Drop the refcount a widget handle can never raise

StrongWidget has no Clone on purpose, so the RefCounter in it never counted
anything: the count stayed at zero, RefCounter::drop always answered true,
and StrongWidget::refs had no caller -- while every widget made paid a heap
allocation for an Arc<AtomicU32> and every one created or dropped paid two
atomic read-modify-writes. The handle is the id, the sender and the type now,
and Drop sends. It is 32 bytes rather than 40, and 40 rather than 48 for a
dyn one.

RefCounter stays for TextureHandle, which does clone -- several widgets
showing one picture share its slot -- and is trimmed to what that needs:
quiet_clone and refs had no callers at all, and new was Default spelt out.

Bryan asked for this in #19 rather than in the review of the code written
before the gate, where the fourteenth sweep had left it.

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
 

core/src/fixed.rs

@@ -1,7 +1,7 @@
 use crate::{UiNum, util::Vec2};
 use std::{
     fmt::{Debug, Display, Formatter},
-    ops::{Add, AddAssign, Div, Mul, Neg, Sub, SubAssign},
+    ops::{Add, AddAssign, Mul, Neg, Sub, SubAssign},
 };
 
 /// A number held as a whole count of `1 / 2^SHIFT`.
@@ -115,7 +115,10 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
     /// that leaves out the thing it was measured from.
     pub const fn ceil_from_f32(v: f32) -> Self {
         let nearest = Self::from_f32(v);
-        match nearest.to_f32() < v {
+        // The top of the grid has no step above it, and stepping past it
+        // wraps to the bottom. A value out there is a caller's mistake, and
+        // the clamp `from_f32` already made is the answer to it.
+        match nearest.to_f32() < v && nearest.0 != Self::MAX.0 {
             true => nearest.next_up(),
             false => nearest,
         }
@@ -131,14 +134,6 @@ 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))
     }
@@ -180,21 +175,6 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
         Self(div_round(self.0 as i64, by as i64) as i32)
     }
 
-    /// Divided by a number on any grid. A zero divisor is a caller bug -- a
-    /// box of no length has no fraction of itself -- and answers with the end
-    /// of the range so that a release build lays out something absurd rather
-    /// than dying.
-    pub const fn div<const BY: u32>(self, by: Fixed<BY>) -> Self {
-        debug_assert!(by.0 != 0, "dividing by a length of zero");
-        if by.0 == 0 {
-            return match self.0 < 0 {
-                true => Self::MIN,
-                false => Self::MAX,
-            };
-        }
-        Self(div_round((self.0 as i64) << BY, by.0 as i64) as i32)
-    }
-
     /// `num / den` on *this* grid rather than on theirs, for weights coarser
     /// than the share they divide.
     pub const fn ratio<const OF: u32>(num: Fixed<OF>, den: Fixed<OF>) -> Self {
@@ -246,16 +226,6 @@ 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() {
@@ -335,14 +305,6 @@ const impl<const SHIFT: u32, const BY: u32> Mul<Fixed<BY>> for Fixed<SHIFT> {
     }
 }
 
-const impl<const SHIFT: u32, const BY: u32> Div<Fixed<BY>> for Fixed<SHIFT> {
-    type Output = Self;
-
-    fn div(self, rhs: Fixed<BY>) -> Self {
-        Fixed::div(self, rhs)
-    }
-}
-
 impl<const SHIFT: u32> Display for Fixed<SHIFT> {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         Display::fmt(&self.to_f32(), f)
@@ -493,19 +455,6 @@ mod tests {
         assert_eq!(Px::ONE.neg() * step_and_a_half, Px::from_raw(-2));
     }
 
-    /// A division rounds to the nearest step, so it cannot put back the
-    /// steps a truncating multiply dropped: a round trip comes back short,
-    /// never long, and by the few steps the two operations gave up.
-    #[test]
-    fn dividing_by_a_fraction_cannot_undo_a_truncating_multiply() {
-        let third = Rel::ONE / Rel::from_int(3);
-        let len = Px::from_int(300);
-        let back = len * third / third;
-        assert!(back <= len, "{back:?} is longer than {len:?}");
-        assert!(len - back <= Px::from_raw(3), "{back:?} against {len:?}");
-        assert_eq!(Px::from_int(100) / Rel::from_f32(0.5), Px::from_int(200));
-    }
-
     /// The bound a greedy line break needs: the width it was measured at is
     /// not on the grid, and the narrowest box the break still holds for is
     /// the step at or above it, never the one below.
@@ -529,15 +478,10 @@ mod tests {
     fn a_number_from_outside_is_clamped_to_the_grid() {
         assert_eq!(Px::from_f32(1e12), Px::MAX);
         assert_eq!(Px::from_f32(-1e12), Px::MIN);
-    }
+        // The ceiling is the other way in from a float, and there is no step
-
+        // above the top of the grid for it to take.
-    #[test]
+        assert_eq!(Px::ceil_from_f32(1e12), Px::MAX);
-    fn a_coarser_grid_rounds_and_a_finer_one_does_not() {
+        assert_eq!(Px::ceil_from_f32(-1e12), Px::MIN);
-        // 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]

core/src/layout_diagnostics.rs

@@ -72,6 +72,7 @@ labelled! {
         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",
     }
@@ -352,7 +353,7 @@ pub(crate) fn reuse(id: WidgetId, outcome: ReuseOutcome) {
 }
 
 /// A drawing that cannot be reused because the box on offer is outside what
-/// it holds for, and which of the three contracts said so. They overlap: a
+/// 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(
@@ -362,24 +363,35 @@ pub(crate) fn outside(
     rel_base: UiVec2,
     window: PxVec2,
 ) {
+    let mut reasons = 0;
+    let mut why = |counter| {
+        bump(counter);
+        reasons += 1;
+    };
     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);
+            why(Counter::OutsidePinnedLen);
         }
-        if !holds.window.contains(window)
+        if !holds.window.contains(window) {
-            || holds
+            why(Counter::OutsideWindow);
+        }
+        if holds
             .rel_base
             .is_some_and(|pinned| pinned != rel_base[axis])
         {
-            bump(Counter::OutsideRelBase);
+            why(Counter::OutsideRelBase);
         }
         if !holds.region.contains(len.to_px(window)) {
-            bump(Counter::OutsideRegion);
+            why(Counter::OutsideRegion);
         }
     }
+    // These four are `AxisHolds::contains`'s four clauses written out again,
+    // because the report wants which one refused rather than that one did. A
+    // clause added there and not here would leave a refusal unexplained.
+    debug_assert!(reasons > 0, "a reuse was refused for no reason counted");
     bump(Counter::ReuseOutside);
     reuse(id, ReuseOutcome::Outside);
 }

core/src/orientation/align.rs

@@ -3,7 +3,7 @@ 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>,
@@ -84,14 +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,
-    };
-}
-
 impl RegionAlign {
     pub const TOP_LEFT: Self = Self::new(AxisAlign::NEG, AxisAlign::NEG);
     pub const TOP_CENTER: Self = Self::new(AxisAlign::CENTER, AxisAlign::NEG);
@@ -159,13 +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))),
+            start: at - self.scale(rel),
-            end: Len::from_parts(at.rel.add(self.rel.mul(rest)), at.px.add(self.px.mul(rest))),
+            end: at + self.scale(Rel::ONE.sub(rel)),
         }
     }
 }
@@ -220,3 +214,4 @@ impl RegionAlign {
 }
 
 impl_axis_index!(RegionAlign => AxisAlign);
+impl_axis_index!(Align => Option<AxisAlign>);

core/src/orientation/axis.rs

@@ -11,15 +11,6 @@ pub enum Axis {
 impl Axis {
     /// Both of them, for the layout code that asks the same question of each.
     pub const BOTH: [Self; 2] = [Self::X, Self::Y];
-
-    /// A per-axis pair with `aligned` on this axis and `ortho` on the other,
-    /// which is what `from_axis` does for a vector.
-    pub fn pair<T>(self, aligned: T, ortho: T) -> [T; 2] {
-        match self {
-            Self::X => [aligned, ortho],
-            Self::Y => [ortho, aligned],
-        }
-    }
 }
 
 impl std::ops::Not for Axis {
@@ -80,50 +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

@@ -23,6 +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())
@@ -148,20 +156,20 @@ impl LayoutLen {
     /// 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 {
+    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 {
+    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> {
+    pub fn declared(&self) -> Option<Len> {
         (self.leftover == Weight::ZERO).then(|| self.without_leftover())
     }
 
@@ -169,7 +177,7 @@ impl LayoutLen {
     /// 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 {
+    pub const fn without_leftover(&self) -> Len {
         Len::from_parts(self.rel, self.px)
     }
 
@@ -239,6 +247,12 @@ impl std::fmt::Display for Size {
 
 impl std::fmt::Display for LayoutLen {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        // A part that is zero is left out, so a length that is zero all
+        // through would print as nothing -- which reads as no length at all
+        // wherever one is printed beside something that has none.
+        if *self == Self::ZERO {
+            return write!(f, "0 px;");
+        }
         if self.px != Px::ZERO {
             write!(f, "{} px;", self.px)?;
         }
@@ -253,3 +267,24 @@ impl std::fmt::Display for LayoutLen {
 }
 
 impl_axis_index!(Size => LayoutLen);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// What a request prints as is how a failing case is read and written out
+    /// again, and a length that printed as nothing could not be told from a
+    /// widget that has no rule at all.
+    #[test]
+    fn every_length_prints_as_something() {
+        for len in [
+            LayoutLen::ZERO,
+            LayoutLen::px(8),
+            LayoutLen::rel(0.5),
+            LayoutLen::LEFTOVER,
+        ] {
+            assert!(!len.to_string().is_empty(), "{len:?} printed as nothing");
+        }
+        assert_eq!(LayoutLen::ZERO.to_string(), "0 px;");
+    }
+}

core/src/orientation/pos.rs

@@ -281,15 +281,6 @@ impl UiSpan {
     pub const fn len(&self) -> Len {
         self.end - self.start
     }
-
-    /// Both ends by the same amount, which is what moving a box without
-    /// changing its length does to every part of it.
-    pub const fn translated(self, by: Len) -> Self {
-        Self {
-            start: self.start + by,
-            end: self.end + by,
-        }
-    }
 }
 
