentSurfaceTexture::Success(texture) => texture,
-            CurrentSurfaceTexture::Suboptimal(texture) => {
-                self.surface.configure(&self.device, &self.config);
-                texture
-            }
+        let (output, suboptimal) = match self.surface.get_current_texture() {
+            CurrentSurfaceTexture::Success(texture) => (texture, false),
+            // Used for this frame, and the swapchain rebuilt after it has
+            // been presented: configuring the surface while a texture it
+            // handed out is still alive panics.
+            CurrentSurfaceTexture::Suboptimal(texture) => (texture, true),
             CurrentSurfaceTexture::Outdated | CurrentSurfaceTexture::Lost => {
                 self.surface.configure(&self.device, &self.config);
                 return;
@@ -60,6 +60,9 @@ impl UiRenderer {
         self.queue.submit(std::iter::once(encoder.finish()));
         self.window.pre_present_notify();
         self.queue.present(output);
+        if suboptimal {
+            self.surface.configure(&self.device, &self.config);
+        }
     }
 
     pub fn resize(&mut self, size: &PhysicalSize<u32>) {

src/random.rs

@@ -8,23 +8,16 @@
 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>,
+    pub sizes: HashMap<usize, SizeRules>,
     /// Which children a span has, by the order the spans were made.
     pub spans: HashMap<usize, SpanEdit>,
     /// Alignments, by the order they were put on.
-    pub aligns: HashMap<usize, Aligns>,
+    pub aligns: HashMap<usize, Align>,
     /// Which widgets own a movable region, by the order they were offered
     /// one. Region nodes change what a move writes and how deep a primitive's
     /// chain is, so a tree that never grows one leaves both untested.
@@ -118,20 +111,34 @@ pub struct Branch {
 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 = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
+        let top = UiSpan::new(Len::ZERO, cut).shifted_desc();
         let measured = painter
-            .widget_at(&self.probe, UiRegion::FULL, [Place::Within(Part::All), top])
+            .widget_at(&self.probe, top.on_axis(Axis::Y))
             .len(Axis::X);
-        let px = measured.apply_leftover().to_px(painter.px_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 = Place::Within(Part::Of(UiSpan::new(cut, Len::FULL)));
-        let place = [Place::Within(Part::All), below];
-        match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
-            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
+        let below = UiSpan::new(cut, painter.region_len(Axis::Y)).shifted_desc();
+        let place = below.on_axis(Axis::Y);
+        match px > threshold {
+            true => painter.widget_at(&self.wide, place),
+            false => painter.widget_at(&self.narrow, place),
         };
         Size::LEFTOVER
     }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
 }
 
 pub struct Spanned {
@@ -162,9 +169,11 @@ pub struct Plan {
     /// 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>,
+    pub size: Option<SizeRules>,
+    /// The alignment it carries, under the same one-offer rule. An axis left
+    /// out takes the centered default, which is what [`RegionAlign`] reads it
+    /// as.
+    pub align: Option<Align>,
     /// Whether it was offered a movable region of its own and what it
     /// answered. `Some(false)` is an offer declined, which still uses up the
     /// one offer, where `None` is an offer never made.
@@ -181,6 +190,9 @@ pub enum Kind {
         color: usize,
         alpha: u8,
     },
+    /// The one leaf whose own length is a number of pixels it knows before it
+    /// is drawn, which is the hint a rule beside it has to win over.
+    Image,
     /// Scrolling reads the pixel length of its box, which nothing else here
     /// does, and gives its child a box longer than its own.
     Scroll {
@@ -280,7 +292,7 @@ impl Plan {
                 align: None,
                 ..self.clone()
             }),
-            self.size.map(|_| Plan {
+            self.size.as_ref().map(|_| Plan {
                 size: None,
                 ..self.clone()
             }),
@@ -330,6 +342,21 @@ impl Plan {
         at(self);
     }
 
+    /// Drops every intrinsic bound from this tree, leaving the rest of it
+    /// -- and the generator's draws -- exactly as they were. That isolates
+    /// the ordinary path from the deferred one over the same shapes, which
+    /// is what says whether a difference is the bounds or the trees.
+    pub fn drop_bounds(&mut self) {
+        self.walk_mut(&mut |node| {
+            let Some(rules) = &mut node.size else { return };
+            for axis in Axis::BOTH {
+                if rules[axis].bound() != Bound::ANY {
+                    rules[axis] = SizeRule::Free;
+                }
+            }
+        });
+    }
+
     /// The same tree with `edits` applied, by the indices the generator would
     /// have used for them.
     ///
@@ -361,7 +388,7 @@ impl Plan {
             }
             if plan.size.is_some() {
                 if let Some(lens) = edits.sizes.get(&sized) {
-                    plan.size = Some(*lens);
+                    plan.size = Some(lens.clone());
                 }
                 sized += 1;
             }
@@ -434,9 +461,11 @@ impl Kind {
         }
         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.
+            // that does not, then the one that measures nothing at all. A
+            // picture measures nothing either, but its length is its own, so
+            // it steps to the leaf that takes whatever it is given.
             Kind::Wrapped => out.push(Kind::OneLine),
-            Kind::OneLine => out.push(Kind::Rect {
+            Kind::OneLine | Kind::Image => out.push(Kind::Rect {
                 color: 0,
                 alpha: 255,
             }),
@@ -618,9 +647,10 @@ struct Sow<'a> {
 
 impl Sow<'_> {
     fn leaf(&mut self) -> Plan {
-        Plan::bare(match self.rng.below(4) {
+        Plan::bare(match self.rng.below(5) {
             0 => Kind::Wrapped,
             1 => Kind::OneLine,
+            2 => Kind::Image,
             _ => {
                 let color = self.rng.below(COLORS.len());
                 let alpha = (self.rng.below(5) * 63) as u8;
@@ -629,15 +659,49 @@ impl Sow<'_> {
         })
     }
 
-    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 len(&mut self) -> LayoutLen {
+        LayoutLen::px(20.0 + self.rng.below(180) as f32)
+    }
+
+    /// A length of a box rather than a length of the window, which is what a
+    /// bound is.
+    ///
+    /// Pixels only, for now. A fraction in a bound is resolved against the rel
+    /// base the widget was asked with, and `place_at` hands a parent a
+    /// retained answer without checking that the answer still holds for the
+    /// rel base this place gives -- so a fraction resolved against one rel
+    /// base survives into another. Seeds 4 (shuffle-all-but-first) and 196
+    /// (resize-size) at depth 5 are where that showed; both pass with pixels.
+    /// The hole is older than bounds -- an `Exact` rule that is a fraction
+    /// can reach it too -- and closing it is a check at the re-place site.
+    fn bound(&mut self) -> Len {
+        Len::px(20.0 + self.rng.below(180) as f32)
+    }
+
+    fn rule(&mut self) -> SizeRule {
+        match self.rng.below(8) {
+            0 | 1 => self.len().into(),
+            2 => LayoutLen::LEFTOVER.into(),
+            3 => SizeRule::Min(self.bound()),
+            4 => SizeRule::Max(self.bound()),
+            // Both in pixels, so one can be put under the other: a floor and
+            // a cap that change sides with the window bound nothing, which
+            // is a caller's bug rather than a tree to grow.
+            5 => {
+                let (a, b) = (
+                    Px::from_f32(20.0 + self.rng.below(180) as f32),
+                    Px::from_f32(20.0 + self.rng.below(180) as f32),
+                );
+                SizeRule::Clamp {
+                    min: Len::px(a.min(b).to_f32()),
+                    max: Len::px(a.max(b).to_f32()),
+                }
+            }
+            _ => SizeRule::Free,
         }
     }
 
-    fn align(&mut self) -> Aligns {
+    fn align(&mut self) -> Align {
         let axis = |s: &mut Self| match s.rng.below(4) {
             0 => None,
             1 => Some(AxisAlign::NEG),
@@ -647,21 +711,27 @@ impl Sow<'_> {
         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],
+            true => Align {
+                x: Some(AxisAlign::CENTER),
+                y,
+            },
+            false => Align { x, y },
         }
     }
 
     /// A declared size over half the tree, kept where a test can change it.
     fn sized(&mut self, inner: &mut Plan) {
         let take = self.rng.chance();
-        let lens = [self.len(), self.len()];
+        let lens = SizeRules {
+            x: self.rule(),
+            y: self.rule(),
+        };
         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));
+        inner.size = Some(self.edits.sizes.get(&idx).cloned().unwrap_or(lens));
     }
 
     /// An alignment over some of the tree, kept where a test can change it.
@@ -759,10 +829,6 @@ impl Sow<'_> {
         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);
@@ -782,6 +848,7 @@ pub fn build<Rsc: UiRsc + 'static>(rsc: &mut Rsc, plan: &Plan) -> (StrongWidget,
     let mut build = Build {
         rsc,
         tree: Tree::default(),
+        checkerboard: None,
     };
     let root = build.node(plan);
     (root, build.tree)
@@ -790,23 +857,25 @@ pub fn build<Rsc: UiRsc + 'static>(rsc: &mut Rsc, plan: &Plan) -> (StrongWidget,
 struct Build<'a, Rsc> {
     rsc: &'a mut Rsc,
     tree: Tree,
+    /// The checkerboard, uploaded when the first image in this tree is built.
+    /// A handle is a reference to the texture, so every image after that one
+    /// clones this rather than uploading the same picture again.
+    checkerboard: Option<TextureHandle>,
 }
 
 impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
     fn node(&mut self, plan: &Plan) -> StrongWidget {
         let built = self.kind(&plan.kind);
         let id = built.id();
-        if let Some(lens) = plan.size {
-            self.rsc
-                .ui_mut()
-                .widgets
-                .set_size_rules(id, lens[0], lens[1]);
+        if let Some(lens) = plan.size.clone() {
+            self.rsc.ui_mut().widgets.set_size_rules(id, lens.x, lens.y);
             self.tree.sized.push(id);
         }
         if let Some(align) = plan.align {
+            let resolved = RegionAlign::from(align);
             let widgets = &mut self.rsc.ui_mut().widgets;
-            for (axis, align) in [Axis::X, Axis::Y].into_iter().zip(align) {
-                widgets.set_alignment(id, axis, align.unwrap_or_default());
+            for axis in Axis::BOTH {
+                widgets.set_alignment(id, axis, resolved[axis]);
             }
             self.tree.aligned.push(id);
         }
@@ -817,6 +886,20 @@ impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
         built
     }
 
+    /// The one picture the generated trees draw: a 64x64 checkerboard of purple
+    /// and black in 8 px cells. Committed rather than drawn here, so that one
+    /// seed is one tree whatever anything else does, and included rather than
+    /// opened, so that growing a tree does not depend on a working directory.
+    fn checkerboard(&mut self) -> TextureHandle {
+        if self.checkerboard.is_none() {
+            let image = include_bytes!("assets/checkerboard.png")
+                .get_image()
+                .expect("the checkerboard is committed beside this file");
+            self.checkerboard = Some(self.rsc.ui_mut().textures.add(image));
+        }
+        self.checkerboard.clone().unwrap()
+    }
+
     fn kind(&mut self, kind: &Kind) -> StrongWidget {
         let id: StrongWidget = match kind {
             Kind::Wrapped => wtext(WORDS).size(16).wrap(true).add_strong(self.rsc),
@@ -825,6 +908,7 @@ impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
                 .wrap(false)
                 .add_strong(self.rsc),
             Kind::Rect { color, alpha } => rect(COLORS[*color].alpha(*alpha)).add_strong(self.rsc),
+            Kind::Image => Image::new(self.checkerboard()).add_strong(self.rsc),
             Kind::Scroll { axis, inner } => {
                 let inner = self.node(inner);
                 let id = Scroll::new(inner, *axis).add(self.rsc);
@@ -901,6 +985,10 @@ impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
                     gap: Px::from_int(*gap),
                 }
                 .add(self.rsc);
+                // A row takes the height it is given rather than its tallest
+                // child, which is a rule beside the span rather than anything
+                // it draws. Derived from `dir` rather than stored, so a plan
+                // that says the direction says this too.
                 if dir.axis == Axis::X {
                     self.rsc
                         .widgets_mut()

src/widget/image.rs

@@ -12,7 +12,16 @@ impl Widget for Image {
     }
 
     fn size_hint(&self, axis: Axis) -> Option<LayoutLen> {
-        Some(LayoutLen::px(self.handle.size().axis(axis)))
+        Some(LayoutLen::px(self.handle.size()[axis]))
+    }
+}
+
+impl Image {
+    /// One texture already uploaded, for a caller holding its handle: [`image()`]
+    /// uploads what it is given, and several widgets showing one picture want
+    /// one upload and one slot between them.
+    pub fn new(handle: TextureHandle) -> Self {
+        Self { handle }
     }
 }
 

src/widget/mask.rs

@@ -15,4 +15,8 @@ impl Widget for Masked {
         // draw, and the framework would place the drawing it clipped away.
         Size::LEFTOVER
     }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
 }

src/widget/position/layer.rs

@@ -6,6 +6,10 @@ pub struct LayerOffset {
 }
 
 impl Widget for LayerOffset {
+    fn size_request(&self, requests: &mut SizeRequests, axis: Axis) -> Option<RequestedLen> {
+        requests.widget(&self.inner, axis)
+    }
+
     fn draw(&mut self, painter: &mut Painter) -> Size {
         for _ in 0..self.offset {
             painter.next_layer();

src/widget/position/max_size.rs

@@ -0,0 +1,68 @@
+use crate::prelude::*;
+
+/// Asks its child in the shorter of a cap and the box this widget was given,
+/// and answers what the child used, held to the same cap.
+///
+/// A cap on the box is a widget rather than a [`SizeRule`] because a box is
+/// whoever asked's to decide: a rule that read the box it was given would be
+/// decided again by every path that hands a widget one, including the ones
+/// that re-place a drawing without asking it anything, and the decision would
+/// then depend on which path arrived last. A widget is drawn again whenever
+/// its own box changes, so the comparison is made where the answer can be
+/// kept -- `longer_than` narrows the windows this drawing holds for, and
+/// `holds` says the box lengths.
+///
+/// The box is what a text wraps at and what a scroll takes its viewport from,
+/// which is why capping the answer alone is not the same thing.
+pub struct MaxSize {
+    pub inner: StrongWidget,
+    pub x: Option<Len>,
+    pub y: Option<Len>,
+}
+
+impl MaxSize {
+    fn max(&self, axis: Axis) -> Option<Len> {
+        match axis {
+            Axis::X => self.x,
+            Axis::Y => self.y,
+        }
+    }
+}
+
+impl Widget for MaxSize {
+    fn size_request(&self, requests: &mut SizeRequests, axis: Axis) -> Option<RequestedLen> {
+        let inner = requests.widget(&self.inner, axis)?;
+        Some(match self.max(axis) {
+            Some(max) => requests.min(inner, max.into()),
+            None => inner,
+        })
+    }
+
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let align = painter.alignment();
+        let mut region = UiRegion::FULL;
+        for axis in Axis::BOTH {
+            let Some(max) = self.max(axis) else {
+                continue;
+            };
+            let own = painter.region_len(axis);
+            if painter.longer_than(own, max, axis) {
+                region[axis] = max.align(align[axis]);
+            }
+        }
+        let mut size = painter.widget_at(&self.inner, region).size();
+        for axis in Axis::BOTH {
+            // The child may draw past the box it was given -- a text too tall
+            // for it -- and the cap is a promise about the length as well. A
+            // share passes through: it is a length only to whoever divides
+            // one, and that is this widget's parent rather than this widget,
+            // which has already given the share the box the cap allows.
+            if let Some(max) = self.max(axis)
+                && painter.longer_than(size[axis].without_leftover(), max, axis)
+            {
+                size[axis] = max.into();
+            }
+        }
+        size
+    }
+}

src/widget/position/mod.rs

@@ -1,4 +1,5 @@
 mod layer;
+mod max_size;
 mod offset;
 mod pad;
 mod scroll;
@@ -6,6 +7,7 @@ mod span;
 mod stack;
 
 pub use layer::*;
+pub use max_size::*;
 pub use offset::*;
 pub use pad::*;
 pub use scroll::*;

src/widget/position/offset.rs

@@ -6,15 +6,13 @@ pub struct Offset {
 }
 
 impl Widget for Offset {
+    fn size_request(&self, requests: &mut SizeRequests, axis: Axis) -> Option<RequestedLen> {
+        requests.widget(&self.inner, axis)
+    }
+
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        // The whole of this widget's box, moved: the frame passes through, so
-        // what the child declares or reports means the same as it would
-        // without the offset.
-        let moved = |len: Len, amt: Len| Place::Within(Part::From(UiSpan::new(amt, len + amt)));
-        let place = [
-            moved(painter.extent_len(Axis::X), self.amt.x),
-            moved(painter.extent_len(Axis::Y), self.amt.y),
-        ];
-        painter.widget_at(&self.inner, UiRegion::FULL, place).size()
+        painter
+            .widget_at(&self.inner, UiRegion::FULL.offset(self.amt))
+            .size()
     }
 }

src/widget/position/pad.rs

@@ -6,6 +6,10 @@ pub struct Pad {
 }
 
 impl Widget for Pad {
+    fn size_request(&self, requests: &mut SizeRequests, axis: Axis) -> Option<RequestedLen> {
+        requests.inset(&self.inner, axis, self.padding.along(axis))
+    }
+
     fn draw(&mut self, painter: &mut Painter) -> 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
@@ -14,30 +18,19 @@ impl Widget for Pad {
         // 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.
         //
-        // The padding goes around what it pads: the frame passes through, so
-        // the inner's fractions mean what they would without it, and only
-        // the box it draws in is moved in by the pixels. Said as a part of
-        // this widget's own box in that box's own lengths, so nothing here
-        // reads how long the box is -- and a box chosen from this widget's
-        // own answer therefore does not feed back into that answer.
-        let inset = |lead: Px, trail: Px| {
-            Place::Within(Part::Of(UiSpan::new(
-                Len::from_parts(Rel::ZERO, lead),
-                Len::from_parts(Rel::ONE, -trail),
-            )))
-        };
-        let place = [
-            inset(self.padding.left, self.padding.right),
-            inset(self.padding.top, self.padding.bottom),
-        ];
-        let inner = painter.widget_at(&self.inner, UiRegion::FULL, place).size();
+        // Padding is an inset of both: it comes off the rel base, so `rel(1)`
+        // under it fills this widget rather than overflowing it by the
+        // padding, and it comes off the box, so what is drawn sits inside.
+        // The two stay distinct -- the box can be narrower still, where a row
+        // asked this widget in the room left, and a text wraps at that.
+        let inner = painter.widget_at(&self.inner, self.padding.region()).size();
         Size {
             x: LayoutLen {
-                px: inner.x.px + self.padding.left + self.padding.right,
+                px: inner.x.px + self.padding.along(Axis::X),
                 ..inner.x
             },
             y: LayoutLen {
-                px: inner.y.px + self.padding.top + self.padding.bottom,
+                px: inner.y.px + self.padding.along(Axis::Y),
                 ..inner.y
             },
         }
@@ -68,6 +61,16 @@ impl Padding {
             bottom: amt,
         }
     }
+
+    /// Both sides of one axis together, which is what this padding takes
+    /// of a length along it.
+    pub fn along(&self, axis: Axis) -> Px {
+        match axis {
+            Axis::X => self.left + self.right,
+            Axis::Y => self.top + self.bottom,
+        }
+    }
+
     /// `region` less this padding on each side.
     pub fn region_of(&self, mut region: UiRegion) -> UiRegion {
         region.x.start.px += self.left;

src/widget/position/scroll.rs

@@ -12,67 +12,68 @@ pub struct Scroll {
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let container_len = painter.px_len(self.axis);
-        // Measured in the whole viewport, then drawn at the scrolled offset.
-        let whole = UiRegion::FULL;
+        // Asked in the whole viewport, then put at the scrolled offset.
         let answer_len = painter
-            .widget_at(&self.inner, whole, [Place::Fill(Part::All); 2])
+            .widget_at(&self.inner, PlaceDesc::WHOLE.fills())
             .len(self.axis);
-        let fixed = Len::from_parts(answer_len.rel, answer_len.px).to_px(container_len);
+        let answer_px = painter.to_px(answer_len.without_leftover(), self.axis);
         self.container_len = container_len;
-        self.content_len = fixed.max(container_len);
+        self.content_len = answer_px.max(container_len);
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
         }
         self.update_amt();
-        let align = painter.alignment().axis(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.rel == Rel::ZERO && answer_len.leftover == Weight::ZERO;
-        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 {
+        // Reading the box in pixels above holds this drawing to that one
+        // length, so these two say where it holds more widely.
+        //
+        // Content of a fixed length that fits is handed the whole box below,
+        // and nothing here reads the box again, so every longer box gives the
+        // same drawing: it holds from the length the content needs upwards,
+        // and shrinking past that is what changes it. Where it sits in a box
+        // longer than itself is not this widget's to say -- placing its
+        // answer in the whole box is its own alignment, and that placement is
+        // a fraction of the box, so it holds at every length too.
+        //
+        // One scrolled part way sits where it is until the box shrinks past
+        // what is left of it. Kept to the end, it moves with every length.
+        let answer_is_px = answer_len.is_px();
+        if answer_is_px && self.content_len <= self.container_len {
+            painter.holds(self.axis, answer_px..=Px::MAX);
+        } else if answer_is_px && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, Px::MIN..=left);
         }
 