 #[repr(C)]
@@ -300,17 +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,

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
     }
@@ -198,41 +194,25 @@ impl TextBuffer {
     }
 
     pub fn shape(&mut self, data: &mut TextData, attrs: &TextAttrs, width: Option<f32>) {
-        let layout_key = LayoutKey {
+        // Asked of the attrs it was given rather than of a copy: copying one
-            attrs: attrs.clone(),
+        // allocates wherever its family is named, and the hit below is what
-            max_width: width,
+        // this cache is for.
-        };
+        let same_shaping = self
-        if self.layout_key.as_ref() == Some(&layout_key) {
+            .layout_key
-            #[cfg(feature = "layout-diagnostics")]
+            .as_ref()
-            diag::bump(Counter::TextShapeHits);
+            .is_some_and(|key| key.attrs == *attrs);
-            return;
+        if same_shaping
-        }
+            && let Some(key) = &self.layout_key
-        // A greedy break at one width is the same break at every width down
+            && self.breaks_the_same(key.max_width, width)
-        // to the longest line it produced: each line still fits, and none can
-        // take a word that would not fit in the wider box. So the layout in
-        // hand already answers, and re-breaking would only be work.
-        //
-        // At the longest line exactly, with no margin below it. A narrower
-        // width really does break differently, so answering one from the
-        // break in hand is how a warm tree keeps lines a cold tree would
-        // never produce. The margin was here because a text reports the
-        // width it used and a parent hands that back; the report is the step
-        // at or above its longest line now, so what comes back fits.
-        if let Some(key) = &self.layout_key
-            && key.attrs == *attrs
-            && let (Some(broke_at), Some(want)) = (key.max_width, width)
-            && want <= broke_at
-            && want >= self.layout.width()
         {
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::TextShapeHits);
             return;
         }
-        let same_shaping = self
+        let old_key = self.layout_key.replace(LayoutKey {
-            .layout_key
+            attrs: attrs.clone(),
-            .as_ref()
+            max_width: width,
-            .is_some_and(|key| key.attrs == *attrs);
+        });
-        let old_key = self.layout_key.replace(layout_key);
         // The glyphs it holds are of the width it held, which the layout may
         // well come back to.
         if let Some(key) = old_key
@@ -272,6 +252,28 @@ impl TextBuffer {
         self.break_lines(width);
     }
 
+    /// Whether the break in hand is the break `want` would make. The attrs
+    /// are the caller's to compare; this is about the width alone.
+    ///
+    /// A greedy break at one width is the same break at every width down to
+    /// the longest line it produced: each line still fits, and none can take a
+    /// word that would not fit in the wider box. So the layout in hand already
+    /// answers, and re-breaking would only be work.
+    ///
+    /// At the longest line exactly, with no margin below it. A narrower width
+    /// really does break differently, so answering one from the break in hand
+    /// is how a warm tree keeps lines a cold tree would never produce. The
+    /// margin was here because a text reports the width it used and a parent
+    /// hands that back; the report is the step at or above its longest line
+    /// now, so what comes back fits.
+    fn breaks_the_same(&self, broke_at: Option<f32>, want: Option<f32>) -> bool {
+        match (broke_at, want) {
+            (broke_at, want) if broke_at == want => true,
+            (Some(broke_at), Some(want)) => want <= broke_at && want >= self.layout.width(),
+            _ => false,
+        }
+    }
+
     fn break_lines(&mut self, width: Option<f32>) {
         self.layout.break_all_lines(width);
         self.layout

core/src/primitive/texture.rs

@@ -66,7 +66,7 @@ impl Textures {
         TextureHandle {
             slot: self.push(image),
             size,
-            counter: RefCounter::new(),
+            counter: RefCounter::default(),
             send: self.send.clone(),
         }
     }

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
+    // Every number both sides count in, written once here rather than a
-    // a second copy of them written into the shader: a grid the two disagree
+    // second time in the shader: a grid the two disagree about puts every
-    // about puts every coordinate somewhere else.
+    // 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,
     )
 }
 
@@ -276,7 +285,9 @@ impl UiRenderNode {
     }
 
     /// What every draw in the ui is given: the window, the masks and the
-    /// move chain every position is resolved through.
+    /// move chain every position is resolved through. The last two are the
+    /// vertex stage's alone -- a mask's rectangle is the same for every
+    /// fragment of one instance, so it is resolved once and handed on.
     fn shared_layout(device: &Device) -> BindGroupLayout {
         device.create_bind_group_layout(&BindGroupLayoutDescriptor {
             entries: &[
@@ -292,7 +303,7 @@ impl UiRenderNode {
                 },
                 BindGroupLayoutEntry {
                     binding: 1,
-                    visibility: ShaderStages::FRAGMENT,
+                    visibility: ShaderStages::VERTEX,
                     ty: BindingType::Buffer {
                         ty: BufferBindingType::Storage { read_only: true },
                         has_dynamic_offset: false,
@@ -302,7 +313,7 @@ impl UiRenderNode {
                 },
                 BindGroupLayoutEntry {
                     binding: 2,
-                    visibility: ShaderStages::VERTEX | ShaderStages::FRAGMENT,
+                    visibility: ShaderStages::VERTEX,
                     ty: BindingType::Buffer {
                         ty: BufferBindingType::Storage { read_only: true },
                         has_dynamic_offset: false,

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:
+// `PX_STEP`, `REL_STEP`, `MASK_NONE`, `MOVE_NONE` and `CHAIN_LIMIT` are
-// what it stores is a whole count of each, both powers of two, so decoding
+// prepended from `iris_core`'s own constants, so none of them is written
-// is exact and the number here is the number the CPU decided.
+// 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
@@ -130,9 +126,25 @@ struct VertexOutput {
     @location(2) uv: vec2<f32>,
     @location(3) @interpolate(flat) mask_idx: u32,
     @location(4) @interpolate(flat) idx: u32,
+    // The mask's rectangle in output pixels, resolved here because it is the
+    // same rectangle for every fragment of one instance and resolving it is a
+    // walk up a chain. Its own chain, not the drawn primitive's, so a
+    // stationary viewport clips content that moves inside it.
+    @location(5) @interpolate(flat) mask_top_left: vec2<f32>,
+    @location(6) @interpolate(flat) mask_bot_right: vec2<f32>,
     @builtin(position) clip_position: vec4<f32>,
 };
 
+// The pixel corners of a region, which is where every coordinate the CPU
+// decided becomes one.
+fn corners(r: Region) -> mat2x2<f32> {
+    let top_left = snap_floor(vec2(r.x.start.rel, r.y.start.rel) * window.dim
+        + vec2(r.x.start.px, r.y.start.px));
+    let bot_right = snap_floor(vec2(r.x.end.rel, r.y.end.rel) * window.dim
+        + vec2(r.x.end.px, r.y.end.px));
+    return mat2x2<f32>(top_left, bot_right);
+}
+
 @vertex
 fn vs_main(
     @builtin(vertex_index) vi: u32,
@@ -145,16 +157,17 @@ fn vs_main(
         UiSpan(scalar_of_pair(in.x_start), scalar_of_pair(in.x_end)),
         UiSpan(scalar_of_pair(in.y_start), scalar_of_pair(in.y_end)),
     );
-    let r = resolve_move(in.move_idx, local);
+    let own = corners(resolve_move(in.move_idx, local));
-    let top_left_rel = vec2(r.x.start.rel, r.y.start.rel);
+    let top_left = own[0];
-    let top_left_px = vec2(r.x.start.px, r.y.start.px);
+    let bot_right = own[1];
-    let bot_right_rel = vec2(r.x.end.rel, r.y.end.rel);
-    let bot_right_px = vec2(r.x.end.px, r.y.end.px);
-
-    let top_left = snap_floor(top_left_rel * window.dim + top_left_px);
-    let bot_right = snap_floor(bot_right_rel * window.dim + bot_right_px);
     let size = bot_right - top_left;
 