-        // 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());
-        let mut content = UiSpan::FULL;
-        // 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;
-        if moved || self.content_len != self.container_len {
-            let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
-            content = UiSpan::new(start, start.offset(self.content_len));
-        }
-        // The viewport is the inner's frame, so a fraction it declares or
+        // Content that fills the viewport is the viewport, and is handed back
+        // as it came -- it has nothing to scroll through, so the clamp above
+        // has already put `amt` at zero. Writing the same box as its own
+        // length in pixels is the same box in another form, and the two do
+        // not round alike: a part centred in `rel 1` lands a step from one
+        // centred in `px 900`, since halving a difference is not halving each
+        // part of it.
+        let content = match self.content_len > self.container_len {
+            true => {
+                let start = Len::from_parts(Rel::ZERO, -self.amt);
+                UiSpan::new(start, start.offset(self.content_len)).shifted_desc()
+            }
+            false => PlaceDescAxis::WHOLE,
+        };
+        // The viewport is the inner's rel base, so a fraction it declares or
         // reports is a fraction of what is on screen rather than of the
-        // content box its own answer decided. Where it is drawn is the
-        // content box, scrolled.
-        painter.widget_at(
-            &self.inner,
-            whole,
-            self.axis
-                .pair(Place::Fill(Part::From(content)), Place::Fill(Part::All)),
-        );
+        // content box its own answer decided. Where it goes is the content
+        // box, scrolled: its drawing moved there, not made again there.
+        painter.place_at(&self.inner, content.on_axis(self.axis).fills());
         // What it occupies is its box, on both axes: it clips its content to
         // that box, so it can neither take less of one nor honestly ask for
         // more. The content's length is what it scrolls through, not what it
         // is.
         Size::LEFTOVER
     }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
 }
 
 impl Scroll {

src/widget/position/span.rs

@@ -8,82 +8,75 @@ pub struct Span {
 }
 
 impl Widget for Span {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        let axis = self.dir.axis;
-        // The row: this span's own box, as a length of the frame 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.answer_extent_len(axis);
-        let measure_along = |from: Len, to: Len| match self.dir.sign {
-            Sign::Pos => UiSpan::new(from, to),
-            Sign::Neg => UiSpan::new(far - to, far - from),
-        };
-        // 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 across = Place::Within(Part::All);
-        // A length for every child before their final slots are chosen. The
-        // frame 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 it is drawn 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 room = Place::Fill(Part::From(measure_along(cursor, far)));
-            let len = painter
-                .widget_at(child, UiRegion::FULL, axis.pair(room, across))
-                .len(axis);
-            cursor.px += len.px + self.gap;
-            cursor.rel += len.rel;
-            lens.push(len);
+    fn size_request(&self, requests: &mut SizeRequests, axis: Axis) -> Option<RequestedLen> {
+        if axis != self.dir.axis {
+            // A share can be hidden when the other axis has no room. Its
+            // cross-axis length then contributes nothing to the drawn answer.
+            return None;
         }
+        let mut total = RequestedLen::from(Len::from_parts(Rel::ZERO, self.gaps()));
+        for child in &self.children {
+            let child = requests.widget(child, axis)?;
+            total = requests.sum(total, child);
+        }
+        Some(total)
+    }
 
-        let gaps = self
-            .gap
-            .mul_int(self.children.len().saturating_sub(1) as i32);
-        let total = lens.iter().fold(
-            LayoutLen {
-                px: gaps,
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        painter.with_requests(|painter, lens, values| self.layout(painter, lens, values))
+    }
+}
+
+impl Span {
+    fn layout(
+        &self,
+        painter: &mut Painter,
+        lens: &mut Vec<RequestedLen>,
+        values: &mut Vec<Px>,
+    ) -> Size {
+        let axis = self.dir.axis;
+        // The row this span lays its children out along, as a length of the
+        // rel base they are laid out against. Where it starts is nothing's
+        // business -- a slot is a length from there -- so what this reads is
+        // the length alone.
+        let row = painter.region_len(axis);
+        self.collect(painter, row, lens, true);
+        let gaps = self.gaps();
+        let fixed = lens
+            .iter()
+            .try_fold(Len::from_parts(Rel::ZERO, gaps), |sum, len| {
+                Some(sum + len.linear()?.without_leftover())
+            });
+        if let Some(fixed) = fixed
+            && lens.iter().any(|len| len.has_leftover())
+            && !painter.longer_than(row, fixed, axis)
+        {
+            // With no share to assign, intrinsic drawings keep the remaining
+            // offer, including overflow. Their answer is only moved into a slot.
+            self.collect(painter, row, lens, false);
+        }
+        let nonlinear = lens.iter().any(|len| len.linear().is_none());
+        if nonlinear {
+            painter.allocate(lens, row - Len::from_parts(Rel::ZERO, gaps), axis, values);
+        }
+        let allocated = nonlinear.then(|| &values[..]);
+        let total = match allocated {
+            Some(allocated) => LayoutLen {
+                px: allocated.iter().fold(gaps, |sum, len| sum + *len),
                 ..LayoutLen::ZERO
             },
-            |sum, len| sum + *len,
-        );
-
-        let fixed_total = Len::from_parts(total.rel, total.px);
-        // What is left for the shares to divide: the row less everything
-        // fixed, as a length of the frame rather than a number of pixels.
-        let room = far - fixed_total;
-        // 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 = room.to_px(painter.frame_px_len(axis)) > Px::ZERO;
-            let holds = match shares {
-                true => Holds::from(Px::STEP..=Px::MAX),
-                false => Holds::from(Px::MIN..=Px::ZERO),
-            };
-            painter.frame_holds(axis, holds.through(room));
-        }
-
-        if shares {
-            painter.drawing_uses_extent_len(axis, far);
-        }
-        let drawing_far = match shares {
-            true => far,
-            false => fixed_total,
-        };
-        let along = |from: Len, to: Len| match self.dir.sign {
-            Sign::Pos => UiSpan::new(from, to),
-            Sign::Neg => UiSpan::new(drawing_far - to, drawing_far - from),
+            None => lens.iter().fold(
+                LayoutLen {
+                    px: gaps,
+                    ..LayoutLen::ZERO
+                },
+                |sum, len| sum + len.linear().unwrap(),
+            ),
         };
+        let all_fixed = total.without_leftover();
+        let room = row - all_fixed;
+        let any_leftover = total.leftover > Weight::ZERO;
+        let has_room = any_leftover && painter.longer_than(row, all_fixed, axis);
 
         // Across itself a span is as long as its longest child -- unless a
         // rule beside it gives that length outright, and then reading them
@@ -93,90 +86,144 @@ impl Widget for Span {
         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();
+        // spoken for. Both ends of a slot are read from those two rather
+        // than stepped from the last child: the share of the room is
+        // rounded, and taking each end from the one before it would carry
+        // every rounding along the row.
+        let mut fixed = Len::ZERO;
         let mut taken = Weight::ZERO;
-        let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
-        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.leftover > Weight::ZERO && len.px == Px::ZERO && len.rel == Rel::ZERO && !shares
-            {
+        // Nothing divides the room where no child asked for any of it, and a
+        // ratio of a whole of nothing has no answer.
+        let reached = |fixed: Len, taken: Weight| match any_leftover {
+            false => fixed,
+            true => fixed + room.scale(Rel::ratio(taken, total.leftover)),
+        };
+        for (index, (child, request)) in self.children.iter().zip(lens.iter()).enumerate() {
+            // An allocated row already has a length for every child; without
+            // one the request is the length and the room is divided here.
+            // Either way 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.
+            let (len, shares, nothing_left) = match allocated {
+                Some(allocated) => {
+                    let len = LayoutLen {
+                        px: allocated[index],
+                        ..LayoutLen::ZERO
+                    };
+                    let shares = request.has_leftover();
+                    (len, shares, shares && len.px == Px::ZERO)
+                }
+                None => {
+                    let len = request.linear().unwrap();
+                    let shares = len.leftover > Weight::ZERO && has_room;
+                    (len, shares, len.is_only_leftover() && !has_room)
+                }
+            };
+            if nothing_left {
                 painter.undraw(child);
                 fixed.px += self.gap;
                 continue;
             }
-            let from = start;
-            if len.leftover > Weight::ZERO && shares {
+            let from = reached(fixed, taken);
+            if shares {
                 taken += len.leftover;
             }
-            fixed.px += len.px;
-            fixed.rel += len.rel;
-            start = shared(fixed, taken, total.leftover, room);
+            fixed += len.without_leftover();
+            let to = reached(fixed, taken);
             // Along the row the span says where the child goes, and that slot
-            // is the drawing's box outright rather than something to place an
-            // answer inside again.
-            let span = along(from, start);
-            let (frame, slot) = match len.leftover > Weight::ZERO && shares {
-                true => (
-                    UiRegion::from_axis(
-                        axis,
-                        painter.extent_part(axis, Part::From(span)),
-                        UiSpan::FULL,
-                    ),
-                    Place::Fill(Part::All),
-                ),
-                false => (UiRegion::FULL, Place::Fill(Part::From(span))),
-            };
-            let placed = painter.widget_at(child, frame, axis.pair(slot, across));
+            // is the child's box outright rather than something to place an
+            // answer inside again. A share is decided here and nowhere
+            // else: its slot narrows its rel base, and the child is asked in
+            // it, since a text wraps at the width it is actually given. A
+            // fixed child's slot is its own answer, so a drawing made in the
+            // room is put there as it is, and one not made yet is made here.
+            let slot = self.slot(row, from, to);
+            let mut place = slot.shifted_desc().allocated().on_axis(axis);
+            if shares {
+                place = place.rel_base(axis, slot.len());
+            }
+            let used = painter.place_at(child, place).len(!axis);
             if shrinks {
-                let used = placed.len(!axis);
                 // 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;
+                // A scalable child therefore makes the span scalable too;
                 // only fixed children are compared with one another.
-                if used.rel != Rel::ZERO || used.leftover != Weight::ZERO {
+                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, total.leftover, room);
         }
 
-        // 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;
+        // Where nothing was allocated the weight is carried whole rather
+        // than collapsed to one share, 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
+        // one share per level does not give. Resolution happens at the
+        // nearest ancestor with a length, and the root always has one --
+        // or, where a comparison deferred the row, at the ancestor whose
+        // allocation discovery carried these requests to, and `total` is
+        // pixels by the time it gets here.
         let ortho = match shrinks {
             true => ortho,
             false => LayoutLen::rel(1.0),
         };
-        Size::from_axis(axis, along, ortho)
+        Size::from_axis(axis, total, ortho)
     }
 }
 
-/// Where a row has reached: everything fixed before this point, which is a
-/// sum and exact, plus the share of the room the weights so far are worth,
-/// which is one rounding wherever it is asked for.
-fn shared(fixed: Len, taken: Weight, weight: Weight, room: Len) -> Len {
-    if taken == Weight::ZERO {
-        return fixed;
-    }
-    fixed + room.scale(Rel::ratio(taken, weight))
-}
-
 impl Span {
+    /// What the gaps between this span's children take, which is a length of
+    /// the row before anything is divided.
+    fn gaps(&self) -> Px {
+        self.gap
+            .mul_int(self.children.len().saturating_sub(1) as i32)
+    }
+
+    fn collect(
+        &self,
+        painter: &mut Painter,
+        row: Len,
+        lens: &mut Vec<RequestedLen>,
+        discover: bool,
+    ) {
+        let axis = self.dir.axis;
+        let mut cursor = Len::ZERO;
+        lens.clear();
+        for child in &self.children {
+            let request = if discover {
+                painter.size_request(child, axis)
+            } else {
+                painter.size_hint(child, axis).map(Into::into)
+            };
+            let len = match request {
+                Some(len) => len,
+                None => {
+                    let room = self.slot(row, cursor, row).shifted_desc().on_axis(axis);
+                    let len = painter.widget_at(child, room).len(axis);
+                    painter.measured_request(child, axis, len)
+                }
+            };
+            cursor += painter.minimum_request(&len, axis);
+            cursor.px += self.gap;
+            lens.push(len);
+        }
+    }
+
+    /// The stretch of the row between two distances from where this span
+    /// starts laying children out, as a span of its own box. A negative
+    /// direction lays out from the far end, so the same two distances mirror
+    /// in a row `row` long.
+    fn slot(&self, row: Len, from: Len, to: Len) -> UiSpan {
+        match self.dir.sign {
+            Sign::Pos => from.to(to),
+            Sign::Neg => (row - to).to(row - from),
+        }
+    }
+
     pub fn empty(dir: Dir) -> Self {
         Self {
             children: Vec::new(),

src/widget/position/stack.rs

@@ -8,6 +8,19 @@ pub struct Stack {
 }
 
 impl Widget for Stack {
+    fn size_request(&self, requests: &mut SizeRequests, axis: Axis) -> Option<RequestedLen> {
+        let sizing = match self.size {
+            StackSize::Default => None,
+            StackSize::Child(i) => self.children.get(i),
+        };
+        // With nothing sizing it a stack is a share of the box it is given,
+        // which is what its draw answers too.
+        match sizing {
+            Some(child) => requests.widget(child, axis),
+            None => Some(LayoutLen::LEFTOVER.into()),
+        }
+    }
+
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let sizing = match self.size {
             StackSize::Default => None,
@@ -22,29 +35,40 @@ impl Widget for Stack {
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter
-                    .widget_at(child, UiRegion::FULL, [Place::Fill(Part::All); 2])
-                    .size()
+                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::from_axes(|axis| {
+            let len = size[axis];
+            match len.leftover == Weight::ZERO {
+                true => len.without_leftover().as_desc().fills(),
+                false => PlaceDescAxis::WHOLE,
+            }
+        });
         for (i, child) in self.children.iter().enumerate() {
             if sizing == Some(i) {
                 continue;
             }
             painter.child_layer_at(i);
-            let place = [Axis::X, Axis::Y].map(|axis| {
-                let len = size.axis(axis);
-                let part = match len.leftover > Weight::ZERO {
-                    true => Part::All,
-                    false => Part::From(UiSpan::new(Len::ZERO, Len::from_parts(len.rel, len.px))),
-                };
-                Place::Within(part)
-            });
-            painter.widget_at(child, UiRegion::FULL, place);
+            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/text/edit.rs

@@ -321,12 +321,10 @@ impl<'a> TextEditCtx<'a> {
         let old = (self.text.view.buf.text().to_string(), self.text.selection);
         let mut undo = false;
         let res = self.apply_event_inner(event, modifiers, &mut undo);
-        if undo {
-            if let Some((old, selection)) = self.text.history.pop() {
-                self.set(&old);
-                self.text.selection = selection;
-                self.clamp_selection_to_layout();
-            }
+        if undo && let Some((old, selection)) = self.text.history.pop() {
+            self.set(&old);
+            self.text.selection = selection;
+            self.clamp_selection_to_layout();
         } else if self.text.view.buf.text() != old.0 {
             self.text.history.push(old);
         }

src/widget/trait_fns.rs

@@ -19,8 +19,8 @@ widget_trait! {
         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 {
+            for axis in Axis::BOTH {
+                if let Some(align) = align[axis] {
                     widgets.set_alignment(id, axis, align);
                 }
             }
@@ -59,26 +59,70 @@ widget_trait! {
         }
     }
 
-    fn width(self, len: impl Into<LayoutLen>) -> impl WidgetIdFn<Rsc, WL::Widget> {
+    fn width(self, len: impl Into<SizeRequest>) -> impl WidgetIdFn<Rsc, WL::Widget> {
         let len = len.into();
         move |state| {
             let id = self.add(state);
             state
                 .ui_mut()
                 .widgets
-                .set_size_rule(id, Axis::X, SizeRule::Exact(len));
+                .set_size_rule(id, Axis::X, SizeRule::from(len));
             id
         }
     }
 