+    var mask = mat2x2<f32>(vec2<f32>(0.0), vec2<f32>(0.0));
+    if in.mask_idx != MASK_NONE {
+        let m = masks[in.mask_idx];
+        mask = corners(resolve_move(m.move_idx, Region(span_of(m.x), span_of(m.y))));
+    }
+
     let uv = vec2<f32>(
         f32(vi % 2u),
         f32(vi / 2u)
@@ -165,26 +178,19 @@ fn vs_main(
     out.top_left = top_left;
     out.bot_right = bot_right;
     out.mask_idx = in.mask_idx;
+    out.mask_top_left = mask[0];
+    out.mask_bot_right = mask[1];
     out.idx = ii;
 
     return out;
 }
 
 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];
+    let top_left = in.mask_top_left;
-    // Its own chain, not the drawn primitive's, so a stationary viewport
+    let bot_right = in.mask_bot_right;
-    // clips content that moves inside it.
-    let m = resolve_move(mask.move_idx, Region(span_of(mask.x), span_of(mask.y)));
-    let tl = vec2(m.x.start.rel, m.y.start.rel);
-    let tl_px = vec2(m.x.start.px, m.y.start.px);
-    let br = vec2(m.x.end.rel, m.y.end.rel);
-    let br_px = vec2(m.x.end.px, m.y.end.px);
-
-    let top_left = snap_floor(tl * window.dim + tl_px);
-    let bot_right = snap_floor(br * window.dim + br_px);
     let pos = in.clip_position.xy;
     if pos.x < top_left.x || pos.x > bot_right.x || pos.y < top_left.y || pos.y > bot_right.y {
         return color * 0.0;

core/src/ui/active.rs

@@ -1,8 +1,25 @@
 use crate::{
-    Declared, LayerId, LayoutHolds, MaskIdx, MoveIdx, PlaceDesc, RegionAlign, RetainedPrimitive,
+    Bounds, Declared, LayerId, LayoutHolds, MaskIdx, MoveIdx, PlaceDesc, RegionAlign,
-    Size, TextureHandle, UiRegion, UiVec2, WidgetId,
+    RetainedPrimitive, Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
+/// One draw of one widget, so that a widget it asked about can carry which
+/// draw that was. Its own type beside the indices here because it names an
+/// occasion rather than a slot: nothing is stored per draw.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct DrawId(u64);
+
+impl DrawId {
+    /// No draw at all, which is what a widget nothing has asked about carries.
+    pub const NONE: Self = Self(0);
+
+    /// The next one after this. Handed out in order and never reused, so a
+    /// note left by an earlier draw can never be read as this one's.
+    pub(crate) fn next(self) -> Self {
+        Self(self.0 + 1)
+    }
+}
+
 /// What is kept of a widget its parent has asked about. `drawn` says whether
 /// it currently draws; one that does not is kept so that a change to it, or
 /// under it, still reaches whoever asked.
@@ -28,6 +45,12 @@ pub struct ActiveData {
     /// The measured answer and its dependencies. A hint-only dependency or
     /// a widget first encountered during placement has no measurement yet.
     pub answer: Option<Answer>,
+    /// The draw that last asked about this widget, which is what says whether
+    /// the draw now running has already asked -- a question the child list can
+    /// only answer by a search, and so in the children a container has rather
+    /// than in one read. Written by whoever asked, so a draw of this widget
+    /// itself carries it across rather than setting it.
+    pub(crate) asked_by: DrawId,
     /// 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
@@ -51,6 +74,8 @@ pub struct ActiveData {
     /// An owned mask holds one reference independently of its primitives.
     pub mask_region: Option<UiRegion>,
     pub children: 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,
@@ -58,6 +83,10 @@ pub struct ActiveData {
     /// A change to one moves a box this widget cannot fix by drawing again,
     /// and comparing them is what says so.
     pub declared: Declared,
+    /// Its 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,
@@ -97,3 +126,12 @@ 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,
@@ -35,11 +92,11 @@ impl Holds {
 
     /// 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 {
+    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 {
+    pub const fn and(&self, other: Self) -> Self {
         Self {
             lo: self.lo.max(other.lo),
             hi: self.hi.min(other.hi),
@@ -57,7 +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

@@ -34,7 +34,7 @@ impl AxisHolds {
         region_len: None,
     };
 
-    pub fn and(self, other: Self) -> Self {
+    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!(
@@ -53,7 +53,7 @@ impl AxisHolds {
         }
     }
 
-    pub fn covers(self, other: Self) -> bool {
+    pub fn covers(&self, other: Self) -> bool {
         self.window.covers(other.window)
             && self.region.covers(other.region)
             && self
@@ -64,7 +64,7 @@ impl AxisHolds {
 
     /// 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 {
+    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))
@@ -87,18 +87,18 @@ impl LayoutHolds {
         y: AxisHolds::ANY,
     };
 
-    pub fn and(self, other: Self) -> Self {
+    pub fn and(&self, other: Self) -> Self {
         Self {
             x: self.x.and(other.x),
             y: self.y.and(other.y),
         }
     }
 
-    pub fn covers(self, other: Self) -> bool {
+    pub fn covers(&self, other: Self) -> bool {
         self.x.covers(other.x) && self.y.covers(other.y)
     }
 
-    pub fn contains(self, window: PxVec2, rel_base: UiVec2, region: UiRegion) -> bool {
+    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()))

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::{PlaceDesc, PlaceDescAxis, RetainedPrimitive};
+pub use place::{PlaceDesc, PlaceDescAxis, PlaceFit, RetainedPrimitive};
 pub use render_state::*;
 
 #[derive(Default)]
@@ -57,8 +57,11 @@ impl Moves {
         }
     }
 
+    /// Frees a slot. Not a change to the entries: the slot keeps the bytes it
+    /// had, nothing names it until it is handed out again, and whoever is
+    /// handed it writes it then -- so re-uploading the array here would send
+    /// the GPU what it already has.
     pub fn remove(&mut self, idx: MoveIdx) {
-        self.changed = true;
         self.arena.remove(Id::preset(idx.idx() as u32));
     }
 
@@ -103,11 +106,7 @@ impl Moves {
     /// the shader's walk costs per primitive.
     pub fn depth(&self, idx: MoveIdx) -> usize {
         let mut depth = 0;
-        let mut at = idx;
+        self.walk(idx, |_| depth += 1);
-        while at != MoveIdx::NONE && depth < CHAIN_LIMIT as usize {
-            at = self.arena[at.idx()].parent;
-            depth += 1;
-        }
         depth
     }
 

core/src/ui/painter.rs

@@ -1,9 +1,10 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Declared, Holds, LayoutHolds, LayoutLen, Len, PlaceDesc, Px, PxVec2, RegionAlign, Rel,
+    Axis, Bound, Bounds, Declared, DrawId, DrawScratch, Holds, LayoutHolds, LayoutLen, Len,
-    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
+    PlaceDesc, PlaceFit, Px, PxVec2, RegionAlign, Rel, RenderedText, RequestArena, RequestedLen,
-    TextureHandle, UiRegion, UiRenderState, UiRsc, UiVec2, Weight, WidgetId, Widgets,
+    RetainedPrimitive, Size, SizeRequests, StrongWidget, TextAttrs, TextBuffer, TextureHandle,
+    UiRegion, UiRenderState, UiRsc, UiVec2, Weight, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
         TexturePrimitive,
@@ -41,6 +42,8 @@ pub struct Painter<'a> {
     pub(super) children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
+    pub(super) request_deps: Vec<WidgetId>,
+    pub(super) scratch: DrawScratch,
     /// What this draw itself reads, as against what its children's drawings
     /// hold for: every window and every length of its own region until it
     /// reads one, then that one unless it says otherwise, and the rel base or
@@ -58,10 +61,137 @@ pub struct Painter<'a> {
     /// counted from however far `layer` has walked.
     pub(super) own_layer: usize,
     pub(super) depth: usize,
+    /// This draw's own id, which it leaves on every widget it asks about.
+    pub(super) draw: DrawId,
     pub(super) id: WidgetId,
 }
 