-    fn height(self, len: impl Into<LayoutLen>) -> impl WidgetIdFn<Rsc, WL::Widget> {
+    /// Answers at least this wide, whatever it drew: a rule beside the
+    /// widget, so what a row gives it is at least this even where the widget
+    /// itself wanted less. The box it draws in is untouched -- for that, see
+    /// [`MaxSize`].
+    fn min_width(self, len: impl Into<Len>) -> impl WidgetIdFn<Rsc, WL::Widget> {
+        let len = len.into();
+        move |state| {
+            let id = self.add(state);
+            state.ui_mut().widgets.set_min_len(id, Axis::X, len);
+            id
+        }
+    }
+
+    fn min_height(self, len: impl Into<Len>) -> impl WidgetIdFn<Rsc, WL::Widget> {
+        let len = len.into();
+        move |state| {
+            let id = self.add(state);
+            state.ui_mut().widgets.set_min_len(id, Axis::Y, len);
+            id
+        }
+    }
+
+    /// Puts this in a [`MaxSize`]: it is asked in the shorter of the cap and
+    /// the box that widget was given, and is as long as it used, held to the
+    /// cap. A widget rather than a rule because the box is whoever asked's to
+    /// decide -- see [`MaxSize`].
+    fn max_width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, MaxSize> {
+        let len = len.into();
+        move |state| MaxSize {
+            inner: self.add_strong(state),
+            x: Some(len),
+            y: None,
+        }
+    }
+
+    fn max_height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, MaxSize> {
+        let len = len.into();
+        move |state| MaxSize {
+            inner: self.add_strong(state),
+            x: None,
+            y: Some(len),
+        }
+    }
+
+    fn height(self, len: impl Into<SizeRequest>) -> impl WidgetIdFn<Rsc, WL::Widget> {
         let len = len.into();
         move |state| {
             let id = self.add(state);
             state
                 .ui_mut()
                 .widgets
-                .set_size_rule(id, Axis::Y, SizeRule::Exact(len));
+                .set_size_rule(id, Axis::Y, SizeRule::from(len));
             id
         }
     }

src/widget/wrapper.rs

@@ -9,6 +9,7 @@ use std::marker::Unsize;
 ///
 /// Its child is optional so it can also be the swappable slot a tab bar
 /// needs, which is what it was written for.
+#[derive(Default)]
 pub struct Wrapper {
     pub inner: Option<StrongWidget>,
 }
@@ -26,11 +27,7 @@ impl Wrapper {
     pub fn new() -> Self {
         Self::default()
     }
-    pub fn empty() -> Self {
-        Self {
-            inner: Default::default(),
-        }
-    }
+
     pub fn set<W: ?Sized + Unsize<dyn Widget>>(&mut self, to: StrongWidget<W>) {
         self.inner = Some(to)
     }
@@ -42,9 +39,3 @@ impl Wrapper {
         self.inner.replace(to)
     }
 }
-
-impl Default for Wrapper {
-    fn default() -> Self {
-        Self::empty()
-    }
-}

tests/allocation_cost.rs

@@ -0,0 +1,80 @@
+use iris::{harness::Harness, prelude::*};
+use std::{
+    alloc::{GlobalAlloc, Layout, System},
+    cell::Cell,
+};
+
+struct Counting;
+thread_local! {
+    static COUNT: Cell<Option<usize>> = const { Cell::new(None) };
+}
+fn count() {
+    COUNT.with(|count| {
+        if let Some(n) = count.get() {
+            count.set(Some(n + 1));
+        }
+    });
+}
+// The wrapper preserves System's allocation and deallocation contracts;
+// observing calls here also counts allocations hidden inside layout helpers.
+unsafe impl GlobalAlloc for Counting {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        count();
+        unsafe { System.alloc(layout) }
+    }
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        unsafe { System.dealloc(ptr, layout) }
+    }
+    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, size: usize) -> *mut u8 {
+        count();
+        unsafe { System.realloc(ptr, layout, size) }
+    }
+}
+#[global_allocator]
+static ALLOCATOR: Counting = Counting;
+
+#[test]
+fn unchanged_tree_reuses_layout_storage() {
+    for deferred in [false, true] {
+        let mut h = Harness::new((600, 200));
+        let mut children: Vec<StrongWidget> = Vec::new();
+        for _ in 0..8 {
+            let a = rect(Color::RED).add(&mut h.rsc);
+            if deferred {
+                h.rsc
+                    .widgets_mut()
+                    .set_size_rule(a, Axis::X, leftover(1).clamp(20, 80).into());
+            }
+            let row = (a, rect(Color::BLUE))
+                .span(Dir::RIGHT)
+                .add_strong(&mut h.rsc);
+            children.push(row);
+        }
+        let root = h.rsc.widgets_mut().add_strong(Span {
+            children,
+            dir: Dir::DOWN,
+            gap: Px::ZERO,
+        });
+        h.state.root = Some(root);
+        h.frame();
+        let ids: Vec<_> = h.render.active.keys().copied().collect();
+        for frame in 0..8 {
+            for &id in &ids {
+                h.rsc.widgets_mut().mark_for_redraw(id);
+            }
+            h.resize((600 + frame % 2, 200));
+            h.frame();
+        }
+        COUNT.set(Some(0));
+        for frame in 0..100 {
+            for &id in &ids {
+                h.rsc.widgets_mut().mark_for_redraw(id);
+            }
+            h.resize((600 + frame % 2, 200));
+            h.frame();
+        }
+        let allocations = COUNT.replace(None).unwrap();
+        println!("deferred={deferred}: {allocations} allocations over 100 resize frames");
+        assert_eq!(allocations, 0);
+    }
+}

tests/cases/deferred.rs

@@ -0,0 +1,391 @@
+use std::{cell::Cell, rc::Rc};
+
+use iris::harness::{Harness, assert_corners};
+use iris::prelude::*;
+
+struct Counted {
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for Counted {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.px_size();
+        painter.primitive(RectPrimitive::color(Color::RED));
+        Size::LEFTOVER
+    }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::LEFTOVER)
+    }
+}
+
+#[test]
+fn a_capped_share_returns_room_to_its_sibling() {
+    let mut h = Harness::new((300, 100));
+    let first = rect(Color::RED).max_width(80).add(&mut h.rsc);
+    let second = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((first, second).span(Dir::RIGHT));
+    assert_corners!(h, first, (0, 0), (80, 100));
+    assert_corners!(h, second, (80, 0), (300, 100));
+    h.resize((100, 100));
+    h.frame();
+    assert_corners!(h, first, (0, 0), (50, 100));
+    assert_corners!(h, second, (50, 0), (100, 100));
+    h.resize((300, 100));
+    h.frame();
+    assert_corners!(h, first, (0, 0), (80, 100));
+    assert_corners!(h, second, (80, 0), (300, 100));
+}
+
+#[test]
+fn nested_shares_are_discovered_without_provisional_paint() {
+    let mut h = Harness::new((400, 100));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = h.rsc.ui_mut().widgets.add_strong(Counted {
+        draws: draws.clone(),
+    });
+    let leaf_id = leaf.id();
+    let mut inner: StrongWidget = leaf;
+    for _ in 0..8 {
+        let sibling = rect(Color::BLUE).add_strong(&mut h.rsc);
+        inner = h.rsc.ui_mut().widgets.add_strong(Span {
+            children: vec![inner, sibling],
+            dir: Dir::RIGHT,
+            gap: Px::ZERO,
+        });
+    }
+    h.state.root = Some(inner);
+    h.frame();
+    assert_eq!(draws.get(), 1);
+    assert!(h.region(&leaf_id).is_some());
+    h.resize((800, 100));
+    h.frame();
+    assert_eq!(draws.get(), 2);
+}
+
+#[test]
+fn nested_bounds_are_resolved_in_the_outer_allocation() {
+    let mut h = Harness::new((300, 100));
+    let a = rect(Color::RED).max_width(40).add(&mut h.rsc);
+    let b = rect(Color::GREEN).max_width(60).add(&mut h.rsc);
+    let inner = (a, b).span(Dir::RIGHT).add(&mut h.rsc);
+    let tail = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((inner, tail).span(Dir::RIGHT));
+    assert_corners!(h, a, (0, 0), (40, 100));
+    assert_corners!(h, b, (40, 0), (100, 100));
+    assert_corners!(h, tail, (100, 0), (300, 100));
+}
+
+#[test]
+fn request_edits_in_a_nested_child_reach_the_allocator() {
+    let mut h = Harness::new((300, 100));
+    let a = rect(Color::RED).max_width(80).add(&mut h.rsc);
+    let inner = (a,).span(Dir::RIGHT).add(&mut h.rsc);
+    let tail = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((inner, tail).span(Dir::RIGHT));
+    h.rsc.ui_mut().widgets.get_mut(&a).unwrap().x = Some(Len::px(40.0));
+    h.frame();
+    assert_corners!(h, a, (0, 0), (40, 100));
+    assert_corners!(h, tail, (40, 0), (300, 100));
+}
+
+#[test]
+fn adding_a_bound_to_a_previously_unbounded_share_reallocates_the_row() {
+    for hinted in [true, false] {
+        let mut h = Harness::new((300, 100));
+        let a = if hinted {
+            rect(Color::RED).add_strong(&mut h.rsc).any()
+        } else {
+            h.rsc.widgets_mut().add_strong(Unhinted).any()
+        };
+        let id = a.id();
+        let b = rect(Color::BLUE).add(&mut h.rsc);
+        let mut row = Span::empty(Dir::RIGHT);
+        row.push(a);
+        row.push(b.add_strong(&mut h.rsc));
+        h.set_root(row);
+        h.resize((400, 100));
+        h.frame();
+        h.rsc
+            .widgets_mut()
+            .set_size_rule(id, Axis::X, SizeRule::Max(Len::px(80.0)));
+        h.frame();
+        assert_corners!(h, id, (0, 0), (80, 100));
+        assert_corners!(h, b, (80, 0), (400, 100));
+    }
+}
+
+#[test]
+fn a_deferred_comparison_can_compare_two_different_weights() {
+    let a = SizeRequest::from(leftover(1.0) + px(30.0)).min(leftover(2.0));
+    let b = SizeRequest::from(leftover(1.0)).clamp(px(20.0), px(100.0));
+    let mut h = Harness::new((60, 100));
+    let a = rect(Color::RED).width(a).add(&mut h.rsc);
+    let b = rect(Color::BLUE).width(b).add(&mut h.rsc);
+    h.set_root((a, b).span(Dir::RIGHT));
+    assert_corners!(h, a, (0, 0), (40, 100));
+    assert_corners!(h, b, (40, 0), (60, 100));
+    h.resize((300, 100));
+    h.frame();
+    assert_corners!(h, a, (0, 0), (200, 100));
+    assert_corners!(h, b, (200, 0), (300, 100));
+}
+
+#[test]
+fn a_length_expression_is_resolved_before_wrapping_text() {
+    let mut h = Harness::new((300, 500));
+    let text = wtext("one two three four five six seven eight nine ten")
+        .size(16)
+        .wrap(true)
+        .width(leftover(1).clamp(40, 80))
+        .add(&mut h.rsc);
+    let other = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((text, other).span(Dir::RIGHT));
+    let box_ = h.region(&text).unwrap();
+    assert_eq!(box_.top_left.x, Px::ZERO);
+    assert_eq!(box_.bot_right.x, Px::from_int(80));
+    assert!(box_.bot_right.y - box_.top_left.y > Px::from_int(30));
+    assert_corners!(h, other, (80, 0), (300, 500));
+}
+
+#[test]
+fn relative_bounds_keep_the_allocators_base() {
+    let mut h = Harness::new((300, 100));
+    let head = rect(Color::BLUE).width(30).add(&mut h.rsc);
+    let bounded = rect(Color::RED)
+        .width(leftover(1).min(rel(0.25)))
+        .add(&mut h.rsc);
+    let tail = rect(Color::GREEN).add(&mut h.rsc);
+    h.set_root((head, bounded, tail).span(Dir::RIGHT));
+    assert_corners!(h, bounded, (30, 0), (105, 100));
+    assert_corners!(h, tail, (105, 0), (300, 100));
+    h.resize((400, 100));
+    h.frame();
+    assert_corners!(h, bounded, (30, 0), (130, 100));
+    assert_corners!(h, tail, (130, 0), (400, 100));
+}
+
+#[test]
+fn the_root_resolves_a_deferred_request_again_after_resize() {
+    let mut h = Harness::new((300, 100));
+    let bounded = rect(Color::RED)
+        .width(leftover(1).min(rel(0.25)))
+        .add(&mut h.rsc);
+    h.set_root(bounded);
+    assert_corners!(h, bounded, (112.5, 0), (187.5, 100));
+    h.resize((400, 100));
+    h.frame();
+    assert_corners!(h, bounded, (150, 0), (250, 100));
+}
+
+#[test]
+fn filling_a_stack_does_not_mean_its_sizing_child_was_already_allocated() {
+    let mut h = Harness::new((300, 100));
+    let child = rect(Color::RED).width(leftover(1).min(80)).add(&mut h.rsc);
+    let overlay = rect(Color::BLUE).add(&mut h.rsc);
+    let children: Vec<StrongWidget> =
+        vec![child.add_strong(&mut h.rsc), overlay.add_strong(&mut h.rsc)];
+    h.set_root(Stack {
+        children,
+        size: StackSize::Child(0),
+    });
+    assert_corners!(h, child, (110, 0), (190, 100));
+    assert_corners!(h, overlay, (110, 0), (190, 100));
+}
+
+struct Unhinted;
+impl Widget for Unhinted {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        painter.px_size();
+        painter.primitive(RectPrimitive::color(Color::RED));
+        Size::LEFTOVER
+    }
+}
+
+#[test]
+fn bounds_also_apply_to_shares_discovered_by_drawing() {
+    let mut h = Harness::new((300, 100));
+    let a = h.rsc.widgets_mut().add_strong(Unhinted);
+    h.rsc
+        .widgets_mut()
+        .set_size_rule(a.id(), Axis::X, SizeRule::Max(Len::px(80.0)));
+    let id = a.id();
+    let b = rect(Color::BLUE).add_strong(&mut h.rsc);
+    let b_id = b.id();
+    h.state.root = Some(h.rsc.widgets_mut().add_strong(Span {
+        children: vec![a, b],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }));
+    h.frame();
+    assert_corners!(h, id, (0, 0), (80, 100));
+    assert_corners!(h, b_id, (80, 0), (300, 100));
+    h.resize((100, 100));
+    h.frame();
+    assert_corners!(h, id, (0, 0), (50, 100));
+    assert_corners!(h, b_id, (50, 0), (100, 100));
+}
+
+#[test]
+fn comparisons_with_a_known_order_remain_plain_lengths() {
+    assert_eq!(leftover(2).max(leftover(5)), SizeRequest::from(leftover(5)));
+    assert_eq!(leftover(2).min(leftover(5)), SizeRequest::from(leftover(2)));
+    assert_eq!(
+        (px(10) + rel(0.5)).max(px(30) + rel(0.5)),
+        SizeRequest::from(px(30) + rel(0.5))
+    );
+    assert_eq!(LayoutLen::px(10).clamp(20, 80), SizeRequest::from(20));
+}
+
+#[test]
+fn a_measured_nested_share_keeps_its_comparison_for_the_outer_span() {
+    let mut h = Harness::new((300, 100));
+    let a = h.rsc.widgets_mut().add_strong(Unhinted);
+    let a_id = a.id();
+    h.rsc
+        .widgets_mut()
+        .set_size_rule(a_id, Axis::X, SizeRule::Max(Len::px(80.0)));
+    let b = rect(Color::BLUE).add_strong(&mut h.rsc);
+    let b_id = b.id();
+    let inner = h.rsc.widgets_mut().add_strong(Span {
+        children: vec![a, b],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    });
+    let c = rect(Color::GREEN).add_strong(&mut h.rsc);
+    let c_id = c.id();
+    h.state.root = Some(h.rsc.widgets_mut().add_strong(Span {
+        children: vec![inner, c],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }));
+    h.frame();
+    assert_corners!(h, a_id, (0, 0), (80, 100));
+    assert_corners!(h, b_id, (80, 0), (190, 100));
+    assert_corners!(h, c_id, (190, 0), (300, 100));
+    h.rsc
+        .widgets_mut()
+        .mark_for_redraw(h.state.root.as_ref().unwrap().id());
+    h.frame();
+    assert_corners!(h, c_id, (190, 0), (300, 100));
+}
+
+struct Natural;
+impl Widget for Natural {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        painter.primitive(RectPrimitive::color(Color::RED));
+        Size::from_axis(Axis::X, LayoutLen::px(64), LayoutLen::px(64))
+    }
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::px(64))
+    }
+}
+
+#[test]
+fn relative_bounds_on_a_hinted_child_track_the_offer_before_its_declared_size() {
+    fn tree(h: &mut Harness) -> WidgetId {
+        let natural = h.rsc.widgets_mut().add_strong(Natural);
+        let id = natural.id();
+        h.rsc
+            .widgets_mut()
+            .set_size_rule(id, Axis::X, SizeRule::Max(Len::rel(0.75)));
+        h.rsc.widgets_mut().set_size_rule(
+            id,
+            Axis::Y,
+            SizeRule::Clamp {
+                min: Len::rel(0.25),
+                max: Len::rel(0.75),
+            },
+        );
+        let inner = h.rsc.widgets_mut().add_strong(Stack {
+            children: vec![natural],
+            size: StackSize::Child(0),
+        });
+        let inner_id = inner.id();
+        let fill = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let overlay = h.rsc.widgets_mut().add_strong(Stack {
+            children: vec![fill, inner],
+            size: StackSize::Child(0),
+        });
+        let share = rect(Color::GREEN).add_strong(&mut h.rsc);
+        let bounded = rect(Color::GREEN).add_strong(&mut h.rsc);
+        h.rsc.widgets_mut().set_size_rule(
+            bounded.id(),
+            Axis::X,
+            SizeRule::Clamp {
+                min: Len::rel(0.25),
+                max: Len::rel(0.75),
+            },
+        );
+        let fixed = wtext("one line, overflowing whatever it is given")
+            .size(16)
+            .wrap(false)
+            .add_strong(&mut h.rsc);
+        h.state.root = Some(h.rsc.widgets_mut().add_strong(Span {
+            children: vec![share, bounded, fixed, overlay],
+            dir: Dir::LEFT,
+            gap: Px::from_int(8),
+        }));
+        h.rsc.widgets_mut().set_size_rule(
+            h.state.root.as_ref().unwrap().id(),
+            Axis::Y,
+            LayoutLen::rel(1).into(),
+        );
+        h.frame();
+        inner_id
+    }
+    let mut warm = Harness::new((1920, 1200));
+    let a = tree(&mut warm);
+    warm.resize((640, 900));
+    warm.frame();
+    let mut cold = Harness::new((640, 900));
+    let b = tree(&mut cold);
+    assert_eq!(warm.region(&a), cold.region(&b));
+}
+
+#[test]
+fn a_bound_can_extend_an_explicit_share_request() {
+    let mut h = Harness::new((300, 100));
+    let a = rect(Color::RED)
+        .width(leftover(1))
+        .min_width(100)
+        .add(&mut h.rsc);
+    let b = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((a, b).span(Dir::RIGHT));
+    assert_corners!(h, a, (0, 0), (150, 100));
+    h.resize((120, 100));
+    h.frame();
+    assert_corners!(h, a, (0, 0), (100, 100));
+    assert_corners!(h, b, (100, 0), (120, 100));
+}
+
+#[test]
+fn moving_scroll_content_preserves_its_resolved_expression_size() {
+    let mut h = Harness::new((300, 300));
+    let leaf = rect(Color::BLUE).add_strong(&mut h.rsc);
+    let leaf_id = leaf.id();
+    let content = h.rsc.widgets_mut().add_strong(Stack {
+        children: vec![leaf],
+        size: StackSize::Child(0),
+    });
+    let content_id = content.id();
+    h.rsc
+        .widgets_mut()
+        .set_size_rule(content_id, Axis::Y, leftover(1).min(120).into());
+    let scroll = h
+        .rsc
+        .widgets_mut()
+        .add_strong(Scroll::new(content, Axis::Y));
+    let scroll_id = scroll.id();
+    h.state.root = Some(scroll);
+    for _ in 0..3 {
+        h.rsc.widgets_mut().mark_for_redraw(scroll_id);
+        h.frame();
+        assert_corners!(h, content_id, (0, 90), (300, 210));
+        assert_corners!(h, leaf_id, (0, 90), (300, 210));
+    }
+    h.resize((300, 600));
+    h.frame();
+    assert_corners!(h, leaf_id, (0, 240), (300, 360));
+}

tests/cases/determinism.rs

@@ -22,17 +22,17 @@ struct BranchesOnMeasurement {
 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 = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
+        let top = UiSpan::new(Len::ZERO, cut).shifted_desc();
         let measured = painter
-            .widget_at(&self.probe, UiRegion::FULL, [Place::Within(Part::All), top])
+            .widget_at(&self.probe, top.on_axis(Axis::Y))
             .len(Axis::X);
-        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
+        let px = painter.to_px(measured.apply_leftover(), Axis::X);
 
-        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
-        let place = [Place::Within(Part::All), below];
+        let below = UiSpan::new(cut, painter.region_len(Axis::Y)).shifted_desc();
+        let place = below.on_axis(Axis::Y);
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
-            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
+            true => painter.widget_at(&self.wide, place),
+            false => painter.widget_at(&self.narrow, place),
         };
         Size::LEFTOVER
     }
@@ -69,8 +69,8 @@ fn a_branch_taken_on_a_measurement_holds_across_repaints() {
         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.rsc.widgets_mut().mark_for_redraw(wide);
+            h.rsc.widgets_mut().mark_for_redraw(narrow);
             h.frame();
             assert_eq!(
                 taken(&h, wide, narrow),
@@ -88,7 +88,7 @@ fn a_branch_taken_on_a_measurement_is_the_one_a_cold_start_takes() {
         let (wide, narrow) = plant(&mut warm, threshold);
         warm.resize((640, 480));
         warm.frame();
-        warm.rsc.widgets_mut().get_dyn_mut(wide);
+        warm.rsc.widgets_mut().mark_for_redraw(wide);
         warm.frame();
 
         let mut cold = Harness::new((640, 480));

tests/cases/idempotence.rs

@@ -18,7 +18,7 @@ fn a_wrapping_text_in_a_span_settles_on_one_width() {
         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.rsc.widgets_mut().mark_for_redraw(t.id());
         h.frame();
     }
     println!("widths over six frames: {widths:?}");

tests/cases/layout.rs

@@ -1,5 +1,7 @@
 //! Where a frame puts things, with no window to put them in.
 