 impl<'a> Painter<'a> {
+    /// Reuses this widget's allocation buffers across draws. A child drawn
+    /// part way through gets a painter of its own, with buffers of its own,
+    /// so nothing it does while this one is mid-row can reach these.
+    pub fn with_requests<T>(
+        &mut self,
+        f: impl FnOnce(&mut Self, &mut Vec<RequestedLen>, &mut Vec<Px>) -> T,
+    ) -> T {
+        let mut requests = std::mem::take(&mut self.scratch.requests);
+        let mut lengths = std::mem::take(&mut self.scratch.lengths);
+        requests.clear();
+        lengths.clear();
+        let result = f(self, &mut requests, &mut lengths);
+        self.scratch.requests = requests;
+        self.scratch.lengths = lengths;
+        result
+    }
+
+    /// Discovers a composable request without painting a provisional box.
+    pub fn size_request<W: ?Sized>(
+        &mut self,
+        child: &StrongWidget<W>,
+        axis: Axis,
+    ) -> Option<RequestedLen> {
+        if let Some(len) = self.size_hint(child, axis) {
+            self.request_deps.push(child.id());
+            return Some(len.into());
+        }
+        let start = self.request_deps.len();
+        let mut requests = SizeRequests {
+            arena: &mut self.state.requests,
+            measured: None,
+            widgets: self.rsc.widgets(),
+            dependencies: &mut self.request_deps,
+            rel_base: self.rel_base[axis],
+        };
+        // Intrinsic fixed content must keep its offered box for wrapping;
+        // only a declaration or a share chooses the box it is drawn in.
+        let request = requests.widget(child, axis).filter(|request| {
+            request.has_leftover()
+                || self.rsc.widgets().size_rules(child.id())[axis]
+                    .request
+                    .is_some()
+        });
+        if request.is_some() {
+            self.rel_base(axis);
+        } else {
+            // A discarded request contributes no dependency: the measured
+            // draw below records the size and box it actually used instead.
+            self.request_deps.truncate(start);
+        }
+        request
+    }
+
+    /// Completes discovery after a child was measured. Only this call may use
+    /// drawn answers: before the ask they could belong to an obsolete box.
+    pub fn measured_request<W: ?Sized>(
+        &mut self,
+        child: &StrongWidget<W>,
+        axis: Axis,
+        len: LayoutLen,
+    ) -> RequestedLen {
+        let start = self.request_deps.len();
+        let bound = self.rsc.widgets().size_rules(child.id())[axis].bound;
+        let mut requests = SizeRequests {
+            arena: &mut self.state.requests,
+            measured: Some(&self.state.active),
+            widgets: self.rsc.widgets(),
+            dependencies: &mut self.request_deps,
+            rel_base: self.rel_base[axis],
+        };
+        if let Some(request) = requests.widget(child, axis)
+            && request.linear().is_none()
+            && request.has_leftover()
+        {
+            self.rel_base(axis);
+            return request;
+        }
+        let shares = len.leftover > Weight::ZERO;
+        let request = match shares {
+            true => requests.bounded(len.into(), bound),
+            false => len.into(),
+        };
+        self.request_deps.truncate(start);
+        // Only a bound that is a fraction was read against the rel base; one
+        // in pixels binds at the same length under any of them.
+        if shares && bound.has_fraction() {
+            self.rel_base(axis);
+        }
+        request
+    }
+
+    /// Divides `room` between `requests`, one length per request in
+    /// `output`. A deferred comparison is decided here, against this window:
+    /// which side of a crossing the solution falls is a question in pixels,
+    /// so the drawing holds only for the window that answered it.
+    pub fn allocate(
+        &mut self,
+        requests: &[RequestedLen],
+        room: Len,
+        axis: Axis,
+        output: &mut Vec<Px>,
+    ) {
+        let window = self.window[axis];
+        self.own[axis].window = self.own[axis].window.and(Holds::at(window));
+        output.clear();
+        output.extend(
+            self.state
+                .requests
+                .allocate(requests, room.to_px(window), window),
+        )
+    }
+
+    /// The least a request can come to, which is what it takes of the row
+    /// before anything is divided. A comparison is read at no share at all.
+    pub fn minimum_request(&mut self, request: &RequestedLen, axis: Axis) -> Len {
+        match request.linear() {
+            Some(len) => len.without_leftover(),
+            None => {
+                let window = self.window[axis];
+                self.own[axis].window = self.own[axis].window.and(Holds::at(window));
+                Len::from_parts(Rel::ZERO, self.state.requests.minimum(*request, window))
+            }
+        }
+    }
+
     fn primitive_at<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
         let kind = self.rsc.ui_mut().primitives.kind::<P>();
         self.write(kind, primitive, region);
@@ -187,19 +317,36 @@ impl<'a> Painter<'a> {
         id: &'s StrongWidget<W>,
         place: impl Into<PlaceDesc>,
     ) -> DrawResult<'s, 'a, W> {
-        let place = self.resolve_rel_base(place.into());
+        let offer = self.resolve_rel_base(place.into());
+        let Ask {
+            rel_base,
+            region,
+            place,
+            declared,
+            bounds,
+            holds: ask_holds,
+            inputs,
+        } = self.placing().ask(
+            self.rsc.widgets(),
+            &mut self.state.requests,
+            self.window,
+            id.id(),
+            offer,
+        );
+        self.own = self.own.and(inputs);
         let region_node = self.rsc.widgets().is_region_node(id.id());
-        let declared = self.declared_lens(id);
-        let align = self.rsc.widgets().alignment(id.id());
-        let (rel_base, region) =
-            place.rel_base_and_region(self.region, self.rel_base, declared, align);
         #[cfg(feature = "layout-diagnostics")]
         if region_node {
             diag::bump(Counter::RegionNodeDraws);
             diag::region_node(id.id(), self.id, region);
         }
         // A child listed twice would be moved twice.
-        let re_asked = self.children.contains(&id.id());
+        let re_asked = self.state.asked_in(id.id(), self.draw);
+        debug_assert_eq!(
+            re_asked,
+            self.children.contains(&id.id()),
+            "the note on a child disagrees with the list it says it is in",
+        );
         if !re_asked {
             self.children.push(id.id());
         }
@@ -215,17 +362,36 @@ impl<'a> Painter<'a> {
                 rel_base,
                 region,
                 placed: place,
-                asked: place,
+                asked: offer,
+                declared,
+                bounds,
+                ask_holds,
                 re_asked,
             },
             None,
             self.rsc,
         );
+        // Written now the draw has happened, since a widget drawn here has no
+        // record of its own until it has.
+        self.state
+            .active
+            .get_mut(&id.id())
+            .expect("a widget that was drawn has a record")
+            .asked_by = self.draw;
         let holds = self.in_parent(drawn.drawing_holds, region, place, declared);
         let answer_holds = self.in_parent(drawn.answer.holds, region, place, declared);
-        match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
+        // Added in step with the child list, so the one search here is the
-            Some((_, kept)) => *kept = holds,
+        // rare case of a child asked about twice.
-            None => self.under.push((id.id(), holds)),
+        match re_asked {
+            true => {
+                let kept = self
+                    .under
+                    .iter_mut()
+                    .find(|(child, _)| *child == id.id())
+                    .expect("a child asked about twice was added the first time");
+                kept.1 = holds;
+            }
+            false => self.under.push((id.id(), holds)),
         }
         DrawResult {
             child: id,
@@ -242,6 +408,11 @@ impl<'a> Painter<'a> {
         self.children.retain(|child| *child != id.id());
         self.under.retain(|(child, _)| *child != id.id());
         self.state.undraw_rec(id.id(), self.rsc);
+        // Taken out of the child list, so the note saying it is in there goes
+        // with it: placing it again is asking again.
+        if let Some(active) = self.state.active.get_mut(&id.id()) {
+            active.asked_by = DrawId::NONE;
+        }
     }
 
     /// Puts a child in `place` of this widget's box, where that box is the
@@ -263,7 +434,7 @@ impl<'a> Painter<'a> {
         let states_rel_base = Axis::BOTH
             .iter()
             .any(|&axis| matches!(place[axis].rel_base, RelBase::Len(_)));
-        if states_rel_base || !self.children.contains(&id.id()) {
+        if states_rel_base || !self.state.asked_in(id.id(), self.draw) {
             return self.widget_at(id, place);
         }
         let at = self.placing();
@@ -282,7 +453,7 @@ impl<'a> Painter<'a> {
     /// This widget as the thing its children are placed within.
     fn placing(&self) -> Placing {
         Placing {
-            id: self.id,
+            id: Some(self.id),
             region: self.region,
             rel_base: self.rel_base,
             depth: self.depth,
@@ -291,27 +462,19 @@ impl<'a> Painter<'a> {
         }
     }
 
-    /// What a widget's rules declare its lengths to be, which whoever draws
-    /// it resolves into its rel base. Reading them depends on nothing -- the box
-    /// that comes of them is kept on the child, and `redraw` compares it
-    /// there.
-    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> Declared {
-        self.rsc.widgets().declared_lens(id.id())
-    }
-
     /// What a child says its length is without being drawn, if it can say,
     /// as the length its draw would report: a fraction in it is resolved
     /// against this widget's rel base, which is the rel base a child asked with
     /// nothing narrowed gets. Asking counts as reading its size.
     pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
-        let widgets = self.rsc.widgets();
+        // A bound is composed into a request rather than applied to a hint,
-        // A rule is the answer where there is one: it wins over whatever the
+        // so a bounded widget cannot say its length without being asked: what
-        // widget would draw, so it has to win over what the widget says too.
+        // it comes to is a comparison only the ask or the allocator makes.
-        let hint = widgets.size_rules(id.id())[axis].exact().or_else(|| {
+        // A miss rather than no read at all, so the counters see it.
-            widgets
+        let bounded = self.rsc.widgets().size_rules(id.id())[axis].bound != Bound::ANY;
-                .get_dyn(id.id())
+        let hint = (!bounded)
-                .and_then(|widget| widget.size_hint(axis))
+            .then(|| self.rsc.widgets().exact_len(id.id(), axis))
-        });
+            .flatten();
         let rel_base = self.rel_base[axis];
         let resolved = hint.map(|hint| hint.within_len(rel_base));
         #[cfg(feature = "layout-diagnostics")]
@@ -335,11 +498,13 @@ impl<'a> Painter<'a> {
         resolved
     }
 
+    /// Records that this draw read a child's length. Listed once per read
+    /// rather than once per child: a child that answered with a hint may have
+    /// no record to note it on until this draw ends, and what the list drives
+    /// asks the same of a widget twice as of it once.
     fn depend_on<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
-        if !self.size_deps.contains(&child.id()) {
         self.size_deps.push(child.id());
     }
-    }
 
     pub fn render_text<'b>(
         &mut self,
@@ -429,7 +594,7 @@ impl<'a> Painter<'a> {
     /// depend on it.
     pub fn has_exact_size(&self, axis: Axis) -> bool {
         self.rsc.widgets().size_rules(self.id)[axis]
-            .exact()
+            .request
             .is_some()
     }
 
@@ -482,6 +647,26 @@ impl<'a> Painter<'a> {
         len.to_px(window)
     }
 
+    /// [`Len::longer_than`], asked on this widget's behalf: the windows the
+    /// comparison comes out the same way on are windows its drawing holds
+    /// for, and nowhere else does it. What a container has left for the
+    /// shares it divides is the one thing that asks.
+    ///
+    /// Narrowed rather than stated, because whatever else this widget read
+    /// about the window is a reason its drawing holds where it does too.
+    pub fn longer_than(&mut self, len: Len, than: Len, axis: Axis) -> bool {
+        let window = self.window[axis];
+        let (longer, holds) = len.longer_than(than, window);
+        debug_assert!(
+            holds.contains(window),
+            "'{}' ({:?}) compared two lengths and kept a range without this window",
+            self.label(),
+            self.id
+        );
+        self.own[axis].window = self.own[axis].window.and(holds);
+        longer
+    }
+
     /// The windows this drawing holds for, stated rather than taken: a
     /// container that branched on a length in pixels says which side of the
     /// boundary it was on, which is wider than the one window reading that
@@ -497,10 +682,6 @@ impl<'a> Painter<'a> {
         self.own[axis].window = holds;
     }
 