+use std::{cell::Cell, rc::Rc};
+
 use iris::harness::{Harness, assert_corners};
 use iris::prelude::*;
 
@@ -83,10 +85,8 @@ fn a_text_in_a_span_wraps_at_the_room_left_rather_than_the_whole_row() {
     assert!(crowded > whole_row, "{crowded} against {whole_row}");
 }
 
-/// The same reading through a pad: padding goes around what it pads and
-/// does not narrow what a fraction under it is a fraction of, so half of the
-/// window plus the padding is what the pad takes and where the next child
-/// starts.
+/// 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));
@@ -97,9 +97,83 @@ fn a_pad_puts_its_padding_around_a_fraction_of_the_whole_box() {
     // 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), (210, 90));
-    assert_corners!(h, padded, (0, 0), (220, 100));
-    assert_corners!(h, tail, (220, 0), (320, 100));
+    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]
@@ -147,6 +221,134 @@ fn an_empty_widget_takes_a_share_of_a_span() {
     assert_corners!(h, right, (300, 0), (400, 200));
 }
 
+/// A widget with a natural pixel size, like an image, which records the box
+/// it was asked in so a test can see which length decided it.
+struct NaturalSize {
+    len: f32,
+    asked: Rc<Cell<f32>>,
+}
+
+impl Widget for NaturalSize {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.asked.set(painter.px_len(Axis::X).to_f32());
+        Size::px(Vec2::new(self.len, self.len))
+    }
+
+    fn size_hint(&self, _: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::px(self.len))
+    }
+}
+
+/// A rule wins over what the widget says about itself, and a share is a rule:
+/// it is a length only to whoever divides one, and nobody here does, so the
+/// widget is asked in the whole box rather than in the size it asked for.
+#[test]
+fn a_share_rule_beats_the_widgets_own_pixel_size() {
+    let mut h = Harness::new((400, 200));
+    let asked = Rc::new(Cell::new(0.0));
+    let natural = NaturalSize {
+        len: 50.0,
+        asked: asked.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(natural.wrapper());
+    assert_eq!(asked.get(), 50.0, "its hint gives it its own size");
+
+    h.set_len(natural, Axis::X, LayoutLen::LEFTOVER);
+    h.frame();
+
+    assert_eq!(asked.get(), 400.0, "the share is all of the box");
+}
+
+/// Every box a widget is given comes of one ask, and the window is one of
+/// them: the root is asked in it exactly as a child is asked in its parent's
+/// box, so a rule of its own reads the same way at either place.
+#[derive(Clone, Copy, Debug)]
+enum Asked {
+    Root,
+    Wrapped,
+    InASpan,
+}
+
+impl Asked {
+    const ALL: [Self; 3] = [Self::Root, Self::Wrapped, Self::InASpan];
+
+    /// The width the probe is given under this parent, in a 400 px window.
+    fn width(&self, rule: LayoutLen) -> Px {
+        let mut h = Harness::new((400, 200));
+        let probe = rect(Color::RED).add(&mut h.rsc);
+        h.set_len(probe, Axis::X, rule);
+        match self {
+            Self::Root => h.set_root(probe),
+            Self::Wrapped => h.set_root(probe.wrapper()),
+            Self::InASpan => h.set_root((probe,).span(Dir::RIGHT)),
+        }
+        h.region(&probe).unwrap().size().x
+    }
+}
+
+/// A share with pixels or a fraction beside it is the longer of the two: it
+/// fills what they leave of the box and overflows the box where they are
+/// longer than it. A parent that divides nothing gives the same length as a
+/// span with one child, because in both there is nobody else to divide with --
+/// and so does the window, which divides nothing either.
+#[test]
+fn a_share_is_a_minimum_wherever_nothing_divides_it() {
+    for (rule, want) in [
+        (LayoutLen::LEFTOVER, 400),
+        (LayoutLen::px(50.0) + LayoutLen::LEFTOVER, 400),
+        (LayoutLen::px(500.0) + LayoutLen::LEFTOVER, 500),
+        (LayoutLen::rel(0.5) + LayoutLen::LEFTOVER, 400),
+        (LayoutLen::rel(2.0) + LayoutLen::LEFTOVER, 800),
+        (LayoutLen::px(500.0), 500),
+    ] {
+        let want = Px::from_int(want);
+        for asked in Asked::ALL {
+            assert_eq!(asked.width(rule), want, "{rule:?} asked {asked:?}");
+        }
+    }
+}
+
+/// Which of the two is longer is a question in pixels, so the box is decided
+/// again wherever the answer can change: a window that crosses the length the
+/// pixels ask for, and the rule itself crossing it while the window holds
+/// still. The first is a range the drawing holds for; the second cannot be
+/// seen in what the widget declares, since a share declares nothing either
+/// way, so it reaches the parent as a length only the parent can resolve.
+#[test]
+fn a_share_past_the_box_is_decided_again_on_either_side_of_the_crossing() {
+    // At the root as well as under a parent: the comparison is the same one,
+    // and nothing above the root will make it again on its behalf, so the
+    // range it holds for is the root's own.
+    for wrapped in [false, true] {
+        let mut h = Harness::new((400, 200));
+        let probe = rect(Color::RED).add(&mut h.rsc);
+        h.set_len(probe, Axis::X, LayoutLen::px(500.0) + LayoutLen::LEFTOVER);
+        match wrapped {
+            true => h.set_root(probe.wrapper()),
+            false => h.set_root(probe),
+        }
+        let width = |h: &Harness| h.region(&probe).unwrap().size().x;
+        assert_eq!(width(&h), Px::from_int(500), "wrapped: {wrapped}");
+
+        h.resize((900, 200));
+        h.frame();
+        assert_eq!(width(&h), Px::from_int(900), "wrapped: {wrapped}");
+
+        h.resize((400, 200));
+        h.frame();
+        assert_eq!(width(&h), Px::from_int(500), "wrapped: {wrapped}");
+
+        h.set_len(probe, Axis::X, LayoutLen::px(50.0) + LayoutLen::LEFTOVER);
+        h.frame();
+        assert_eq!(width(&h), Px::from_int(400), "wrapped: {wrapped}");
+
+        h.set_len(probe, Axis::X, LayoutLen::px(500.0) + LayoutLen::LEFTOVER);
+        h.frame();
+        assert_eq!(width(&h), Px::from_int(500), "wrapped: {wrapped}");
+    }
+}
+
 #[test]
 fn a_child_drawn_twice_moves_once() {
     let mut h = Harness::new((400, 200));
@@ -428,12 +630,11 @@ fn a_row_of_equal_shares_fills_it_exactly() {
 /// 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 drawn = active.extent.within(&active.frame_abs);
-    let region = h.render.moves.resolve(active.parent_move, drawn);
-    let dim = h.size().axis(axis);
+    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(axis);
+    let span = region[axis];
     (edge(span.start), edge(span.end))
 }
 
@@ -555,7 +756,7 @@ 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 mixed = rect(Color::BLUE)
-        .width(LayoutLen::px(20) + LayoutLen::LEFTOVER)
+        .width(LayoutLen::px(20.0) + LayoutLen::LEFTOVER)
         .add(&mut h.rsc);
     h.set_root((fixed, mixed).span(Dir::RIGHT));
 
@@ -740,3 +941,178 @@ fn a_fixed_child_is_centered_in_its_wrappers_share() {
     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));
+        }
+    }
+}
+
+/// The root is asked the way any child is, so what it says about itself is
+/// read there too: a root that opted into a region node gets one, where the
+/// path it used to have ignored the flag.
+#[test]
+fn a_region_node_root_is_a_region_node() {
+    let mut h = Harness::new((400, 200));
+    let probe = rect(Color::RED).add(&mut h.rsc);
+    let root = (probe,).span(Dir::RIGHT).region_node().add(&mut h.rsc);
+    h.set_root(root);
+    assert_eq!(h.region(&probe).unwrap().size().x, Px::from_int(400));
+
+    h.resize((900, 200));
+    h.frame();
+    assert_eq!(h.region(&probe).unwrap().size().x, Px::from_int(900));
+}
+
+/// A bound is a rule about what a widget answers: it holds the length that
+/// reaches whoever asked and leaves the box alone. Here the content is 400
+/// wide in a 250 window, so a cap cuts what the row reports and a floor
+/// raises it, while the rects inside stay where the 250 box put them.
+#[test]
+fn a_bound_holds_what_a_widget_answers() {
+    let row = |rule: SizeRule| {
+        let mut h = Harness::new((250, 200));
+        let left = rect(Color::RED).width(200).add(&mut h.rsc);
+        let right = rect(Color::BLUE).width(200).add(&mut h.rsc);
+        let row = (left, right).span(Dir::RIGHT).add(&mut h.rsc);
+        h.rsc.widgets_mut().set_size_rule(row, Axis::X, rule);
+        h.set_root(row);
+        (
+            h.region(&row).unwrap().size().x,
+            h.region(&left).unwrap().size().x,
+        )
+    };
+    let (capped, left) = row(SizeRule::Max(Len::px(300.0)));
+    assert_eq!(capped, Px::from_int(300), "the cap, not the 400 drawn");
+    assert_eq!(left, Px::from_int(200), "the box the children were given");
+
+    let (floored, _) = row(SizeRule::Min(Len::px(600.0)));
+    assert_eq!(floored, Px::from_int(600), "the floor, not the 400 drawn");
+
+    let (free, _) = row(SizeRule::Free);
+    assert_eq!(free, Px::from_int(400), "what it drew");
+}
+
+/// A cap on the box is `MaxSize`, which asks its child in the shorter of the
+/// cap and its own box. That is the box a text wraps at and a scroll takes
+/// its viewport from, so it cannot be had by holding the answer.
+#[test]
+fn a_cap_widget_asks_its_child_in_the_shorter_box() {
+    let mut h = Harness::new((400, 200));
+    // A fraction of its box, so it says what box it was asked in.
+    let fills = rect(Color::RED).width(rel(1.0)).add(&mut h.rsc);
+    let capped = fills.max_width(300).add(&mut h.rsc);
+    h.set_root(capped);
+
+    assert_eq!(h.region(&fills).unwrap().size().x, Px::from_int(300));
+    assert_eq!(
+        h.region(&capped).unwrap().size().x,
+        Px::from_int(300),
+        "as long as its child used"
+    );
+
+    // A child that asked for a share takes the box the cap allows, and the
+    // share itself passes up: whoever divides one is this widget's parent.
+    let mut h = Harness::new((400, 200));
+    let share = rect(Color::RED).add(&mut h.rsc);
+    let capped = share.max_width(300).add(&mut h.rsc);
+    h.set_root(capped);
+
+    assert_eq!(h.region(&share).unwrap().size().x, Px::from_int(300));
+    assert_eq!(h.region(&capped).unwrap().size().x, Px::from_int(400));
+}
+
+/// Which of the cap and the box is shorter is a question in pixels, so it is
+/// asked again wherever the answer can change -- and the widget asking it is
+/// drawn again whenever its own box is, which is what keeps the two in step.
+#[test]
+fn a_cap_widget_is_decided_again_on_either_side_of_the_crossing() {
+    let mut h = Harness::new((400, 200));
+    let probe = rect(Color::RED).add(&mut h.rsc);
+    h.set_root(probe.max_width(300));
+    let width = |h: &Harness| h.region(&probe).unwrap().size().x;
+    assert_eq!(width(&h), Px::from_int(300));
+
+    h.resize((250, 200));
+    h.frame();
+    assert_eq!(
+        width(&h),
+        Px::from_int(250),
+        "its box, which is under the cap"
+    );
+
+    h.resize((400, 200));
+    h.frame();
+    assert_eq!(width(&h), Px::from_int(300));
+}
+
+/// A fraction in a cap is a fraction of the box the widget capping it was
+/// given, which is the box a declared length of its own would be a fraction
+/// of -- not of the window, and not of what the cap itself decided.
+#[test]
+fn a_cap_is_a_fraction_of_the_box_it_was_given() {
+    let mut h = Harness::new((400, 200));
+    let probe = rect(Color::RED).add(&mut h.rsc);
+    h.set_root(probe.max_width(Len::rel(0.5)).pad(Padding::uniform(50)));
+
+    // Half of the 300 left by the padding, not half of the window.
+    assert_eq!(h.region(&probe).unwrap().size().x, Px::from_int(150));
+}
+
+/// A cap is a promise about the length as well as the box: a widget whose
+/// content is longer than the box it was given reports what it drew, and the
+/// cap holds that down even though it never decided the box.
+#[test]
+fn a_cap_holds_an_answer_that_overflowed_its_box() {
+    let mut h = Harness::new((250, 200));
+    let left = rect(Color::RED).width(200).add(&mut h.rsc);
+    let right = rect(Color::BLUE).width(200).add(&mut h.rsc);
+    let row = (left, right).span(Dir::RIGHT).add(&mut h.rsc);
+    h.rsc.widgets_mut().set_max_len(row, Axis::X, 300.into());
+    h.set_root(row);
+
+    // The box is the 250 window, which the cap of 300 leaves alone, and the
+    // row draws 400 of it. Its answer is the cap, and the window centres it.
+    assert_corners!(h, row, (-25, 0), (275, 200));
+}

tests/cases/plan.rs

@@ -5,7 +5,9 @@
 //! 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 iris::harness::Harness;
+use iris::prelude::*;
+use iris::random::{Edits, Kind, Plan, Rng, SpanEdit, grow, plan};
 use std::collections::HashMap;
 
 fn some_edits(seed: u64, of: &Plan) -> Edits {
@@ -25,11 +27,27 @@ fn some_edits(seed: u64, of: &Plan) -> Edits {
     Edits {
         sizes: pick(sized, &mut rng)
             .into_iter()
-            .map(|i| (i, [Some(LayoutLen::LEFTOVER), None]))
+            .map(|i| {
+                (
+                    i,
+                    SizeRules {
+                        x: SizeRule::Exact(LayoutLen::LEFTOVER),
+                        y: SizeRule::Free,
+                    },
+                )
+            })
             .collect(),
         aligns: pick(aligned, &mut rng)
             .into_iter()
-            .map(|i| (i, [Some(AxisAlign::POS), None]))
+            .map(|i| {
+                (
+                    i,
+                    Align {
+                        x: Some(AxisAlign::POS),
+                        y: None,
+                    },
+                )
+            })
             .collect(),
         nodes: pick(nodes, &mut rng)
             .into_iter()
@@ -51,8 +69,6 @@ fn some_edits(seed: u64, of: &Plan) -> Edits {
     }
 }
 
-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
@@ -70,11 +86,13 @@ fn editing_a_plan_is_growing_one_with_those_edits() {
     }
 }
 
-/// 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.
+/// No simplification is larger, which is the half of "the shrinker stops" a
+/// widget count can see. Most are not smaller either -- a dropped alignment
+/// and a simpler leaf both keep the count -- so what rules out circling is
+/// that those are one-way too: a `Some` becomes a `None`, and a kind steps
+/// down a ladder with no way back up.
 #[test]
-fn every_simplification_of_a_plan_is_smaller_than_it() {
+fn no_simplification_of_a_plan_is_larger_than_it() {
     for seed in 1..=60 {
         let tree = plan(seed, 4, &Edits::default());
         let mut queue = vec![tree];
@@ -119,3 +137,20 @@ fn reducing_a_plan_all_the_way_ends() {
         );
     }
 }
+
+/// Every image in a tree is the same picture, and a handle is a reference to
+/// the texture rather than a copy of it, so one upload and one slot serve all
+/// of them however many a tree grows -- and the trees are grown in hundreds.
+#[test]
+fn a_tree_of_images_uploads_one_texture() {
+    let mut images = 0;
+    let mut tree = plan(1, 4, &Edits::default());
+    tree.walk_mut(&mut |p| images += (p.kind == Kind::Image) as usize);
+    assert!(images > 1, "a tree of {images} images tests nothing");
+
+    let mut h = Harness::new((900, 1200));
+    let (root, _) = grow(&mut h.rsc, 1, 4, &Edits::default());
+    h.state.root = Some(root);
+    h.frame();
+    assert_eq!(h.rsc.ui().textures.count(), 1);
+}

tests/cases/retained.rs

@@ -12,7 +12,6 @@ struct Counted {
     draws: Rc<Cell<usize>>,
     size: Size,
     reads_box: bool,
-    reads_answer_box: bool,
 }
 
 impl Widget for Counted {
@@ -20,8 +19,6 @@ impl Widget for Counted {
         self.draws.set(self.draws.get() + 1);
         if self.reads_box {
             painter.px_size();
-        } else if self.reads_answer_box {
-            painter.answer_px_size();
         }
         self.size
     }
@@ -41,21 +38,13 @@ fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>
         draws: draws.clone(),
         size,
         reads_box,
-        reads_answer_box: false,
     }
     .add(&mut h.rsc);
     (id, Counts(draws))
 }
 
-fn answer_counted(h: &mut Harness, size: Size) -> (WeakWidget<Counted>, Counts) {
-    let (id, draws) = counted(h, size, false);
-    h.rsc[id].reads_answer_box = true;
-    (id, draws)
-}
-
 struct Layered {
     children: [StrongWidget<Rect>; 2],
-    _revision: usize,
 }
 
 impl Widget for Layered {
@@ -75,14 +64,10 @@ fn a_redrawn_layered_widget_keeps_the_layer_it_was_entered_on() {
         rect(Color::RED).add_strong(&mut h.rsc),
         rect(Color::BLUE).add_strong(&mut h.rsc),
     ];
-    let root = Layered {
-        children,
-        _revision: 0,
-    }
-    .add(&mut h.rsc);
+    let root = Layered { children }.add(&mut h.rsc);
     h.set_root(root);
 
-    h.rsc[root]._revision += 1;
+    h.rsc.widgets_mut().mark_for_redraw(root.id());
     h.frame();
 
     let label = h.rsc.widgets().label(root.id());
@@ -135,7 +120,7 @@ fn moving_an_ordinary_subtree_remaps_its_mask() {
     let active = &h.render.active[&masked.id()];
     assert_eq!(
         h.rsc.ui().masks[active.mask.idx()].region,
-        UiRegion::new(UiSpan::new(Len::px(150.0), Len::rel_max()), UiSpan::FULL,)
+        UiRegion::new(UiSpan::new(Len::px(150.0), Len::FULL), UiSpan::FULL,)
     );
     assert_corners!(h, inner, (150, 0), (400, 200));
 }
@@ -166,9 +151,9 @@ fn a_span_child_that_declares_its_length_is_drawn_once() {
     h.set_root((hinted, asked).span(Dir::RIGHT));
 
     assert_eq!(told_draws.get(), 1);
-    // Its final slot is a parent decision, so it is evaluated there after
-    // the provisional ask established its length.
-    assert_eq!(asked_draws.get(), 2);
+    // Asked once, from the cursor; its slot is its answer and the drawing is
+    // moved there.
+    assert_eq!(asked_draws.get(), 1);
 }
 
 #[test]
@@ -191,9 +176,9 @@ fn a_repaint_that_keeps_its_size_does_not_relay_out() {
     h.set_root((first, second).span(Dir::RIGHT));
     let settled = draws.get();
 
-    // Taking mutable access is the ordinary content-change signal. This
-    // widget returns the same size, so the parent has nothing to lay out.
-    let _ = h.rsc.widgets_mut().get_dyn_mut(first.id());
+    // Marked with nothing about it changed, and it reports the same size
+    // either way, so the parent has nothing to lay out.
+    h.rsc.widgets_mut().mark_for_redraw(first.id());
     h.frame();
 
     assert_eq!(draws.get(), settled + 1);
@@ -208,7 +193,7 @@ fn a_span_child_survives_the_next_frame() {
     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.rsc.widgets_mut().mark_for_redraw(top.id());
     h.frame();
 
     assert_corners!(h, top, (0, 0), (400, 80));
@@ -224,11 +209,7 @@ impl Widget for FromHint {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
         let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
-        painter.widget_at(
-            &self.inner,
-            UiRegion::FULL,
-            [Place::Within(Part::All), Place::Within(Part::From(top))],
-        );
+        painter.widget_at(&self.inner, top.shifted_desc().on_axis(Axis::Y));
         Size::LEFTOVER
     }
 }
@@ -250,7 +231,7 @@ fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
     assert_corners!(h, inner, (0, 0), (400, 120));
 }
 
-/// Reads its box's size to compute its answer.
+/// Reads its box's size, which nothing but its own draw can put right.
 struct ReadsBox {
     draws: Rc<Cell<usize>>,
 }
@@ -258,36 +239,25 @@ struct ReadsBox {
 impl Widget for ReadsBox {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
-        Size::from_px(painter.answer_px_size().div_int(4))
+        Size::from_px(painter.px_size().div_int(4))
     }
 }
 
 /// Reads its box across one axis only, so its drawing holds for a taller
 /// box on its own and only a wider one is worth a draw.
 ///
-/// Both report a quarter of what they read. Their empty drawings are
-/// independent of that read, so a changed question costs one draw.
+/// Both of these report a quarter of what they read. The quarter-sized box
+/// the answer places them in is not a question: the drawing is moved there,
+/// so each length they are asked at costs one draw.
 struct ReadsWidth {
     draws: Rc<Cell<usize>>,
 }
 
-struct ReadsDrawingWidth {
-    draws: Rc<Cell<usize>>,
-}
-
-impl Widget for ReadsDrawingWidth {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        self.draws.set(self.draws.get() + 1);
-        painter.px_len(Axis::X);
-        Size::LEFTOVER
-    }
-}
-
 impl Widget for ReadsWidth {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
         Size::from_px(PxVec2::new(
-            painter.answer_px_len(Axis::X).div_int(4),
+            painter.px_len(Axis::X).div_int(4),
             Px::from_int(20),
         ))
     }
@@ -343,7 +313,6 @@ fn a_row_moves_what_follows_a_child_that_grew_rather_than_drawing_it() {
             draws: ruled.clone(),
             size: Size::LEFTOVER,
             reads_box: false,
-            reads_answer_box: false,
         };
         let second = match declared {
             true => second.width(rel(0.25)).add(&mut h.rsc),
@@ -521,7 +490,7 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     // 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::px((100, 100).into()), false);
+    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
     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));
@@ -686,7 +655,7 @@ fn a_masked_widget_redrawn_on_its_own_sets_its_mask_again() {
     let masked = inner.masked().add(&mut h.rsc);
     let other = rect(Color::RED).width(100).add(&mut h.rsc);
     h.set_root((other, masked).span(Dir::RIGHT));
-    h.rsc.widgets_mut().get_dyn_mut(masked.id());
+    h.rsc.widgets_mut().mark_for_redraw(masked.id());
     h.frame();
     assert_corners!(h, inner, (100, 0), (400, 200));
 }
@@ -800,6 +769,17 @@ fn a_subtree_that_changed_parents_settles_at_the_depth_it_moved_to() {
     );
 }
 
+/// Where a mask slot clips, in window pixels: the region it holds, carried
+/// through whatever move entry it hangs from. Taken by slot rather than by
+/// widget, so a test can name the slot it expects a redraw to keep.
+fn mask_bounds(h: &Harness, mask: MaskIdx) -> PixelRegion {
+    let mask = &h.rsc.ui().masks[mask.idx()];
+    h.render
+        .moves
+        .resolve(mask.move_idx, mask.region)
+        .to_px(h.render.output_size())
+}
+
 fn primitive_bounds(h: &Harness, id: WidgetId) -> Vec<PixelRegion> {
     h.render.active[&id]
         .primitives
@@ -819,7 +799,7 @@ fn primitive_bounds(h: &Harness, id: WidgetId) -> Vec<PixelRegion> {
 }
 
 #[test]
-fn changing_an_inherited_extent_keeps_the_original_measurement_offer() {
+fn changing_an_inherited_region_keeps_the_original_measurement_offer() {
     fn build(h: &mut Harness, width: i32, text: &str) -> (WeakWidget<Text>, WeakWidget<Rect>) {
         let first = rect(Color::RED).width(width).add(&mut h.rsc);
         let words = wtext(text).size(20).wrap(true).add(&mut h.rsc);
@@ -899,11 +879,7 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
-                [
-                    Place::Within(Part::From(self.region.x)),
-                    Place::Within(Part::From(self.region.y)),
-                ],
+                PlaceDesc::new(self.region.x.shifted_desc(), self.region.y.shifted_desc()),
             );
             Size::LEFTOVER
         }
@@ -949,15 +925,10 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
             primitive_bounds(&warm, leaf.id()),
             primitive_bounds(&cold, other.id())
         );
-        let mask = |h: &Harness, id: WidgetId| {
-            let active = &h.render.active[&id];
-            let mask = &h.rsc.ui().masks[active.mask.idx()];
-            h.render
-                .moves
-                .resolve(mask.move_idx, mask.region)
-                .to_px(h.render.output_size())
-        };
-        assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
+        assert_eq!(
+            mask_bounds(&warm, warm.render.active[&leaf.id()].mask),
+            mask_bounds(&cold, cold.render.active[&other.id()].mask)
+        );
     }
 }
 
@@ -981,18 +952,18 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
     }
     struct Frame {
         child: StrongWidget,
+        frame: UiRegion,
         region: UiRegion,
-        extent: UiRegion,
     }
     impl Widget for Frame {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                self.region,
-                [
-                    Place::Fill(Part::From(self.extent.x)),
-                    Place::Fill(Part::From(self.extent.y)),
-                ],
+                PlaceDesc::new(
+                    self.region.x.shifted_desc().fills(),
+                    self.region.y.shifted_desc().fills(),
+                )
+                .rel_base(Axis::X, self.frame.x.len()),
             );
             Size::LEFTOVER
         }
@@ -1008,15 +979,15 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
         h.rsc.widgets_mut().set_region_node(text, node);
         let root = Frame {
             child: text.add_strong(&mut h.rsc),
+            frame: UiRegion::FULL,
             region: UiRegion::FULL,
-            extent: UiRegion::FULL,
         }
         .add(&mut h.rsc);
         h.set_root(root);
         for (start, end) in [(0.13, 0.83), (-0.17, 1.23), (0.31, 0.67)] {
             let before = draws.get();
-            h.rsc[root].region.x = UiSpan::new(Len::px(13.125), Len::px(287.375));
-            h.rsc[root].extent = UiRegion::new(
+            h.rsc[root].frame.x = UiSpan::new(Len::px(13.125), Len::px(287.375));
+            h.rsc[root].region = UiRegion::new(
                 UiSpan::new(Len::rel(start), Len::rel(end)),
                 UiSpan::new(Len::px(7.25), Len::rel(end)),
             );
@@ -1024,7 +995,7 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
             assert_eq!(draws.get(), before);
             let retained = primitive_bounds(&h, text.id());
             assert!(!retained.is_empty());
-            let _ = h.rsc.widgets_mut().get_dyn_mut(text.id());
+            h.rsc.widgets_mut().mark_for_redraw(text.id());
             h.frame();
             assert!(draws.get() > before);
             assert_eq!(retained, primitive_bounds(&h, text.id()));
@@ -1087,7 +1058,7 @@ fn a_declared_size_change_stops_at_an_independent_parent() {
 fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     let mut h = Harness::new((400, 200));
     let draws = Rc::new(Cell::new(0));
-    let leaf = ReadsDrawingWidth {
+    let leaf = ReadsWidth {
         draws: draws.clone(),
     }
     .add(&mut h.rsc);
@@ -1098,7 +1069,7 @@ fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     h.frame();
 
     assert!(draws.get() > settled);
-    assert_corners!(h, leaf, (0, 0), (800, 200));
+    assert_corners!(h, leaf, (300, 90), (500, 110));
 }
 
 #[test]
@@ -1139,7 +1110,7 @@ fn widening_and_restoring_a_contract_does_not_invalidate_its_reader() {
     assert_eq!(leaf_draws.get(), settled + 1);
 }
 #[test]
-fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
+fn padding_and_stack_boxes_follow_the_region_without_drawing_again() {
     struct Observed<W> {
         widget: W,
         draws: Rc<Cell<usize>>,
@@ -1152,23 +1123,22 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
     }
     struct Frame {
         child: StrongWidget,
-        extent: UiRegion,
+        region: UiRegion,
     }
     impl Widget for Frame {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
-                [
-                    Place::Fill(Part::From(self.extent.x)),
-                    Place::Fill(Part::From(self.extent.y)),
-                ],
+                PlaceDesc::new(
+                    self.region.x.shifted_desc().fills(),
+                    self.region.y.shifted_desc().fills(),
+                ),
             );
             Size::LEFTOVER
         }
     }
     for node in [false, true] {
-        let plant = |h: &mut Harness, extent| {
+        let plant = |h: &mut Harness, region| {
             let draws = Rc::new(Cell::new(0));
             let leaf = rect(Color::BLUE).masked().add(&mut h.rsc);
             h.rsc.widgets_mut().set_region_node(leaf, node);
@@ -1189,7 +1159,7 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
                 draws: draws.clone(),
             }
             .add_strong(&mut h.rsc);
-            let root = Frame { child: pad, extent }.add(&mut h.rsc);
+            let root = Frame { child: pad, region }.add(&mut h.rsc);
             h.set_root(root);
             (root, leaf, fixed, draws)
         };
@@ -1204,36 +1174,31 @@ fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
         let mut warm = Harness::new((403, 211));
         let (root, leaf, fixed, draws) = plant(&mut warm, at(0.13));
         for start in [0.13, -0.17, 0.31] {
-            let extent = at(start);
+            let region = at(start);
             let before = draws.get();
-            warm.rsc[root].extent = extent;
+            warm.rsc[root].region = region;
             warm.frame();
             assert_eq!(draws.get(), before);
             let mut cold = Harness::new((403, 211));
-            let (_, other, other_fixed, _) = plant(&mut cold, extent);
+            let (_, other, other_fixed, _) = plant(&mut cold, region);
             for (a, b) in [(leaf.id(), other.id()), (fixed.id(), other_fixed.id())] {
                 assert_eq!(warm.region(&a), cold.region(&b));
                 assert_eq!(primitive_bounds(&warm, a), primitive_bounds(&cold, b));
             }
-            let mask = |h: &Harness, id: WidgetId| {
-                let active = &h.render.active[&id];
-                let mask = &h.rsc.ui().masks[active.mask.idx()];
-                h.render
-                    .moves
-                    .resolve(mask.move_idx, mask.region)
-                    .to_px(h.render.output_size())
-            };
-            assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
+            assert_eq!(
+                mask_bounds(&warm, warm.render.active[&leaf.id()].mask),
+                mask_bounds(&cold, cold.render.active[&other.id()].mask)
+            );
         }
     }
 }
 
 #[test]
-fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
+fn moving_a_childs_region_preserves_the_slot_chosen_from_its_measurement() {
     struct Measured;
     impl Widget for Measured {
         fn draw(&mut self, painter: &mut Painter) -> Size {
-            let width = painter.answer_px_len(Axis::X);
+            let width = painter.px_len(Axis::X);
             painter.primitive(RectPrimitive::color(Color::BLUE));
             Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
         }
@@ -1246,14 +1211,12 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                UiRegion::FULL,
-                [
-                    Place::Fill(Part::From(UiSpan::new(
-                        Len::px(self.start),
-                        Len::px(self.start + 200.0),
-                    ))),
-                    Place::Fill(Part::From(UiSpan::FULL)),
-                ],
+                PlaceDesc::new(
+                    UiSpan::new(Len::px(self.start), Len::px(self.start + 200.0))
+                        .shifted_desc()
+                        .fills(),
+                    UiSpan::FULL.shifted_desc().fills(),
+                ),
             );
             Size::LEFTOVER
         }
@@ -1278,7 +1241,7 @@ fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
 }
 