-    pub fn text_data(&mut self) -> &mut TextData {
-        &mut self.rsc.ui_mut().text
-    }
-
     pub fn child_layer(&mut self) {
         self.layer = self.state.layers.child(self.layer);
     }
@@ -651,36 +832,211 @@ impl Painter<'_> {
 }
 
 impl Widgets {
-    /// What a widget's box is where a rule or its own hint says so outright.
+    /// What says a widget's length on one axis without drawing it, if anything
-    pub(super) fn declared_lens(&self, id: WidgetId) -> Declared {
+    /// does. A rule is the answer where there is one: it wins over whatever the
-        let rules = self.size_rules(id);
+    /// widget would draw, so it has to win over what the widget says too -- a
-        let widget = self.get_dyn(id);
+    /// share included, since a share is a length only to whoever divides one,
-        Declared::from_axes(|axis| {
+    /// and that is the parent rather than this widget.
-            rules[axis].declared().or_else(|| {
+    fn exact_len(&self, id: WidgetId, axis: Axis) -> Option<LayoutLen> {
-                // A hint still narrows the box where no rule does, which is
+        // A request is a length the rule gives, and the hint below must not
-                // how a widget with a natural pixel size -- an image, a gap
+        // narrow the box in its place: what the request comes to is not known
-                // -- gets that size rather than the whole offer. That is the
+        // until the parent allocates, and it is the parent's answer, not this
-                // offer's business rather than a declaration's, and this
+        // widget's.
-                // falls away once a widget occupies its reported size inside
+        let rule = &self.size_rules(id)[axis];
-                // the box it was offered.
+        if rule.deferred().is_some() {
-                widget
+            return None;
-                    .and_then(|widget| widget.size_hint(axis))
+        }
-                    .and_then(LayoutLen::declared)
+        rule.exact().or_else(|| {
-            })
+            // A hint still narrows the box where no rule does, which is how a
+            // widget with a natural pixel size -- an image, a gap -- gets that
+            // size rather than the whole offer. That is the offer's business
+            // rather than a declaration's, and this falls away once a widget
+            // occupies its reported size inside the box it was offered.
+            self.get_dyn(id)?.size_hint(axis)
         })
     }
 }
 
+/// One ask of a widget: the box it draws in, what its fractions are of, and
+/// what deciding those read.
+pub(super) struct Ask {
+    pub rel_base: UiVec2,
+    pub region: UiRegion,
+    /// The place the ask came to, which a rule of the widget's own can take
+    /// past the box its parent offered.
+    pub place: PlaceDesc,
+    /// What the widget's box is on each axis where something says so
+    /// outright: its rule, its hint, or a bound the offer fell outside.
+    /// An intrinsic answer can still occupy less than a capped box.
+    pub declared: Declared,
+    /// Its bounds, resolved against the rel base its rules were resolved
+    /// against, for the answer to be held to where the box was not.
+    pub bounds: Bounds,
+    /// What the ask itself holds for, kept on the widget asked about: a rule
+    /// compared against the offer in pixels holds only for the windows on its
+    /// side of the crossing, and that range reaches whoever asked through the
+    /// drawing it is part of. Kept on the widget asked about rather than on
+    /// the asker because the root has no asker.
+    pub holds: LayoutHolds,
+    /// Inputs read against the parent before declarations choose a new base.
+    /// These belong to the asker; the widget's own holds describe its output box.
+    pub inputs: LayoutHolds,
+}
+
+impl Placing {
+    /// Asks about a widget at `place` of this box, with the widget's own
+    /// rules applied to what the place offers it. `place` is resolved: what
+    /// a rel base of the asker's is a fraction of, the asker worked out.
+    ///
+    /// Every ask is this one, the root's included -- there the box is the
+    /// window and nothing above narrowed it, which is what [`Self::WINDOW`]
+    /// says.
+    pub(super) fn ask(
+        &self,
+        widgets: &Widgets,
+        requests: &mut RequestArena,
+        window: PxVec2,
+        id: WidgetId,
+        mut place: PlaceDesc,
+    ) -> Ask {
+        let align = widgets.alignment(id);
+        let rules = widgets.size_rules(id);
+        let mut holds = LayoutHolds::ANY;
+        let mut inputs = LayoutHolds::ANY;
+        let mut declared = Declared::NONE;
+        let mut bounds = Bounds::ANY;
+        for axis in Axis::BOTH {
+            let base = place.base(axis, self.rel_base);
+            let stated = widgets.exact_len(id, axis);
+            if let Some(len) = stated.and_then(|len| len.declared()) {
+                if len.rel != Rel::ZERO {
+                    inputs[axis].rel_base = Some(self.rel_base[axis]);
+                }
+                declared[axis] = Some(len.within_len(base));
+            }
+            if let Some(request) = rules[axis].deferred() {
+                inputs[axis].rel_base = Some(self.rel_base[axis]);
+                inputs[axis].region_len = Some(self.region[axis].len());
+                inputs[axis].window = Holds::at(window[axis]);
+                let offer = place.of(self.region, align)[axis].len();
+                let px = if place[axis].fit == PlaceFit::Allocated {
+                    offer.to_px(window[axis])
+                } else {
+                    let request = requests.import(request, base);
+                    let request = requests.bounded(request, rules[axis].bound.within_len(base));
+                    holds[axis].window = Holds::at(window[axis]);
+                    requests
+                        .allocate(&[request], offer.to_px(window[axis]), window[axis])
+                        .next()
+                        .unwrap()
+                };
+                let len = Len::from_parts(Rel::ZERO, px);
+                place[axis].rel_base = RelBase::Len(len);
+                declared[axis] = Some(len);
+                holds[axis].rel_base = Some(len);
+                continue;
+            }
+            // A share fills what the pixels and fraction beside it leave of
+            // the box and overflows where they are longer, which is the rule
+            // a span follows with one child. Only the overflow is a box of
+            // the widget's own: a share that fits is the box it was given,
+            // which is what this place already says.
+            let (share, kept) = self.share_past_the_offer(stated, window[axis], place, align, axis);
+            holds[axis].window = holds[axis].window.and(kept);
+            if let Some(len) = share {
+                place[axis] = len.as_desc().fills();
+            }
+            let bound = rules[axis].bound;
+            if bound != Bound::ANY {
+                if bound.has_fraction() {
+                    inputs[axis].rel_base = Some(self.rel_base[axis]);
+                }
+                // A solved slot may narrow the widget's rel base, but the
+                // allocator evaluated its bounds against this parent's base.
+                let bound_base = if place[axis].fit == PlaceFit::Allocated {
+                    self.rel_base[axis]
+                } else {
+                    base
+                };
+                bounds[axis] = bound.within_len(bound_base);
+                let offer =
+                    declared[axis].unwrap_or_else(|| place.of(self.region, align)[axis].len());
+                let (held, kept) = bounds[axis].outside(offer, window[axis]);
+                holds[axis].window = holds[axis].window.and(kept);
+                // The comparison reads the incoming box, before a bound
+                // replaces it. Placement keeps that decision; only an ask
+                // may compare a new offer.
+                if declared[axis].is_none() {
+                    inputs[axis].region_len = Some(self.region[axis].len());
+                }
+                if let Some(len) = held {
+                    declared[axis] = Some(len);
+                }
+            }
+        }
+        let (rel_base, region) =
+            place.rel_base_and_region(self.region, self.rel_base, declared, align);
+        Ask {
+            rel_base,
+            region,
+            place,
+            declared,
+            bounds,
+            holds,
+            inputs,
+        }
+    }
+
+    /// The box a widget's own share asks for where that is longer than the
+    /// box `place` gives it, and nothing where the share fits -- with the
+    /// windows that answer holds for, which is a range either way.
+    ///
+    /// A share is a length only to whoever divides one, and nothing divides a
+    /// box handed to one child: what is left of it after the pixels and the
+    /// fraction beside the share is what the share takes, so the length comes
+    /// to the whole box until those are longer than it and to them once they
+    /// are. Only that second case is a box its parent did not give, and the
+    /// crossing between them is a question in pixels.
+    fn share_past_the_offer(
+        &self,
+        stated: Option<LayoutLen>,
+        window: Px,
+        place: PlaceDesc,
+        align: RegionAlign,
+        axis: Axis,
+    ) -> (Option<Len>, Holds) {
+        // A place that is the widget's placement outright is a box its parent
+        // decided, and a parent that divides one has already given the share
+        // whatever it was owed. Only an offer -- a box with the answer still
+        // to be placed inside it -- is a box a share reads.
+        if place[axis].fit.fills() {
+            return (None, Holds::ANY);
+        }
+        // A share with nothing beside it is the box whatever the box is, so
+        // there is no comparison to make and no range to keep for one.
+        let Some(stated) = stated else {
+            return (None, Holds::ANY);
+        };
+        if stated.leftover == Weight::ZERO || stated.is_only_leftover() {
+            return (None, Holds::ANY);
+        }
+        let fixed = stated
+            .without_leftover()
+            .within_len(place.base(axis, self.rel_base));
+        let offer = place.of(self.region, align)[axis].len();
+        let (longer, holds) = fixed.longer_than(offer, window);
+        (longer.then_some(fixed), holds)
+    }
+}
+
 impl LayoutLen {
     /// Whether what a widget reported along an axis is the whole of the box
     /// it is in rather than a part to be placed inside it. A share fills,
     /// because a share is a length only to whoever divides one, and whoever
-    /// did is the one that handed down this box. A declared axis does too:
+    /// did is the one that handed down this box. An axis the parent decided
-    /// the rule already gave the region its length, and the rule's length is
+    /// from the answer is the answer already.
-    /// what the widget reports there. And an axis the parent decided from
+    pub(super) fn fills(&self, decided: bool) -> bool {
-    /// the answer is the answer already.
+        self.leftover != Weight::ZERO || decided
-    pub(super) fn fills(self, declared: Option<Len>, decided: bool) -> bool {
-        self.leftover != Weight::ZERO || declared.is_some() || decided
     }
 }
 