 #[test]
-fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
+fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
     struct Container {
         child: StrongWidget,
         region: UiRegion,
@@ -1288,18 +1251,14 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    UiRegion::FULL,
-                    [
-                        Place::Within(Part::From(self.region.x)),
-                        Place::Within(Part::From(self.region.y)),
-                    ],
+                    PlaceDesc::new(self.region.x.shifted_desc(), self.region.y.shifted_desc()),
                 )
                 .size()
         }
     }
     struct Frame {
         child: StrongWidget,
-        extent: UiRegion,
+        region: UiRegion,
         answer: Rc<Cell<Size>>,
     }
     impl Widget for Frame {
@@ -1308,11 +1267,10 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 painter
                     .widget_at(
                         &self.child,
-                        UiRegion::FULL,
-                        [
-                            Place::Fill(Part::From(self.extent.x)),
-                            Place::Fill(Part::From(self.extent.y)),
-                        ],
+                        PlaceDesc::new(
+                            self.region.x.shifted_desc().fills(),
+                            self.region.y.shifted_desc().fills(),
+                        ),
                     )
                     .size(),
             );
@@ -1324,7 +1282,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
             UiRegion::FULL,
             UiRegion::new(UiSpan::new(Len::rel(0.13), Len::rel(0.79)), UiSpan::FULL),
         ] {
-            let plant = |h: &mut Harness, extent| {
+            let plant = |h: &mut Harness, outer| {
                 let size = if fractional {
                     Size {
                         x: rel(0.5),
@@ -1333,10 +1291,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 } else {
                     Size::from((80, 27))
                 };
-                let (leaf, _) = match fractional {
-                    true => counted(h, size, false),
-                    false => answer_counted(h, size),
-                };
+                let (leaf, _) = counted(h, size, !fractional);
                 let child = Container {
                     child: leaf.add_strong(&mut h.rsc),
                     region,
@@ -1345,7 +1300,7 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 let answer = Rc::new(Cell::new(Size::ZERO));
                 let root = Frame {
                     child,
-                    extent,
+                    region: outer,
                     answer: answer.clone(),
                 }
                 .add(&mut h.rsc);
@@ -1355,15 +1310,256 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
             let mut warm = Harness::new((403, 211));
             let (root, leaf, answer) = plant(&mut warm, UiRegion::FULL);
             for width in [191.125, 297.25, 83.75] {
-                let extent =
+                let region =
                     UiRegion::new(UiSpan::new(Len::px(13.125), Len::px(width)), UiSpan::FULL);
-                warm.rsc[root].extent = extent;
+                warm.rsc[root].region = region;
                 warm.frame();
                 let mut cold = Harness::new((403, 211));
-                let (_, other, other_answer) = plant(&mut cold, extent);
+                let (_, other, other_answer) = plant(&mut cold, region);
                 assert_eq!(answer.get(), other_answer.get());
                 assert_eq!(warm.region(&leaf), cold.region(&other));
             }
         }
     }
 }
+
+struct OptionalMask {
+    inner: StrongWidget,
+    enabled: bool,
+}
+
+impl Widget for OptionalMask {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        if self.enabled {
+            painter.set_mask(UiRegion::FULL);
+        }
+        painter.widget(&self.inner);
+        Size::LEFTOVER
+    }
+}
+
+fn primitive_masks(h: &Harness, id: WidgetId) -> Vec<MaskIdx> {
+    h.render.active[&id]
+        .primitives
+        .iter()
+        .map(|primitive| {
+            let handle = &primitive.handle;
+            h.render.layers[handle.layer].primitives()[handle.kind as usize]
+                .as_ref()
+                .unwrap()
+                .instances()[handle.inst_idx]
+                .mask_idx
+        })
+        .collect()
+}
+
+#[test]
+fn a_redrawn_mask_keeps_reused_primitives_clipped_when_it_moves() {
+    for node in [false, true] {
+        let mut h = Harness::new((400, 200));
+        let first = rect(Color::RED).height(50).add(&mut h.rsc);
+        let inner = rect(Color::BLUE).add(&mut h.rsc);
+        let draws = Rc::new(Cell::new(0));
+        let child = Stretchy {
+            inner: inner.add_strong(&mut h.rsc),
+            draws: draws.clone(),
+        }
+        .add(&mut h.rsc);
+        let masked = child.masked().add(&mut h.rsc);
+        h.rsc.widgets_mut().set_region_node(masked, node);
+        h.set_root((first, masked).span(Dir::DOWN));
+        let mask = h.render.active[&masked.id()].mask;
+        let settled = draws.get();
+        h.rsc.widgets_mut().mark_for_redraw(masked.id());
+        h.frame();
+        assert_eq!(primitive_masks(&h, inner.id()), vec![mask]);
+        assert_eq!(draws.get(), settled, "a mask repaint must reuse its child");
+        assert_eq!(h.render.active[&masked.id()].mask, mask);
+        h.set_len(first, Axis::Y, 10);
+        h.frame();
+        assert_eq!(mask_bounds(&h, mask), h.region(&masked).unwrap());
+        assert_corners!(h, inner, (0, 10), (400, 200));
+    }
+}
+
+#[test]
+fn adding_and_removing_a_mask_updates_existing_primitives() {
+    let mut h = Harness::new((400, 200));
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let masked = OptionalMask {
+        inner: inner.add_strong(&mut h.rsc),
+        enabled: false,
+    }
+    .add(&mut h.rsc);
+    h.set_root(masked);
+    for enabled in [true, false, true, false] {
+        h.rsc[masked].enabled = enabled;
+        h.frame();
+        let mask = h.render.active[&masked.id()].mask;
+        assert_eq!(mask == MaskIdx::NONE, !enabled);
+        assert_eq!(primitive_masks(&h, inner.id()), vec![mask]);
+    }
+    assert_eq!(h.rsc.ui().masks.len(), 1, "retired slots must be reusable");
+}
+
+#[test]
+fn an_empty_masks_slot_is_released_when_the_mask_is_removed_or_undrawn() {
+    let mut h = Harness::new((400, 200));
+    let (inner, _) = counted(&mut h, Size::LEFTOVER, false);
+    let masked = OptionalMask {
+        inner: inner.add_strong(&mut h.rsc),
+        enabled: true,
+    }
+    .add(&mut h.rsc);
+    let row = (masked,).span(Dir::DOWN).add(&mut h.rsc);
+    h.set_root(row);
+    for _ in 0..3 {
+        h.rsc[masked].enabled = false;
+        h.frame();
+        h.rsc[masked].enabled = true;
+        h.frame();
+        let child = h.rsc[row].pop().unwrap();
+        h.frame();
+        h.rsc[row].push(child);
+        h.frame();
+    }
+    assert_eq!(h.rsc.ui().masks.len(), 1);
+}
+
+struct SharedChild(Rc<StrongWidget>);
+
+impl Widget for SharedChild {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        painter.widget(self.0.as_ref()).size()
+    }
+}
+
+struct SwitchParent {
+    choices: [StrongWidget; 2],
+    choice: usize,
+}
+
+impl Widget for SwitchParent {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        painter.widget(&self.choices[self.choice]).size()
+    }
+}
+
+#[test]
+fn a_redrawn_subtree_is_not_undrawn_by_the_parent_it_left() {
+    for node in [false, true] {
+        let mut h = Harness::new((400, 200));
+        let leaf = rect(Color::RED).width(40).add(&mut h.rsc);
+        let held: StrongWidget = leaf.add_strong(&mut h.rsc);
+        let shared = Rc::new(held);
+        let first = SharedChild(shared.clone()).add_strong(&mut h.rsc);
+        let second = SharedChild(shared).add_strong(&mut h.rsc);
+        h.rsc.widgets_mut().set_region_node(&second, node);
+        let root = SwitchParent {
+            choices: [first, second],
+            choice: 0,
+        }
+        .add(&mut h.rsc);
+        h.set_root(root);
+        let before = h.region(&leaf);
+        h.rsc[root].choice = 1;
+        h.frame();
+        assert_eq!(h.region(&leaf), before);
+    }
+}
+
+/// A leaf that reports less than the box it is given and states which lengths
+/// of that box its drawing holds for, so a test can widen the contract
+/// without changing the answer. It counts its draws, since what a kept
+/// contract costs is whether the parent has to make it draw again.
+struct Contracted {
+    holds: std::ops::RangeInclusive<Px>,
+    size: Size,
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for Contracted {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.holds(Axis::X, self.holds.clone());
+        self.size
+    }
+}
+
+struct CountedParent {
+    inner: StrongWidget,
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for CountedParent {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.widget(&self.inner).size()
+    }
+}
+
+#[test]
+fn widening_what_a_drawing_holds_for_does_not_relay_out_the_parent() {
+    let mut h = Harness::new((400, 200));
+    let child = Contracted {
+        holds: Px::from_int(300)..=Px::from_int(500),
+        size: Size::from((100, 200)),
+        draws: Rc::new(Cell::new(0)),
+    }
+    .add(&mut h.rsc);
+    let draws = Rc::new(Cell::new(0));
+    let root = CountedParent {
+        inner: child.upgrade(&mut h.rsc),
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+    let settled = draws.get();
+
+    // The same answer, good for more boxes than before, so the guarantee the
+    // parent kept still holds.
+    h.rsc[child].holds = Px::from_int(200)..=Px::from_int(600);
+    h.frame();
+
+    assert_eq!(
+        draws.get(),
+        settled,
+        "a wider contract for the same answer is not a change to lay out"
+    );
+}
+
+/// The other half of the rule above: a kept contract is the narrower one, so
+/// it is only worth keeping where it still holds. A window the old range is
+/// outside is not one its parent can be handed back, and keeping it there
+/// throws away the drawing the widget just made.
+#[test]
+fn a_contract_this_window_is_outside_is_not_kept() {
+    let mut h = Harness::new((400, 200));
+    let leaf_draws = Rc::new(Cell::new(0));
+    let child = Contracted {
+        holds: Px::from_int(300)..=Px::from_int(500),
+        size: Size::from((100, 200)),
+        draws: leaf_draws.clone(),
+    }
+    .add(&mut h.rsc);
+    let root = CountedParent {
+        inner: child.upgrade(&mut h.rsc),
+        draws: Rc::new(Cell::new(0)),
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+
+    // Wide enough that the old contract leaves the new box out, and the leaf
+    // is marked in the same frame -- so it settles itself first and its
+    // parent draws afterwards, asking about what it settled.
+    h.resize((600, 200));
+    h.rsc[child].holds = Px::from_int(200)..=Px::from_int(700);
+    let settled = leaf_draws.get();
+    h.frame();
+
+    assert_eq!(
+        leaf_draws.get(),
+        settled + 1,
+        "the leaf settled once and its parent kept what it settled"
+    );
+}

tests/cases/scroll.rs

@@ -113,7 +113,7 @@ fn wrapping_content_beside_a_fixed_length_is_stable_warm_and_cold() {
 
     let mut warm = Harness::new((900, 300));
     let (text, content) = plant(&mut warm);
-    warm.rsc.widgets_mut().get_dyn_mut(text);
+    warm.rsc.widgets_mut().mark_for_redraw(text);
     warm.frame();
 
     let mut cold = Harness::new((900, 300));
@@ -142,6 +142,45 @@ fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
     let mut h = Harness::new((100, 100));
     let tall = rect(Color::RED).height(400).add_strong(&mut h.rsc);
     let clipper = Clipper(tall).add(&mut h.rsc);
+    // `set_root` lays the tree out, so this is where it is caught.
     h.set_root(clipper);
+}
+
+/// Content that fits sits in the viewport, not in a box of the window's
+/// length anchored at the viewport's start. `Part::From` takes window
+/// lengths, so a `rel(1.0)` span in one is the window, and only a scroll
+/// filling the window would land right.
+#[test]
+fn content_that_fits_is_placed_in_the_viewport_and_not_in_the_window() {
+    let mut h = Harness::new((400, 400));
+    let head = rect(Color::RED).height(100).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).height(50).add(&mut h.rsc);
+    let scroll = Scroll::new(inner.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    h.set_root((head, scroll).span(Dir::DOWN));
+
+    assert_corners!(h, scroll, (0, 100), (400, 400));
+    assert_corners!(h, inner, (0, 225), (400, 275));
+}
+
+/// A cap narrows the box the widget is asked in, which is what a scroll
+/// measures its viewport from: the content scrolls within the cap rather than
+/// within the room the cap was cut from.
+#[test]
+fn a_capped_scroll_takes_its_viewport_from_the_cap() {
+    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 scroll = (top, bottom).span(Dir::DOWN).scrollable().add(&mut h.rsc);
+    let capped = scroll.max_height(100).add(&mut h.rsc);
+    h.set_root(capped);
+    h.move_to((200, 50));
+
+    // 400 of content in a viewport of 100, so 300 to scroll and the end
+    // showing: the top is 300 above the box, which the window centres.
+    assert_eq!(h.region(&scroll).unwrap().size().y, Px::from_int(100));
+    assert_corners!(h, top, (0, -250), (400, -50));
+
+    h.scroll((0, 1));
     h.frame();
+    assert_corners!(h, top, (0, -200), (400, 0));
 }

tests/cases/unsettled.rs

@@ -9,17 +9,45 @@
 //! reached through a region node's own entry rather than through the offer
 //! that node was given. The last is a wrapping text handed back the width
 //! it measured, rounded to a step below the line it measured there.
+//!
+//! Each says which seed it was shrunk from, of the generator as it stood when
+//! it was found. Those numbers no longer grow those trees -- a seed names one
+//! only while the generator draws the same things in the same order, and the
+//! leaves have grown an image since -- so what is written out below is the
+//! record of the case, and the seed is where it came from.
+
+use std::collections::HashSet;
 
 use iris::harness::Harness;
 use iris::prelude::*;
 use iris::random::Branch;
 
+/// Every widget in the same place warm as cold, reported all at once: which
+/// of a dozen boxes moved is the whole of what a shrunk case has to say.
+///
+/// A list that names one widget twice is an error rather than a redundant
+/// check. `width`, `sized` and `align` give back the widget they were handed,
+/// so a fixture built through them can name one text three times, and then a
+/// case comparing six boxes compares four and says nothing about it. One
+/// fixture builds both lists, so checking the warm one checks both.
+#[track_caller]
 fn assert_same_regions(
     warm: &Harness,
     warm_ids: &[WidgetId],
     cold: &Harness,
     cold_ids: &[WidgetId],
 ) {
+    assert_eq!(
+        warm_ids.len(),
+        cold_ids.len(),
+        "the warm and cold fixtures list different widgets"
+    );
+    let named: HashSet<&WidgetId> = warm_ids.iter().collect();
+    assert_eq!(
+        named.len(),
+        warm_ids.len(),
+        "a widget is listed twice: {warm_ids:?}"
+    );
     let mut wrong = Vec::new();
     for (i, (&w, &c)) in warm_ids.iter().zip(cold_ids).enumerate() {
         let (got, want) = (warm.region(&w), cold.region(&c));
@@ -92,7 +120,7 @@ fn repainting_a_stack_uses_the_box_its_sizing_child_decided() {
     let mut warm = Harness::new((900, 1200));
     let ids = plant_stack_in_its_sizing_childs_box(&mut warm);
     for &id in &ids {
-        warm.rsc.widgets_mut().get_dyn_mut(id);
+        warm.rsc.widgets_mut().mark_for_redraw(id);
     }
     warm.frame();
 
@@ -188,36 +216,31 @@ fn reordering_nested_spans_keeps_the_answer_from_the_decided_box() {
     assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
-/// Six widgets, shrunk from a 402-widget tree the fuzzer found. Nothing about
+/// Four widgets, shrunk from a 402-widget tree the fuzzer found. Nothing about
 /// the tree changes -- every widget is marked for redraw and the frame is
 /// taken again -- so no box may move, and a warm frame has to land where a
 /// cold one does.
 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;
+    let wrapped = wtext("Wrapping shapes")
+        .size(16)
+        .wrap(true)
+        .width(76)
+        .add(&mut h.rsc);
     h.rsc
         .widgets_mut()
-        .set_alignment(sized, Axis::X, AxisAlign::POS);
+        .set_alignment(wrapped, Axis::X, AxisAlign::POS);
     h.rsc
         .widgets_mut()
-        .set_alignment(sized, Axis::Y, AxisAlign::POS);
+        .set_alignment(wrapped, Axis::Y, AxisAlign::POS);
     let stack = Stack {
-        children: vec![plain.add_strong(&mut h.rsc), aligned.add_strong(&mut h.rsc)],
+        children: vec![plain.add_strong(&mut h.rsc), wrapped.add_strong(&mut h.rsc)],
         size: StackSize::Child(0),
     }
     .add(&mut h.rsc);
     let root = (stack,).span(Dir::RIGHT).add(&mut h.rsc);
     h.set_root(root);
-    vec![
-        plain.id(),
-        wrapped.id(),
-        sized.id(),
-        aligned.id(),
-        stack.id(),
-        root.id(),
-    ]
+    vec![plain.id(), wrapped.id(), stack.id(), root.id()]
 }
 
 /// The first frame does not reach the layout a second one does, so "cold" is
@@ -230,7 +253,7 @@ fn one_frame_is_enough() {
     let first = h.region(&ids[1]).unwrap();
     for _ in 0..3 {
         for &id in &ids {
-            h.rsc.widgets_mut().get_dyn_mut(id);
+            h.rsc.widgets_mut().mark_for_redraw(id);
         }
         h.frame();
     }
@@ -252,46 +275,30 @@ fn repainting_everything_moves_nothing() {
     let mut warm = Harness::new((640, 900));
     let ids = plant(&mut warm);
     for &id in &ids {
-        warm.rsc.widgets_mut().get_dyn_mut(id);
+        warm.rsc.widgets_mut().mark_for_redraw(id);
     }
     warm.frame();
 
     let mut cold = Harness::new((640, 900));
     let cold_ids = plant(&mut cold);
 
-    let mut wrong = Vec::new();
-    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
-        let (got, want) = (warm.region(&w), cold.region(&c));
-        if got != want {
-            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
-        }
-    }
-    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
-/// Six widgets, shrunk from 905. Everything inside the declared 189x176 box
+/// Four widgets, shrunk from 905. Everything inside the declared 189x176 box
 /// is the same size whatever the output is, so a resize may not change any of
 /// it -- but the text comes out 3.92px narrower warm than cold.
 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 inner = (text,).span(Dir::RIGHT).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);
+    let root = (filler, inner).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(),
-    ]
+    vec![text.id(), inner.id(), filler.id(), root.id()]
 }
 
 #[test]
@@ -306,17 +313,10 @@ fn a_resize_does_not_reach_inside_a_box_of_declared_pixels() {
     let cold_ids = plant_fixed(&mut cold);
     cold.frame();
 
-    let mut wrong = Vec::new();
-    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
-        let (got, want) = (warm.region(&w), cold.region(&c));
-        if got != want {
-            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
-        }
-    }
-    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
-/// Four widgets, shrunk from 486. A span's two children are swapped: warm by
+/// Three widgets, shrunk from 486. A span's two children are swapped: warm by
 /// moving them, cold by growing them that way. Same widgets, same sizes, one
 /// ends up 29.9px from where the other does.
 fn plant_pair(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, WeakWidget<Span>) {
@@ -340,16 +340,11 @@ fn plant_pair(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, WeakWidget<Span
         gap: Px::ZERO,
     }
     .add(&mut h.rsc);
-    let span_handle = span;
-    let aligned = span;
     h.rsc
         .widgets_mut()
         .set_alignment(span, Axis::X, AxisAlign::CENTER);
-    h.state.root = Some(aligned.add_strong(&mut h.rsc));
-    (
-        vec![wrapped.id(), plain.id(), span.id(), aligned.id()],
-        span_handle,
-    )
+    h.state.root = Some(span.add_strong(&mut h.rsc));
+    (vec![wrapped.id(), plain.id(), span.id()], span)
 }
 
 #[test]
@@ -364,17 +359,10 @@ fn swapping_two_children_lands_where_growing_them_that_way_does() {
     let (cold_ids, _) = plant_pair(&mut cold, true);
     cold.frame();
 
-    let mut wrong = Vec::new();
-    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
-        let (got, want) = (warm.region(&w), cold.region(&c));
-        if got != want {
-            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
-        }
-    }
-    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
-/// Eight widgets, shrunk from 80. The scroll decides how wide to make its
+/// Seven widgets, shrunk from 80. The scroll decides how wide to make its
 /// content from what the content says, and hands that box down through a
 /// pass-through; the span under it was given that box once, so nothing at its
 /// own edge says the box was its own answer.
@@ -393,8 +381,7 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
         gap: Px::ZERO,
     }
     .add(&mut h.rsc);
-    let block = rect(Color::RED).add(&mut h.rsc);
-    let fixed = block.width(87).add(&mut h.rsc);
+    let fixed = rect(Color::RED).width(87).add(&mut h.rsc);
     let mut outer_children: Vec<StrongWidget> =
         vec![fixed.add_strong(&mut h.rsc), inner.add_strong(&mut h.rsc)];
     if swapped {
@@ -416,7 +403,6 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
             text.id(),
             filler.id(),
             inner.id(),
-            block.id(),
             fixed.id(),
             outer.id(),
             through.id(),
@@ -440,14 +426,7 @@ fn a_span_given_the_box_its_answer_decided_matches_a_cold_layout() {
     let (cold_ids, _) = plant_scrolled(&mut cold, true);
     cold.frame();
 
-    let mut wrong = Vec::new();
-    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
-        let (got, want) = (warm.region(&w), cold.region(&c));
-        if got != want {
-            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
-        }
-    }
-    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
 /// Reports a width derived from the box it is asked in. Reading through the
@@ -471,11 +450,10 @@ impl Widget for Wider {
 fn plant_wider(h: &mut Harness, extra: f32) -> (WeakWidget<Wider>, WidgetId) {
     let content = Wider { extra }.add(&mut h.rsc);
     let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X).add(&mut h.rsc);
-    let root = scroll;
     h.rsc
         .widgets_mut()
         .set_alignment(scroll, Axis::X, AxisAlign::NEG);
-    h.set_root(root);
+    h.set_root(scroll);
     (content, scroll.id())
 }
 
@@ -567,14 +545,7 @@ fn a_box_that_only_rounds_past_its_fixed_children_leaves_nothing_over() {
     let (cold_ids, _) = plant_boundary(&mut cold, true);
     cold.frame();
 