@@ -694,17 +1050,11 @@ impl PlaceDesc {
     /// part. That is what makes a fraction the same fraction wherever the part
     /// it is placed in sits and however long it is -- the fraction is resolved
     /// once, here, against the rel base it was reported of.
-    pub(super) fn placement(
+    pub(super) fn placement(self, region: UiRegion, size: Size, align: RegionAlign) -> UiRegion {
-        self,
-        region: UiRegion,
-        size: Size,
-        declared: Declared,
-        align: RegionAlign,
-    ) -> UiRegion {
         let mut placed = region;
         for axis in Axis::BOTH {
             let reported = size[axis];
-            if reported.fills(declared[axis], self[axis].fills) {
+            if reported.fills(self[axis].fit.fills()) {
                 continue;
             }
             placed[axis] = placed[axis].place(reported.without_leftover(), align[axis]);
@@ -717,11 +1067,8 @@ impl PlaceDesc {
     ///
     /// `own` is that widget's own box, and `place` what of it the child is
     /// given, including any rel base it states -- a row's slot, or padding's rel
-    /// base less its pixels. That is a window length, like every other length
+    /// base less its pixels. The ask has already resolved declarations into
-    /// here, since a slot of a row is not a fraction of anything the row can
+    /// window lengths, so placement only aligns them inside the given box.
-    /// name. The child's declaration is a fraction of whichever reached it, and
-    /// is the only one that also places the box: a box the caller decided is
-    /// what `place` names.
     pub(super) fn rel_base_and_region(
         self,
         own: UiRegion,
@@ -733,13 +1080,8 @@ impl PlaceDesc {
         let mut rel_base = parent_rel_base;
         let mut region = given;
         for axis in Axis::BOTH {
-            let base = match self[axis].rel_base {
+            let base = self.base(axis, parent_rel_base);
-                RelBase::Len(len) => len,
+            let len = declared[axis].unwrap_or(base);
-                RelBase::Inherit | RelBase::WithRegion => parent_rel_base[axis],
-            };
-            let len = declared[axis]
-                .map(|len| len.within_len(base))
-                .unwrap_or(base);
             rel_base[axis] = len;
             if declared[axis].is_some() {
                 region[axis] = given[axis].place(len, align[axis]);

core/src/ui/place.rs

@@ -1,5 +1,5 @@
 use crate::util::impl_axis_index;
-use crate::{Axis, AxisAlign, Len, PrimitiveHandle, RegionAlign, UiRegion, UiSpan};
+use crate::{Axis, AxisAlign, Len, PrimitiveHandle, RegionAlign, UiRegion, UiSpan, UiVec2};
 
 /// How a child's region along one axis comes from the region of the widget
 /// asking, and what its fractions are of.
@@ -12,10 +12,24 @@ use crate::{Axis, AxisAlign, Len, PrimitiveHandle, RegionAlign, UiRegion, UiSpan
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct PlaceDescAxis {
     pub span: PlaceSpan,
-    pub fills: bool,
+    pub fit: PlaceFit,
     pub rel_base: RelBase,
 }
 
+#[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),
@@ -44,7 +58,13 @@ impl PlaceDescAxis {
     /// it again. A container uses it where it hands back exactly what the
     /// child asked for -- a row placing a child at the length it reported.
     pub const fn fills(mut self) -> Self {
-        self.fills = true;
+        self.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
     }
 
@@ -123,6 +143,16 @@ impl PlaceDesc {
         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))
@@ -143,7 +173,7 @@ impl UiSpan {
     pub const fn within_desc(self) -> PlaceDescAxis {
         PlaceDescAxis {
             span: PlaceSpan::Within(self),
-            fills: false,
+            fit: PlaceFit::Align,
             rel_base: RelBase::WithRegion,
         }
     }
@@ -161,7 +191,7 @@ impl UiSpan {
     pub const fn shifted_desc(self) -> PlaceDescAxis {
         PlaceDescAxis {
             span: PlaceSpan::Shifted(self),
-            fills: false,
+            fit: PlaceFit::Align,
             rel_base: RelBase::Inherit,
         }
     }
@@ -175,7 +205,7 @@ impl Len {
     pub const fn as_desc(self) -> PlaceDescAxis {
         PlaceDescAxis {
             span: PlaceSpan::Sized(self),
-            fills: false,
+            fit: PlaceFit::Align,
             rel_base: RelBase::Len(self),
         }
     }

core/src/ui/render_state.rs

@@ -1,9 +1,10 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::{
-    ActiveData, Answer, Axis, Declared, DrawLayers, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx,
+    ActiveData, Answer, Axis, Bounds, Declared, DrawId, DrawLayers, IdLike, LayoutHolds, LayoutLen,
-    MoveIdx, Moves, Painter, PixelRegion, PlaceDesc, PxVec2, Rel, Size, StrongWidget, UiRegion,
+    Len, MaskIdx, MoveIdx, Moves, Painter, PixelRegion, PlaceDesc, PxVec2, Size, StrongWidget,
-    UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    UiRegion, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    ui::painter::Ask,
     util::{HashMap, Vec2},
 };
 
@@ -23,11 +24,18 @@ pub(super) struct DrawInfo {
     /// The box the widget is asked in, in its parent region node's
     /// coordinates.
     pub region: UiRegion,
-    /// Where the widget is put, and where it was asked, as parts of the
+    /// Where the widget is put, and what its parent offered it, as parts of
-    /// parent's box. See [`PlaceDesc`]. The two are one ask's place until the
+    /// the parent's box. See [`PlaceDesc`]. The two are one place until a
-    /// parent puts the answer somewhere else.
+    /// rule of the widget's own takes it past the offer, or the parent puts
+    /// the answer somewhere else.
     pub placed: PlaceDesc,
     pub asked: PlaceDesc,
+    /// What the ask made of the widget's own rules. See [`Ask::declared`]
+    /// and [`Ask::bounds`].
+    pub declared: Declared,
+    pub bounds: Bounds,
+    /// What the ask that gave it those two holds for. See [`Ask::holds`].
+    pub ask_holds: LayoutHolds,
     /// Whether the parent already asked about this widget in this draw.
     pub re_asked: bool,
 }
@@ -43,7 +51,8 @@ pub(super) struct Drawn {
 /// What a widget's children are placed in: its own box, the coordinates its
 /// drawing is in, and what else one ask of a child is decided from.
 pub(super) struct Placing {
-    pub id: WidgetId,
+    /// The widget whose box this is, and nothing for the window.
+    pub id: Option<WidgetId>,
     pub region: UiRegion,
     pub rel_base: UiVec2,
     pub depth: usize,
@@ -51,6 +60,21 @@ pub(super) struct Placing {
     pub mask: MaskIdx,
 }
 
+impl Placing {
+    /// The window, which is what the root is placed within. Nothing above the
+    /// root narrowed a box or chose where it goes, so it is asked in the whole
+    /// output and its fractions are of the whole output -- an ordinary ask,
+    /// from the one box nobody drew.
+    pub const WINDOW: Self = Self {
+        id: None,
+        region: UiRegion::FULL,
+        rel_base: UiVec2::FULL_SIZE,
+        depth: 0,
+        move_idx: MoveIdx::NONE,
+        mask: MaskIdx::NONE,
+    };
+}
+
 pub struct UiRenderState {
     pub active: HashMap<WidgetId, ActiveData>,
     pub layers: DrawLayers,
@@ -70,7 +94,14 @@ pub struct UiRenderState {
     deferred: crate::util::HashSet<WidgetId>,
     /// What the walk has left to settle, deepest last. Ordered rather than
     /// searched for, so finding the next one is not a pass over the marks.
-    pending: std::collections::BTreeSet<(usize, WidgetId)>,
+    pending: std::collections::BinaryHeap<(usize, WidgetId)>,
+    pub(super) requests: crate::RequestArena,
+    changed: Vec<WidgetId>,
+    /// The last draw id handed out. Each draw takes a fresh one and leaves it
+    /// on every widget it asks about, which is how it knows in one read
+    /// whether it has asked already.
+    last_draw: DrawId,
+    request_readers: HashMap<WidgetId, crate::util::HashSet<WidgetId>>,
     pub moves: Moves,
 }
 
@@ -84,6 +115,10 @@ impl UiRenderState {
             slots: Default::default(),
             deferred: Default::default(),
             pending: Default::default(),
+            requests: Default::default(),
+            changed: Vec::new(),
+            last_draw: DrawId::NONE,
+            request_readers: Default::default(),
             moves: Default::default(),
             resized: false,
         }
@@ -124,20 +159,23 @@ impl UiRenderState {
         }
     }
 
-    /// The root is asked about in the output. Its own rules narrow both its
+    /// The root's first draw: the ask [`Placing::WINDOW`] answered, with the
-    /// rel base and box; nothing above it chose a different one.
+    /// bookkeeping a widget with no parent carries.
-    fn root_info(&self, rel_base: UiVec2, region: UiRegion) -> DrawInfo {
+    fn root_info(&self, ask: &Ask, region_node: bool) -> DrawInfo {
         DrawInfo {
             layer: 0,
             parent: None,
-            depth: 1,
+            depth: Placing::WINDOW.depth + 1,
             parent_move: MoveIdx::NONE,
-            region_node: false,
+            region_node,
             mask: MaskIdx::NONE,
-            rel_base,
+            rel_base: ask.rel_base,
-            region,
+            region: ask.region,
-            placed: PlaceDesc::WHOLE,
+            placed: ask.place,
             asked: PlaceDesc::WHOLE,
+            declared: ask.declared,
+            bounds: ask.bounds,
+            ask_holds: ask.holds,
             re_asked: false,
         }
     }
@@ -167,6 +205,7 @@ impl UiRenderState {
                 weak widgets: {all:#?}"
             );
         }
+        self.requests.reset();
         let root = root.into();
         if self.root_changed(root) {
             self.redraw_all(root, rsc);
@@ -184,24 +223,18 @@ impl UiRenderState {
         let _layout = diag::timer(TimerKind::FullLayout);
         self.clear(rsc);
         if let Some(id) = root {
-            let (rel_base, region) = Self::root_layout(id.id(), rsc.widgets());
+            let ask = Placing::WINDOW.ask(
-            let info = self.root_info(rel_base, region);
+                rsc.widgets(),
+                &mut self.requests,
+                self.output_size,
+                id.id(),
+                PlaceDesc::WHOLE,
+            );
+            let info = self.root_info(&ask, rsc.widgets().is_region_node(id.id()));
             self.draw_inner(id.id(), info, None, rsc);
         }
     }
 