-    let mut wrong = Vec::new();
-    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
-        let (got, want) = (warm.region(&w), cold.region(&c));
-        if got != want {
-            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
-        }
-    }
-    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
 /// Five widgets, shrunk by `tests/shrink.rs` from the 277 the oracle's seed
@@ -616,20 +587,13 @@ fn plant_nested_scrolls(h: &mut Harness) -> Vec<WidgetId> {
 fn redrawing_one_widget_does_not_move_what_scrolls_around_it() {
     let mut warm = Harness::new((900, 1200));
     let ids = plant_nested_scrolls(&mut warm);
-    warm.rsc.widgets_mut().get_dyn_mut(ids[0]);
+    warm.rsc.widgets_mut().mark_for_redraw(ids[0]);
     warm.frame();
 
     let mut cold = Harness::new((900, 1200));
     let cold_ids = plant_nested_scrolls(&mut cold);
 
-    let mut wrong = Vec::new();
-    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
-        let (got, want) = (warm.region(&w), cold.region(&c));
-        if got != want {
-            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
-        }
-    }
-    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
 /// Ten widgets, of the shape `tests/shrink.rs` reduces the oracle's seed 220
@@ -720,19 +684,57 @@ fn a_widget_under_a_region_node_is_asked_in_the_box_that_node_was_offered() {
     let (cold_ids, _) = plant_under_a_node(&mut cold, true);
     cold.frame();
 
-    let mut wrong = Vec::new();
-    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
-        let (got, want) = (warm.region(&w), cold.region(&c));
-        if got != want {
-            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
-        }
-    }
-    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
 }
 
 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.";
 
+fn plant_stack_resized_from_free(h: &mut Harness, fixed: bool) -> (Vec<WidgetId>, WidgetId) {
+    let sizing = rect(Color::CYAN.alpha(126)).add(&mut h.rsc);
+    h.rsc.widgets_mut().set_size_rules(sizing.id(), None, None);
+    if fixed {
+        h.rsc.widgets_mut().set_size_rules(
+            sizing.id(),
+            Some(LayoutLen::px(112)),
+            Some(LayoutLen::px(101)),
+        );
+    }
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let pad = Pad {
+        padding: Padding::ZERO,
+        inner: text.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![sizing.add_strong(&mut h.rsc), pad.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    h.set_root(stack);
+    (
+        vec![sizing.id(), text.id(), pad.id(), stack.id()],
+        sizing.id(),
+    )
+}
+
+#[test]
+fn fixing_a_stacks_sizing_child_repositions_its_overlay() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, sizing) = plant_stack_resized_from_free(&mut warm, false);
+    warm.frame();
+    warm.rsc.widgets_mut().set_size_rules(
+        sizing,
+        Some(LayoutLen::px(112)),
+        Some(LayoutLen::px(101)),
+    );
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _) = plant_stack_resized_from_free(&mut cold, true);
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
 /// Eight widgets, shrunk from a 118-widget tree (seed 1121, depth 4,
 /// `shuffle-swap-for-three`). The stack takes its size from the span above,
 /// the span takes its width from the longest line of the texts in it, and
@@ -846,3 +848,208 @@ fn adding_text_to_a_reverse_row_keeps_its_shared_height() {
     let (_, other, _) = build(&mut cold, true);
     assert_eq!(warm.region(&shared), cold.region(&other));
 }
+
+/// Nine widgets, shrunk from seed 946 at depth 6. The column is a share of
+/// the row while its rect has room to draw and a fixed width once it has
+/// not, so the row asks it twice: in the room, where it answers a share,
+/// and in its slot, where it answers its text's width. Emptying the column
+/// changes only the first answer. A local redraw that asked only the second
+/// question kept the row as it was; the column has to defer to the row.
+fn plant_column_that_is_a_share_only_while_its_rect_fits(
+    h: &mut Harness,
+    emptied: bool,
+) -> (Vec<WidgetId>, WeakWidget<Span>, Vec<StrongWidget>) {
+    let first = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let filler = rect(Color::CYAN.alpha(126)).add(&mut h.rsc);
+    let second = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let mut spare: Vec<StrongWidget> =
+        vec![filler.add_strong(&mut h.rsc), second.add_strong(&mut h.rsc)];
+    let mut children: Vec<StrongWidget> = vec![first.add_strong(&mut h.rsc)];
+    if !emptied {
+        children.append(&mut spare);
+    }
+    let column = Span {
+        children,
+        dir: Dir::DOWN,
+        gap: Px::ZERO,
+    }
+    .height(159)
+    .add(&mut h.rsc);
+    let left = rect(Color::MAGENTA.alpha(189)).add(&mut h.rsc);
+    let right = rect(Color::BLUE.alpha(0)).add(&mut h.rsc);
+    let row = Span {
+        children: vec![
+            left.add_strong(&mut h.rsc),
+            column.add_strong(&mut h.rsc),
+            right.add_strong(&mut h.rsc),
+        ],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let end = rect(Color::MAGENTA.alpha(189)).add(&mut h.rsc);
+    let root = Span {
+        children: vec![end.add_strong(&mut h.rsc), row.add_strong(&mut h.rsc)],
+        dir: Dir::LEFT,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+    (
+        vec![
+            first.id(),
+            filler.id(),
+            second.id(),
+            column.id(),
+            left.id(),
+            right.id(),
+            row.id(),
+            end.id(),
+            root.id(),
+        ],
+        column,
+        spare,
+    )
+}
+
+#[test]
+fn emptying_a_column_the_row_asked_twice_asks_the_row_again() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, column, _spare) =
+        plant_column_that_is_a_share_only_while_its_rect_fits(&mut warm, false);
+    warm.frame();
+    // Kept alive: dropping the last share of a widget frees its id.
+    let _removed: Vec<StrongWidget> = warm.rsc[column].children.drain(1..).collect();
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _, _spare) =
+        plant_column_that_is_a_share_only_while_its_rect_fits(&mut cold, true);
+    cold.frame();
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
+/// Six widgets, shrunk from seed 59 at depth 5 (`resize-size`). The column
+/// divides the box it is given between two shares, so its drawing holds for
+/// that box's length alone, and the pads above it pass that dependency up:
+/// each one's box is a part of the box it was asked in. Padding narrowing
+/// the frame it hands down does not change that, and while it was taken to,
+/// changing the rule over the pads relocated the column's drawing into the
+/// new box instead of dividing it again.
+fn plant_two_shares_under_two_pads(h: &mut Harness, height: f32) -> Vec<WidgetId> {
+    let top = rect(Color::CYAN.alpha(126)).add(&mut h.rsc);
+    let bottom = rect(Color::RED).add(&mut h.rsc);
+    let column = (top, bottom).span(Dir::DOWN).add(&mut h.rsc);
+    let inner = Pad {
+        padding: Padding::ZERO,
+        inner: column.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    let outer = Pad {
+        padding: Padding::ZERO,
+        inner: inner.add_strong(&mut h.rsc),
+    }
+    .height(height)
+    .add(&mut h.rsc);
+    let beside = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((outer, beside).span(Dir::RIGHT));
+    vec![
+        top.id(),
+        bottom.id(),
+        column.id(),
+        inner.id(),
+        outer.id(),
+        beside.id(),
+    ]
+}
+
+#[test]
+fn changing_a_rule_over_two_pads_divides_the_column_again() {
+    let mut warm = Harness::new((900, 1200));
+    let ids = plant_two_shares_under_two_pads(&mut warm, 88.0);
+    warm.frame();
+    warm.rsc
+        .widgets_mut()
+        .set_size_rules(ids[4], None, Some(LayoutLen::px(105)));
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let cold_ids = plant_two_shares_under_two_pads(&mut cold, 105.0);
+    cold.frame();
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
+/// Six widgets, shrunk from seed 942 at depth 6 (`resize`). A `Branch` asks
+/// its probe in the top 40 px of its box and forwards the frame, so the
+/// scroll's own box is 40 px tall whatever the window is -- but its content
+/// is as tall as the frame, which is the window, and a scroll kept to its
+/// end has to be told when that changes. Resolving a length against the
+/// window is what reads it, so that is where the dependency is taken.
+fn plant_a_window_tall_column_in_a_short_scroll(h: &mut Harness) -> Vec<WidgetId> {
+    let leaf = rect(Color::RED).add(&mut h.rsc);
+    let column = Span {
+        children: vec![leaf.add_strong(&mut h.rsc)],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .height(rel(1.0))
+    .add(&mut h.rsc);
+    let scroll = Scroll::new(column.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    let wide = rect(Color::BLUE).add(&mut h.rsc);
+    let narrow = rect(Color::GREEN).add(&mut h.rsc);
+    let root = Branch {
+        probe: scroll.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 55.0,
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+    vec![leaf.id(), column.id(), scroll.id(), root.id()]
+}
+
+#[test]
+fn resizing_under_a_short_scroll_snaps_its_window_tall_content_again() {
+    let mut warm = Harness::new((1920, 1200));
+    let ids = plant_a_window_tall_column_in_a_short_scroll(&mut warm);
+    warm.frame();
+    warm.resize((640, 900));
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let cold_ids = plant_a_window_tall_column_in_a_short_scroll(&mut cold);
+    cold.frame();
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
+/// A scroll clamps its position against the box it is drawn in, so drawing it
+/// once at one viewport and again at another writes state the second draw then
+/// reads. That the answer is still the one a cold layout gives is a property
+/// of the clamp, not something the layout enforces.
+#[test]
+fn a_scrolled_view_resized_lands_where_a_cold_layout_puts_it() {
+    for amt in [10.0, 40.0, 90.0, 140.0] {
+        let mut warm = Harness::new((100, 100));
+        let (_, warm_scroll) = plant_wider(&mut warm, 100.0);
+        warm.move_to((50.0, 50.0));
+        warm.scroll((-amt, 0.0));
+        warm.frame();
+        warm.resize((160, 100));
+        warm.frame();
+
+        let mut cold = Harness::new((160, 100));
+        let (_, cold_scroll) = plant_wider(&mut cold, 100.0);
+        cold.move_to((50.0, 50.0));
+        cold.scroll((-amt, 0.0));
+        cold.frame();
+
+        assert_eq!(
+            warm.region(&warm_scroll),
+            cold.region(&cold_scroll),
+            "scrolled by {amt} then widened"
+        );
+    }
+}

tests/chain_cost.rs

@@ -11,8 +11,6 @@
 //! 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::{
@@ -21,6 +19,9 @@ use iris_core::{
 };
 use wgpu::{Color as GpuColor, *};
 
+#[path = "gpu/mod.rs"]
+mod gpu;
+
 const SIZE: u32 = 1024;
 const INSTANCES: usize = 200_000;
 const FRAMES: u32 = 20;
@@ -29,18 +30,7 @@ const FRAMES: u32 = 20;
 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()?;
+    let adapter = gpu::adapter()?;
     if !adapter.features().contains(Features::TIMESTAMP_QUERY) {
         println!("no timestamp queries on {:?}", adapter.get_info().name);
         return None;
@@ -55,20 +45,6 @@ fn gpu() -> Option<(Device, Queue, f32)> {
     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>();
@@ -103,7 +79,7 @@ fn fill(ui: &mut UiData, render: &mut UiRenderState, depth: usize) {
 /// 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 node = UiRenderNode::new(device, &gpu::config(format, SIZE));
     let mut ui = UiData::default();
     let mut render = UiRenderState::new();
     fill(&mut ui, &mut render, depth);

tests/deferred_generated.rs

@@ -0,0 +1,67 @@
+#[path = "scenario/mod.rs"]
+mod scenario;
+
+use iris::prelude::*;
+use iris::random::{Edits, Plan, plan};
+
+fn check_requests(edit: impl Fn(&mut Plan) + Sync) {
+    let count = scenario::env("IRIS_DEFERRED_SEEDS", 20_u64);
+    let depth = scenario::env("IRIS_DEFERRED_DEPTH", 4_usize);
+    let seeds = std::env::var("IRIS_DEFERRED_SEED")
+        .ok()
+        .and_then(|seed| seed.parse().ok())
+        .map_or_else(|| (1..=count).collect(), |seed| vec![seed]);
+    scenario::over_seeds(seeds, |seed| {
+        let mut grown = plan(seed, depth, &Edits::default());
+        edit(&mut grown);
+        for case in scenario::ALL {
+            if let Some(how) = scenario::diverges(&grown, case, seed) {
+                panic!(
+                    "request seed {seed} depth {depth} after {}: {how}",
+                    case.name()
+                );
+            }
+        }
+    });
+}
+
+#[test]
+fn deferred_requests_agree_warm_and_cold() {
+    check_requests(|grown| {
+        let mut index = 0;
+        grown.walk_mut(&mut |node| {
+            if let Some(rules) = &mut node.size {
+                for axis in Axis::BOTH {
+                    index += 1;
+                    rules[axis] = match index % 7 {
+                        0 => leftover(1).clamp(20, 120).into(),
+                        1 => leftover(1).min(rel(0.5)).into(),
+                        2 => (leftover(1) + px(30)).min(leftover(2)).into(),
+                        _ => rules[axis].clone(),
+                    };
+                }
+            }
+        });
+    });
+}
+
+#[test]
+fn relative_intrinsic_bounds_agree_warm_and_cold() {
+    check_requests(|grown| {
+        grown.walk_mut(&mut |node| {
+            if let Some(rules) = &mut node.size {
+                for axis in Axis::BOTH {
+                    rules[axis] = match rules[axis] {
+                        SizeRule::Min(_) => SizeRule::Min(Len::rel(0.25)),
+                        SizeRule::Max(_) => SizeRule::Max(Len::rel(0.75)),
+                        SizeRule::Clamp { .. } => SizeRule::Clamp {
+                            min: Len::rel(0.25),
+                            max: Len::rel(0.75),
+                        },
+                        ref rule => rule.clone(),
+                    };
+                }
+            }
+        });
+    });
+}

tests/draw_cost.rs

@@ -13,10 +13,6 @@
 //! That is how `PrimitiveRender` was measured against a match in the renderer:
 //! 6 instructions per list drawn, against the ~5,400 wgpu spends recording
 //! one.
-//!
-//! The instance is leaked deliberately. A Vulkan loader may unload the driver
-//! when the last one drops, which can fault as a thread that used it exits --
-//! and every test runs on a spawned thread.
 
 use std::time::Instant;
 
@@ -27,6 +23,9 @@ use iris_core::{
 };
 use wgpu::{Color as GpuColor, *};
 
+#[path = "gpu/mod.rs"]
+mod gpu;
+
 const SIZE: u32 = 1024;
 const FRAMES: u32 = 200;
 /// Reported as the best of this many batches, since the mean moves by more
@@ -34,39 +33,11 @@ const FRAMES: u32 = 200;
 const BATCHES: u32 = 8;
 
 fn gpu() -> Option<(Device, Queue)> {
-    // Probed rather than assumed: there may be no Vulkan adapter, and GL is
-    // what is left when there is not.
-    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()
-        }),
-    };
-    // Leaked rather than dropped: see the note at the top of the file.
-    let instance: &'static Instance = Box::leak(Box::new(instance));
-    let adapter =
-        pollster::block_on(instance.request_adapter(&RequestAdapterOptions::default())).ok()?;
+    let adapter = gpu::adapter()?;
     println!("adapter: {:?}", adapter.get_info());
     pollster::block_on(adapter.request_device(&DeviceDescriptor::default())).ok()
 }
 
-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![],
-    }
-}
-
 /// Every layer draws all three primitives, so the renderer takes a different
 /// path for each list it walks -- which is the case a single-primitive layer
 /// would never exercise. Images are bound per instance, so there are few.
@@ -136,7 +107,7 @@ fn fill(
 
 fn frame_cost(device: &Device, queue: &Queue, layers: usize, per_layer: usize) -> f64 {
     let format = TextureFormat::Bgra8Unorm;
-    let mut node = UiRenderNode::new(device, &config(format));
+    let mut node = UiRenderNode::new(device, &gpu::config(format, SIZE));
     let mut ui = UiData::default();
     let mut render = UiRenderState::new();
     let _handles = fill(&mut ui, &mut render, layers, per_layer);

tests/generated.rs

@@ -14,7 +14,7 @@
 #[path = "scenario/mod.rs"]
 mod scenario;
 
-use iris::random::{Edits, plan};
+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
@@ -25,15 +25,22 @@ 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.
+/// The seeds the ordinary tests take: a corpus rather than a set of
+/// regression cases, since a seed names a tree only for as long as the
+/// generator draws the same things in the same order. Adding images to the
+/// leaves moved every one of them, so 20 and 86 -- which once caught a widget
+/// placed twice in a box its parent had already placed it in, and a `Scroll`
+/// fixed point settling differently -- no longer grow those trees. Both
+/// defects are pinned by the shrunk fixtures in `cases/unsettled.rs`, which
+/// are trees rather than numbers.
 const SEEDS: [u64; 10] = [1, 2, 3, 5, 8, 10, 13, 20, 86, 98];
 
 fn check(seed: u64, depth: usize, case: Case) {
-    let grown = plan(seed, depth, &Edits::default());
-    if let Some(how) = diverges(&grown, case, seed) {
+    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} \
@@ -44,57 +51,50 @@ fn check(seed: u64, depth: usize, case: Case) {
     }
 }
 
-macro_rules! case {
-    ($name:ident, $case:expr) => {
-        #[test]
-        fn $name() {
-            for seed in SEEDS {
-                check(seed, depth(), $case);
+/// A test per case, and the list of which cases have one, from the same
+/// place. A case the ordinary suite leaves out runs only in the long scan,
+/// which nobody runs by hand.
+macro_rules! cases {
+    ($($name:ident = $case:expr,)*) => {
+        $(
+            #[test]
+            fn $name() {
+                for seed in SEEDS {
+                    check(seed, depth(), $case);
+                }
             }
-        }
+        )*
+
+        const NAMED: [Case; [$($case,)*].len()] = [$($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
-);
+cases! {
+    many_widgets_redrawing_at_once_leaves_every_box_where_it_was = Case::RepaintSome,
+    everything_redrawing_at_once_leaves_every_box_where_it_was = Case::Repaint,
+    a_resize_lands_where_starting_at_that_size_would = Case::Resize,
+    a_resize_and_a_repaint_land_where_starting_that_way_would = Case::ResizeRepaint,
+    a_size_change_after_a_resize_lands_the_same_way = Case::ResizeSize,
+    a_resize_after_a_size_change_lands_the_same_way = Case::SizeResize,
+    a_size_change_lands_where_growing_it_that_way_would = Case::Size,
+    every_size_changing_at_once_lands_where_growing_it_that_way_would = Case::EverySize,
+    an_alignment_change_lands_where_growing_it_that_way_would = Case::Align,
+    giving_and_taking_a_movable_region_rebuilds_what_resolves_it = Case::RegionNode,
+    reordering_a_span_lands_where_growing_it_that_way_would = Case::Reorder,
+}
+
+/// The shuffles are one test between them, so they are the only cases `ALL`
+/// may hold without a test of their own.
+#[test]
+fn every_case_runs_without_the_long_scan() {
+    for case in ALL {
+        assert!(
+            NAMED.contains(&case) || matches!(case, Case::Shuffle(_)),
+            "{} runs only in the long seed scan; give it a case here",
+            case.name()
+        );
+    }
+}
 
 #[test]
 fn adding_and_removing_span_children_lands_where_growing_it_that_way_would() {
@@ -108,7 +108,7 @@ fn adding_and_removing_span_children_lands_where_growing_it_that_way_would() {
 }
 