-    /// The root's rel base and box: the window, taken in by the root's own
-    /// rules. Nothing above it narrowed anything or chose where it goes, so
-    /// its declaration is the whole of what decides either.
-    fn root_layout(id: WidgetId, widgets: &Widgets) -> (UiVec2, UiRegion) {
-        PlaceDesc::WHOLE.rel_base_and_region(
-            UiRegion::FULL,
-            UiVec2::FULL_SIZE,
-            widgets.declared_lens(id),
-            widgets.alignment(id),
-        )
-    }
-
     pub(super) fn draw_inner(
         &mut self,
         id: WidgetId,
@@ -226,7 +259,6 @@ impl UiRenderState {
             );
         }
         let align = rsc.widgets().alignment(id);
-        let declared = rsc.widgets().declared_lens(id);
         // Nothing this widget measured can be dirty while it draws: layout is
         // one bottom-up walk, so anything deeper has settled or deferred to
         // its own parent, and a deferred one leaves that parent marked.
@@ -240,7 +272,7 @@ impl UiRenderState {
             .then(|| self.retained_answer(id, region, info))
             .flatten()
             .and_then(|answer| {
-                let placed = info.placed.placement(region, answer.size, declared, align);
+                let placed = info.placed.placement(region, answer.size, align);
                 self.try_reuse(id, region, placed, info, rsc)
                     .then_some(answer)
             });
@@ -252,7 +284,7 @@ impl UiRenderState {
             // Where the drawing goes: the part its parent gave it, with the
             // answer placed inside that part on any axis the parent left
             // open.
-            let placed = info.placed.placement(region, answer.size, declared, align);
+            let placed = info.placed.placement(region, answer.size, align);
             if placed != region {
                 self.relocate(id, placed, info, rsc);
             }
@@ -295,6 +327,14 @@ impl UiRenderState {
         old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
     ) -> Answer {
+        let draw = self.next_draw();
+        // Whoever asked about this widget wrote this, and a draw of the widget
+        // itself is not that: the record is rebuilt below, so it is carried
+        // across rather than reset.
+        let asked_by = old
+            .as_ref()
+            .or_else(|| self.active.get(&id))
+            .map_or(DrawId::NONE, |active| active.asked_by);
         let rel_base = info.rel_base;
         let (move_idx, region, retired_move) = match info.region_node {
             // A node entry is only a translation. Its local box keeps the
@@ -311,7 +351,27 @@ impl UiRenderState {
         let mask_slot = old
             .as_ref()
             .and_then(|old| old.mask_region.map(|_| old.mask));
-        let old_children = old.map_or_else(Vec::new, |old| old.children);
+        let (mut old_children, textures, primitives, request_deps, mut scratch) = match old {
+            Some(old) => (
+                old.children,
+                old.textures,
+                old.primitives,
+                old.request_deps,
+                old.scratch,
+            ),
+            None => Default::default(),
+        };
+        // Every one of these is a buffer this widget's last draw filled and
+        // `remove` emptied, kept for its capacity alone. A drawing whose
+        // primitives were still in it would record them twice.
+        debug_assert!(
+            textures.is_empty() && primitives.is_empty() && request_deps.is_empty(),
+            "'{}' ({id:?}) was drawn again over what its last draw left",
+            rsc.widgets().label(id)
+        );
+        let children = std::mem::take(&mut scratch.children);
+        let size_deps = std::mem::take(&mut scratch.size_deps);
+        let under = std::mem::take(&mut scratch.under);
         rsc.widgets_mut().needs_redraw.remove(&id);
         let window = self.output_size;
         let mut painter = Painter {
@@ -323,17 +383,20 @@ impl UiRenderState {
             layer: info.layer,
             own_layer: info.layer,
             id,
-            textures: Vec::new(),
+            textures,
-            primitives: Vec::new(),
+            primitives,
             mask_region: None,
             mask_slot,
-            children: Vec::new(),
+            children,
-            size_deps: Vec::new(),
+            size_deps,
+            request_deps,
+            scratch,
             own: LayoutHolds::ANY,
-            under: Vec::new(),
+            under,
             answer_under: LayoutHolds::ANY,
             depth: info.depth,
             move_idx,
+            draw,
             rsc,
         };
 
@@ -362,12 +425,15 @@ impl UiRenderState {
             own,
             answer_under,
             children,
-            size_deps,
+            mut size_deps,
-            under,
+            request_deps,
+            mut scratch,
+            mut under,
             move_idx,
             layer,
             own_layer: _,
             depth: _,
+            draw: _,
             id,
         } = painter;
 
@@ -379,11 +445,15 @@ impl UiRenderState {
         // A rule wins on the axis it names, and the draw answers the rest.
         // Applied here so it is one place rather than every widget that could
         // carry one, and so the widget under a rule never learns of it. The
-        // rel base is the answer where the rule gave a length outright: it was
+        // rel base is the answer wherever the ask declared a length: it was
-        // resolved into the rel base when the child was asked, and resolving it
+        // resolved into the rel base when the widget was asked, and resolving
-        // again here would take the fraction of a fraction.
+        // it again here would take the fraction of a fraction.
         let rules = rsc.widgets().size_rules(id);
-        let ruled = |axis: Axis, reported: LayoutLen| match rules[axis].exact() {
+        let ruled = |axis: Axis, reported: LayoutLen| {
+            if rules[axis].deferred().is_some() {
+                return info.rel_base[axis].into();
+            }
+            match rules[axis].exact() {
                 None => reported,
                 Some(len) if len.leftover == Weight::ZERO => LayoutLen {
                     rel: info.rel_base[axis].rel,
@@ -391,11 +461,50 @@ impl UiRenderState {
                     leftover: Weight::ZERO,
                 },
                 Some(len) => len.within_len(info.rel_base[axis]),
+            }
         };
-        let size = Size {
+        let mut size = Size {
             x: ruled(Axis::X, size.x),
             y: ruled(Axis::Y, size.y),
         };
+        // What the drawing read is combined with what the ask decided rather
+        // than replacing it: a widget may widen its own ranges, and cannot
+        // widen the ask's.
+        let mut own_holds = own.and(info.ask_holds);
+        for axis in Axis::BOTH {
+            // A rule that is a fraction of the rel base is answered with the
+            // rel base's own length, so the answer is that rel base's and not
+            // just that many pixels of this window -- the same pin a widget
+            // that read its rel base took for its drawing. A bound counts:
+            // which side of it the box fell was decided against this rel
+            // base, and the same box of a different one can fall on the other.
+            if rules[axis].has_fraction() {
+                own_holds[axis].rel_base = Some(info.rel_base[axis]);
+            }
+            // A bound is a promise about the length as well as about the box:
+            // a widget that drew past the box it was given -- a text too tall
+            // for it, an image at its own size under a cap -- is still held to
+            // what its rule allows.
+            //
+            // Held here rather than taken from the box, even where the bound
+            // decided that box. What a widget answers is its own, and a bound
+            // that replaced the answer would make a share into a fixed length
+            // the moment a box was long enough -- which is a length the span
+            // dividing that box decided from this answer, so the two would
+            // choose each other. A share is left alone here for the same
+            // reason: it is a length only to whoever divides one, and the box
+            // that divider gives is a box this widget is asked in, where the
+            // bound is applied to it.
+            let answer = size[axis];
+            if answer.leftover != Weight::ZERO {
+                continue;
+            }
+            let (held, kept) = info.bounds[axis].outside(answer.without_leftover(), window[axis]);
+            own_holds[axis].window = own_holds[axis].window.and(kept);
+            if let Some(held) = held {
+                size[axis] = held.into();
+            }
+        }
         // A widget that clipped its contents to its box drew nothing outside
         // it, so reporting more than the box asks to be placed at a length it
         // does not occupy -- and its parent would place the part it cut off.
@@ -413,7 +522,7 @@ impl UiRenderState {
             region.to_px(window),
         );
         for c in &old_children {
-            if !children.contains(c) {
+            if !self.asked_in(*c, draw) {
                 self.undraw_rec(*c, rsc);
             }
         }
@@ -423,21 +532,10 @@ impl UiRenderState {
         if let Some(idx) = retired_move {
             self.moves.remove(idx);
         }
-        // A rule that is a fraction of the rel base is answered with the
-        // rel base's own length, so the answer is that rel base's and not just
-        // that many pixels of this window -- the same pin a widget that read
-        // its rel base took for its drawing.
-        let mut own_holds = own;
-        for axis in Axis::BOTH {
-            let fraction = rules[axis].exact().is_some_and(|len| len.rel != Rel::ZERO);
-            if fraction {
-                own_holds[axis].rel_base = Some(info.rel_base[axis]);
-            }
-        }
         let answer_holds = own_holds.and(answer_under);
         let holds = under
-            .into_iter()
+            .iter()
-            .fold(answer_holds, |holds, (_, child)| holds.and(child));
+            .fold(answer_holds, |holds, (_, child)| holds.and(*child));
         debug_assert!(
             holds.contains(self.output_size, info.rel_base, region),
             "'{}' ({id:?}) drew in {}, outside the ranges it reported: {holds:?}",
@@ -448,7 +546,7 @@ impl UiRenderState {
         // and a change there has to reach it. Asking answered whatever mark
         // it had: a hint is read live, and a drawing is not kept past one.
         for &dep in &size_deps {
-            if !children.contains(&dep) {
+            if !self.asked_in(dep, draw) {
                 self.asked(
                     dep,
                     DrawInfo {
@@ -462,6 +560,9 @@ impl UiRenderState {
                         region: UiRegion::FULL,
                         placed: PlaceDesc::WHOLE,
                         asked: PlaceDesc::WHOLE,
+                        declared: Declared::NONE,
+                        bounds: Bounds::ANY,
+                        ask_holds: LayoutHolds::ANY,
                         re_asked: false,
                     },
                     rsc,
@@ -470,6 +571,15 @@ impl UiRenderState {
             }
         }
 