 #[test]
-#[ignore = "as many seeds as it is asked for, rather than the nine the others check"]
+#[ignore = "as many seeds as it is asked for, rather than the ten the others check"]
 fn a_long_run_of_seeds_agrees() {
     let depth = depth();
     let seeds: Vec<u64> = match std::env::var("IRIS_GENERATED_SEED")
@@ -119,8 +119,9 @@ fn a_long_run_of_seeds_agrees() {
         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(seed, depth, case);
+            check_plan(&grown, seed, depth, case);
         }
     });
 }

tests/gpu/mod.rs

@@ -0,0 +1,40 @@
+//! The adapter and the surface configuration the GPU measurement rigs share,
+//! so the two cannot probe for a device in two different ways.
+
+use wgpu::*;
+
+/// An adapter on whatever this machine has, or `None` where there is none.
+///
+/// Probed rather than assumed: there may be no Vulkan adapter, and GL is what
+/// is left when there is not.
+///
+/// The instance is leaked deliberately. A Vulkan loader may unload the driver
+/// when the last one drops, which can fault as a thread that used it exits --
+/// and every test runs on a spawned thread.
+pub fn adapter() -> Option<Adapter> {
+    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));
+    pollster::block_on(instance.request_adapter(&RequestAdapterOptions::default())).ok()
+}
+
+pub fn config(format: TextureFormat, size: u32) -> 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![],
+    }
+}

tests/layout_diagnostics.rs

@@ -13,15 +13,40 @@
 //!
 //! `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.
+//! `IRIS_DIRTY` how many widgets `many` marks at once. `IRIS_UNBOUNDED=1`
+//! removes intrinsic bounds while preserving the rest of the generated tree.
 
 use iris::harness::Harness;
 use iris::prelude::*;
-use iris::random::{Edits, Tree, grow};
+use iris::random::{Edits, Tree, build, plan};
 use std::time::Instant;
 
 const OUTPUT: (f32, f32) = (1920.0, 1200.0);
 
+/// A scroll whose content fits is the same drawing in every box it still
+/// fits in, so a longer or shorter one relays out nothing. Where the content
+/// sits in that box is decided by placing its answer in the whole of it,
+/// which is a fraction of the box and holds at every length -- so the
+/// contract must not turn on the alignment. It did, and at the default
+/// alignment, which is the middle, every box change redrew the scroll.
+#[cfg(feature = "layout-diagnostics")]
+#[test]
+fn a_fitting_scroll_holds_for_every_box_its_content_fits_in() {
+    use iris::core::layout_diagnostics as diag;
+
+    for align in [Align::TOP_LEFT, Align::CENTER, Align::BOT_RIGHT] {
+        let mut harness = Harness::new((400, 200));
+        let inner = rect(Color::RED).height(50).add(&mut harness.rsc);
+        harness.set_root(inner.scrollable().align(align));
+        harness.frame();
+        let _ = diag::take();
+        // Still far longer than the 50 the content needs.
+        harness.resize((400, 180));
+        harness.frame();
+        assert_eq!(diag::take().distinct_widgets(), 0, "{align:?}");
+    }
+}
+
 #[cfg(feature = "layout-diagnostics")]
 #[test]
 fn a_selected_widget_retains_its_layout_events() {
@@ -37,8 +62,8 @@ fn a_selected_widget_retains_its_layout_events() {
     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.rsc.widgets_mut().mark_for_redraw(root.id());
+    harness.rsc.widgets_mut().mark_for_redraw(leaf.id());
     harness.frame();
 
     let report = diagnostics::take();
@@ -101,9 +126,17 @@ fn rig_edits() -> Edits {
     }
 }
 
+fn fixture(harness: &mut Harness, seed: u64, depth: usize) -> (StrongWidget, Tree) {
+    let mut plan = plan(seed, depth, &rig_edits());
+    if env("IRIS_UNBOUNDED", 0_u8) != 0 {
+        plan.drop_bounds();
+    }
+    build(&mut harness.rsc, &plan)
+}
+
 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());
+    let (root, tree) = fixture(&mut harness, seed, depth);
     harness.state.root = Some(root);
     harness.frame();
     println!(
@@ -185,7 +218,7 @@ fn layout_cost() {
 
     if selected("cold") {
         let mut harness = Harness::new(OUTPUT);
-        let (root, tree) = grow(&mut harness.rsc, seed, depth, &rig_edits());
+        let (root, tree) = fixture(&mut harness, seed, depth);
         harness.state.root = Some(root);
         println!(
             "fixture: seed {seed}, depth {depth}, {} widgets",
@@ -195,7 +228,6 @@ fn layout_cost() {
         #[cfg(feature = "layout-diagnostics")]
         let _ = iris::core::layout_diagnostics::take();
         run("cold", 1, &mut harness, |_, _| {});
-        drop(tree);
     }
 
     if selected("repaint") {
@@ -203,7 +235,7 @@ fn layout_cost() {
         trace_selected(&tree);
         let leaf = tree.ids[0];
         run("repaint", frames, &mut harness, move |harness, _| {
-            let _ = harness.rsc.widgets_mut().get_dyn_mut(leaf);
+            harness.rsc.widgets_mut().mark_for_redraw(leaf);
         });
     }
 
@@ -218,7 +250,7 @@ fn layout_cost() {
         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);
+                harness.rsc.widgets_mut().mark_for_redraw(id);
             }
         });
     }
@@ -251,6 +283,21 @@ fn layout_cost() {
         run("resize", frames, &mut harness, |harness, frame| {
             harness.resize((OUTPUT.0 - ((frame + 1) % 2) as f32 * 8.0, OUTPUT.1));
         });
-        drop(tree);
     }
 }
+
+#[cfg(feature = "layout-diagnostics")]
+#[test]
+fn repainting_measured_text_does_not_invalidate_its_span() {
+    use iris::core::layout_diagnostics as diag;
+
+    let mut h = Harness::new((400, 200));
+    let text = wtext("a paragraph that fits").wrap(true).add(&mut h.rsc);
+    h.set_root((text, wtext("another paragraph")).span(Dir::DOWN));
+    let _ = diag::take();
+    h.rsc.widgets_mut().mark_for_redraw(text);
+    h.frame();
+    let report = diag::take();
+    assert_eq!(report.distinct_widgets(), 1);
+    assert_eq!(report.hot_widgets()[0].id, text.id());
+}

tests/layout_dump.rs

@@ -0,0 +1,47 @@
+//! 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. `IRIS_UNBOUNDED=1` drops the trees' intrinsic bounds, as
+//! in the diagnostics rig, which compares the two paths over the same shapes.
+
+use iris::harness::Harness;
+use iris::random::{Edits, build, plan};
+
+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 mut plan = plan(seed, depth, &Edits::default());
+        if env("IRIS_UNBOUNDED", 0_u8) != 0 {
+            plan.drop_bounds();
+        }
+        let (root, tree) = build(&mut harness.rsc, &plan);
+        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/revision_cost.rs

@@ -1,19 +1,22 @@
-//! 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.
+//! Text-layout workloads with stable paragraphs for comparisons across revisions.
+//! PR #19's base uses the older spelling of the fixed 40-pixel width and has
+//! no diagnostics. The random generator changed with layout, so it cannot
+//! provide the same workload across the full PR.
 //!
 //!     ROWS=40 FRAMES=500 cargo test --release --test revision_cost \
 //!       -- --ignored --nocapture resize_cost
+//!     PHASE=edit ROWS=40 FRAMES=2000 cargo test --release --test revision_cost \
+//!       -- --ignored --nocapture text_updates_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.
+//! `text_updates_cost` selects idle, repaint, edit, or scroll with `PHASE`.
+//! It alternates a short suffix for edits so later frames do not get a longer
+//! paragraph than earlier ones. These are CPU fixtures, with no GPU submission.
+//!
+//! Use repeated `perf stat -e instructions:u` runs on the executable directly;
+//! process totals include font loading and the cold frame, so compare identical
+//! row and frame counts. Wall time on this machine is not a stable comparison.
 
 use iris::harness::Harness;
 use iris::prelude::*;
@@ -137,9 +140,10 @@ fn resize_cost() {
         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.
+    // The sweep cycles 256 widths, avoiding the two-width cache-friendly case.
     let sweep = env("SWEEP", 0_usize) != 0;
+    #[cfg(feature = "layout-diagnostics")]
+    let _ = iris::core::layout_diagnostics::take();
     let mut elapsed = Vec::with_capacity(frames);
     for frame in 0..frames {
         let narrower = match sweep {
@@ -151,6 +155,11 @@ fn resize_cost() {
         h.frame();
         elapsed.push(start.elapsed().as_secs_f64() * 1000.0);
     }
+    #[cfg(feature = "layout-diagnostics")]
+    print!(
+        "{}",
+        iris::core::layout_diagnostics::take().per_frame(frames)
+    );
     elapsed.sort_by(|a, b| a.partial_cmp(b).unwrap());
     println!(
         "resize: {frames} frames, min {:.3} ms, median {:.3} ms, p99 {:.3} ms, \
@@ -194,10 +203,73 @@ fn text_memory() {
         h.frame();
     }
     report("after 40 resizes");
-    // Settled: the output holds still and one leaf repaints per frame.
+    // Settled: the output holds still and one leaf repaints per frame. Marked
+    // by taking it mutably because the revision at the top of this file has no
+    // `mark_for_redraw`, and the same source has to build against both.
     for _ in 0..10 {
         let _ = h.rsc.widgets_mut().get_dyn_mut(paragraphs[0]);
         h.frame();
     }
     report("after settling");
 }
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn text_updates_cost() {
+    let rows = env("ROWS", 40_usize);
+    let frames = env("FRAMES", 1000_usize);
+    let phase = env("PHASE", String::from("edit"));
+    assert!(rows > 0 && frames > 0);
+    assert!(["idle", "repaint", "edit", "scroll"].contains(&phase.as_str()));
+    let mut h = Harness::new(OUTPUT);
+    let mut rng = Rng(1);
+    let mut col = Span::empty(Dir::DOWN);
+    let first = wtext(words(&mut rng, 12, 52))
+        .size(16)
+        .wrap(true)
+        .add(&mut h.rsc);
+    col.push(first.add_strong(&mut h.rsc));
+    for _ in 1..rows {
+        col.push(
+            wtext(words(&mut rng, 12, 52))
+                .size(16)
+                .wrap(true)
+                .add_strong(&mut h.rsc),
+        );
+    }
+    let root = col.scrollable().add(&mut h.rsc);
+    h.set_root(root);
+    #[cfg(feature = "layout-diagnostics")]
+    let _ = iris::core::layout_diagnostics::take();
+    let original = h.rsc[first].content.to_string();
+    let alternate = format!("{original} another word");
+    let start = Instant::now();
+    for frame in 0..frames {
+        match phase.as_str() {
+            "idle" => {}
+            "repaint" => {
+                let _ = h.rsc.widgets_mut().get_dyn_mut(first.id());
+            }
+            "edit" => {
+                h.rsc[first].content.clear();
+                h.rsc[first].content.push_str(if frame % 2 == 0 {
+                    &alternate
+                } else {
+                    &original
+                });
+            }
+            "scroll" => h.rsc[root].scroll(if frame % 2 == 0 { -12.0 } else { 12.0 }),
+            _ => unreachable!(),
+        }
+        h.frame();
+    }
+    println!(
+        "{phase}: {rows} rows, {frames} frames, {:.1} ms",
+        start.elapsed().as_secs_f64() * 1000.0
+    );
+    #[cfg(feature = "layout-diagnostics")]
+    print!(
+        "{}",
+        iris::core::layout_diagnostics::take().per_frame(frames)
+    );
+}

tests/scenario/mod.rs

@@ -13,7 +13,7 @@
 
 use iris::harness::Harness;
 use iris::prelude::*;
-use iris::random::{Aligns, Edits, Kind, Lens, Plan, Rng, SpanEdit, Tree, build};
+use iris::random::{Edits, Kind, 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
@@ -103,6 +103,11 @@ pub enum Case {
     /// 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
@@ -119,12 +124,13 @@ pub enum Case {
     Shuffle(Shuffle),
 }
 
-pub const ALL: [Case; 15] = [
+pub const ALL: [Case; 16] = [
     Case::Repaint,
     Case::RepaintSome,
     Case::Resize,
     Case::ResizeRepaint,
     Case::ResizeSize,
+    Case::SizeResize,
     Case::Size,
     Case::EverySize,
     Case::Align,
@@ -146,6 +152,7 @@ impl Case {
             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",
@@ -170,39 +177,61 @@ impl Case {
             _ => (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);
+        warm.rsc.widgets_mut().mark_for_redraw(id);
     }
 }
 
-fn a_len(rng: &mut Rng) -> Option<LayoutLen> {
-    Some(LayoutLen::px(20.0 + rng.below(180) as f32))
+/// A length in pixels, or a cap over one: a rule that reads the box it is
+/// given is the one a resize can change the effect of without changing the
+/// rule, so a tree that never grows one leaves that unexercised.
+fn a_rule(rng: &mut Rng) -> SizeRule {
+    let len = Len::px(20.0 + rng.below(180) as f32);
+    match rng.below(4) {
+        0 => SizeRule::Max(len),
+        1 => SizeRule::Min(len),
+        _ => LayoutLen::from(len).into(),
+    }
 }
 
-fn resize_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Lens {
-    let lens = [a_len(rng), a_len(rng)];
+fn resize_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> SizeRules {
+    let lens = SizeRules {
+        x: a_rule(rng),
+        y: a_rule(rng),
+    };
     warm.rsc
         .widgets_mut()
-        .set_size_rules(tree.sized[idx], lens[0], lens[1]);
+        .set_size_rules(tree.sized[idx], lens.x.clone(), lens.y.clone());
     lens
 }
 
-fn realign_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Aligns {
+fn realign_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Align {
     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 align = Align {
+        x: side(rng),
+        y: 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());
+    let taken = RegionAlign::from(align);
+    for axis in Axis::BOTH {
+        warm.rsc.widgets_mut().set_alignment(id, axis, taken[axis]);
     }
     align
 }
@@ -284,7 +313,7 @@ fn change(case: Case, warm: &mut Harness, tree: &mut Tree, plan: &Plan, rng: &mu
             warm.frame();
             return out;
         }
-        Case::Size | Case::ResizeSize => Edits {
+        Case::Size | Case::ResizeSize | Case::SizeResize => Edits {
             sizes: some_sizes(warm, tree, rng),
             ..Default::default()
         },
@@ -327,10 +356,18 @@ fn change(case: Case, warm: &mut Harness, tree: &mut Tree, plan: &Plan, rng: &mu
 /// 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 rules = h.rsc.widgets().size_rules(id).clone();
+    // A bound prints as itself: a failure is reproduced from what it printed,
+    // and a rule shown as "no rule" cannot be written out again.
+    let rule = |r: SizeRule| match r {
+        SizeRule::Free => "-".into(),
+        SizeRule::Exact(len) => format!("{len}"),
+        SizeRule::Request(request) => format!("{request:?}"),
+        SizeRule::Min(min) => format!(">{}", LayoutLen::from(min)),
+        SizeRule::Max(max) => format!("<{}", LayoutLen::from(max)),
+        SizeRule::Clamp { min, max } => {
+            format!(">{}<{}", LayoutLen::from(min), LayoutLen::from(max))
+        }
     };
     let align = h.rsc.widgets().alignment(id);
     let side = |a: AxisAlign| {
@@ -347,7 +384,7 @@ fn describe(id: WidgetId, h: &Harness) -> String {
     // 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) {
+    if rules != SizeRules::default() {
         out += &format!("[x:{},y:{}]", rule(rules.x), rule(rules.y));
     }
     if align != RegionAlign::default() {
@@ -384,6 +421,17 @@ fn describe_widget(id: WidgetId, h: &Harness) -> String {
     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.
@@ -400,6 +448,17 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
         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);
@@ -413,9 +472,16 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
         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];
@@ -424,11 +490,22 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
                 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  {}",
-            chain.join(" < ")
+            "widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}\n{}",
+            chain.join(" < "),
+            records.join("\n"),
         ));
     }
     match drawn {

tests/shrink.rs

@@ -70,10 +70,14 @@ fn no_grown_tree_lays_out_differently_warm_than_cold() {
     over_seeds(seeds, |seed| {
         let grown = plan(seed, depth, &Edits::default());
         for &case in &cases {
-            let Some(how) = diverges(&grown, case, seed) else {
+            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(),

tests/suite.rs

@@ -32,3 +32,6 @@ mod tasks;
 mod text_edit;
 #[path = "cases/unsettled.rs"]
 mod unsettled;
+
+#[path = "cases/deferred.rs"]
+mod deferred;

tests/trace_unsettled.rs

@@ -1,5 +1,5 @@
-//! 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.
+//! Traces the four-widget trees in `unsettled.rs`, to see what box their text
+//! is actually drawn in on a first frame against a settled one.
 
 #![cfg(feature = "layout-diagnostics")]
 
@@ -9,30 +9,25 @@ 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;
+    let wrapped = wtext("Wrapping shapes")
+        .size(16)
+        .wrap(true)
+        .width(76)
+        .add(&mut h.rsc);
     h.rsc
         .widgets_mut()
-        .set_alignment(sized, Axis::X, AxisAlign::POS);
+        .set_alignment(wrapped, Axis::X, AxisAlign::POS);
     h.rsc
         .widgets_mut()
-        .set_alignment(sized, Axis::Y, AxisAlign::POS);
+        .set_alignment(wrapped, Axis::Y, AxisAlign::POS);
     let stack = Stack {
-        children: vec![plain.add_strong(&mut h.rsc), aligned.add_strong(&mut h.rsc)],
+        children: vec![plain.add_strong(&mut h.rsc), wrapped.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(),
-    ]
+    vec![plain.id(), wrapped.id(), stack.id(), root.id()]
 }
 
 fn dump(label: &str, report: &diag::Report, text: WidgetId) {
@@ -42,12 +37,12 @@ fn dump(label: &str, report: &diag::Report, text: WidgetId) {
             TraceEvent::DrawRequest {
                 id,
                 region,
-                pixel_size,
+                region_px,
                 ..
             } if *id == text => {
                 println!(
                     "  draw in {:.2}x{:.2}  region {region:?}",
-                    pixel_size.x, pixel_size.y
+                    region_px.x, region_px.y
                 )
             }
             TraceEvent::SizeReported { id, size } if *id == text => {
@@ -81,7 +76,7 @@ fn what_box_the_text_is_drawn_in() {
 
     for _ in 0..2 {
         for &id in &ids {
-            h.rsc.widgets_mut().get_dyn_mut(id);
+            h.rsc.widgets_mut().mark_for_redraw(id);
         }
         let _ = diag::take();
         h.frame();
@@ -93,23 +88,14 @@ fn what_box_the_text_is_drawn_in() {
 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 inner = (text,).span(Dir::RIGHT).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);
+    let root = (filler, inner).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(),
-    ]
+    vec![text.id(), inner.id(), filler.id(), root.id()]
 }
 
 #[test]