+        for &dep in &request_deps {
+            self.request_readers.entry(dep).or_default().insert(id);
+        }
+        old_children.clear();
+        size_deps.clear();
+        under.clear();
+        scratch.children = old_children;
+        scratch.size_deps = size_deps;
+        scratch.under = under;
         let active = ActiveData {
             id,
             placement: region,
@@ -479,6 +589,7 @@ impl UiRenderState {
             region,
             // Whoever asked writes the answer.
             answer: None,
+            asked_by,
             re_asked: info.re_asked,
             size,
             holds,
@@ -489,7 +600,10 @@ impl UiRenderState {
             primitives,
             mask_region,
             children,
-            declared: rsc.widgets().declared_lens(id),
+            request_deps,
+            scratch,
+            declared: info.declared,
+            bounds: info.bounds,
             own_align: rsc.widgets().alignment(id),
             move_idx,
             parent_move: info.parent_move,
@@ -505,6 +619,21 @@ impl UiRenderState {
         }
     }
 
+    fn next_draw(&mut self) -> DrawId {
+        self.last_draw = self.last_draw.next();
+        self.last_draw
+    }
+
+    /// Whether `draw` has asked about this widget, which is what it left on
+    /// the widget's own record when it did. One widget is asked about by one
+    /// container, since the handle a container holds a child by cannot be
+    /// cloned, so one note per widget is enough to answer this.
+    pub(super) fn asked_in(&self, id: WidgetId, draw: DrawId) -> bool {
+        self.active
+            .get(&id)
+            .is_some_and(|active| active.asked_by == draw)
+    }
+
     /// Keeps a region node's entry across redraws because descendants retain
     /// its index.
     fn move_slot(&mut self, id: WidgetId, parent: MoveIdx, region: UiRegion) -> MoveIdx {
@@ -534,6 +663,7 @@ impl UiRenderState {
         if !active.drawn
             || active.is_region_node() != info.region_node
             || active.parent_move != info.parent_move
+            || active.bounds != info.bounds
         {
             return None;
         }
@@ -659,6 +789,9 @@ impl UiRenderState {
         let active = self.active.get_mut(&id).unwrap();
         active.rel_base = info.rel_base;
         active.placed = info.placed;
+        // What the ask made of its rules, which a re-place decides again.
+        active.declared = info.declared;
+        active.bounds = info.bounds;
         #[cfg(feature = "layout-diagnostics")]
         {
             let (counter, outcome) = match (moved, is_region_node) {
@@ -692,12 +825,11 @@ impl UiRenderState {
         let placed = place.placement(
             region,
             active.measured().unwrap_or(active.size),
-            active.declared,
             active.own_align,
         );
         let info = DrawInfo {
             layer: active.layer,
-            parent: Some(at.id),
+            parent: at.id,
             depth: at.depth + 1,
             parent_move: at.move_idx,
             region_node: active.is_region_node(),
@@ -706,6 +838,10 @@ impl UiRenderState {
             region,
             placed: place,
             asked: active.asked,
+            declared: active.declared,
+            bounds: active.bounds,
+            // Placing decides no box: this is the one the ask already gave.
+            ask_holds: LayoutHolds::ANY,
             re_asked: active.re_asked,
         };
         self.relocate(child, placed, info, rsc);
@@ -713,8 +849,8 @@ impl UiRenderState {
 
     /// The rel base and the box a widget already drawn is given at `place` of
     /// the box its parent is being taken as. What narrowed its rel base and what
-    /// it declared are its own record's, so both are resolved against that
+    /// it declared are its own record's. Declared lengths stay as the ask
-    /// parent's rel base again exactly as the first ask resolved them.
+    /// resolved them; the destination only decides where they sit.
     fn ask_again(active: &ActiveData, at: &Placing, place: PlaceDesc) -> (UiVec2, UiRegion) {
         place.rel_base_and_region(at.region, at.rel_base, active.declared, active.own_align)
     }
@@ -734,7 +870,7 @@ impl UiRenderState {
             rsc.ui_mut().masks.get_mut(active.mask).region = mask_region.within(&placed);
         }
         let at = Placing {
-            id,
+            id: Some(id),
             region: placed,
             rel_base: info.rel_base,
             depth: info.depth,
@@ -783,6 +919,11 @@ impl UiRenderState {
     fn remove(&mut self, id: WidgetId, undraw: bool, rsc: &mut dyn UiRsc) -> Option<ActiveData> {
         let mut active = self.active.remove(&id);
         if let Some(active) = &mut active {
+            for dep in active.request_deps.drain(..) {
+                if let Some(readers) = self.request_readers.get_mut(&dep) {
+                    readers.remove(&id);
+                }
+            }
             for primitive in &active.primitives {
                 let mask = self.layers.free(&primitive.handle);
                 if mask != MaskIdx::NONE {
@@ -846,6 +987,7 @@ impl UiRenderState {
                 asked: PlaceDesc::WHOLE,
                 region: UiRegion::FULL,
                 answer: None,
+                asked_by: DrawId::NONE,
                 re_asked: false,
                 size,
                 holds: LayoutHolds::ANY,
@@ -856,8 +998,11 @@ impl UiRenderState {
                 primitives: Vec::new(),
                 mask_region: None,
                 children: Vec::new(),
+                request_deps: Vec::new(),
+                scratch: Default::default(),
                 move_idx: info.parent_move,
                 declared: Declared::NONE,
+                bounds: Bounds::ANY,
                 own_align: rsc.widgets().alignment(id),
                 parent_move: info.parent_move,
                 mask: info.mask,
@@ -868,6 +1013,7 @@ impl UiRenderState {
     }
 
     fn clear(&mut self, rsc: &mut dyn UiRsc) {
+        self.request_readers.clear();
         for (_, active) in self.active.drain() {
             if active.drawn {
                 rsc.on_undraw(&active);
@@ -888,6 +1034,7 @@ impl UiRenderState {
             rsc.on_remove(id);
             self.remove(id, true, rsc);
             self.drop_slot(id);
+            self.request_readers.remove(&id);
         }
         rsc.ui_mut().textures.free();
     }
@@ -895,6 +1042,18 @@ impl UiRenderState {
     pub fn redraw_updates(&mut self, rsc: &mut dyn UiRsc) {
         #[cfg(feature = "layout-diagnostics")]
         let _layout = diag::timer(TimerKind::IncrementalLayout);
+        self.changed.clear();
+        self.changed
+            .extend(rsc.widgets().needs_redraw.iter().copied());
+        while let Some(id) = self.changed.pop() {
+            if let Some(readers) = self.request_readers.get(&id) {
+                for &reader in readers {
+                    if rsc.widgets_mut().needs_redraw.insert(reader) {
+                        self.changed.push(reader);
+                    }
+                }
+            }
+        }
         // Deepest first, and strictly: a widget that cannot settle where it
         // is defers to its parent rather than drawing the parent from
         // inside itself. It marks the parent, stays marked, and waits here
@@ -906,21 +1065,21 @@ impl UiRenderState {
         // something below is about to change it -- which is the whole class
         // of defect where a widget settles inside its parent's draw, clears
         // its mark there, and tells nobody its answer moved.
-        // The queue is that set, ordered: a mark made while the walk runs
+        // A mark made while the walk runs queues itself through `mark`. What
-        // queues itself through `mark`. What ends the walk is still the set
+        // ends the walk is the marks being spent rather than the queue being
-        // being spent, not the queue, so a mark that reached it another way
+        // empty, so a mark that reached the queue twice, or that was settled
-        // cannot be left for the next frame.
+        // another way, costs a pop and nothing else.
         loop {
             for &id in rsc.widgets().needs_redraw.iter() {
                 if !self.deferred.contains(&id) {
                     let depth = self.depth(id);
-                    self.pending.insert((depth, id));
+                    self.pending.push((depth, id));
                 }
             }
             if self.pending.is_empty() {
                 break;
             }
-            while let Some((depth, id)) = self.pending.pop_last() {
+            while let Some((depth, id)) = self.pending.pop() {
                 // Settled inside an ancestor's draw, or deferred to one,
                 // since the mark that queued it.
                 if self.deferred.contains(&id) || !rsc.widgets().needs_redraw.contains(&id) {
@@ -931,7 +1090,7 @@ impl UiRenderState {
                 // depth it had under the parent it left.
                 let now = self.depth(id);
                 if now != depth {
-                    self.pending.insert((now, id));
+                    self.pending.push((now, id));
                     continue;
                 }
                 #[cfg(feature = "layout-diagnostics")]
@@ -948,7 +1107,7 @@ impl UiRenderState {
     fn mark(&mut self, id: WidgetId, widgets: &mut Widgets) {
         if widgets.needs_redraw.insert(id) && !self.deferred.contains(&id) {
             let depth = self.depth(id);
-            self.pending.insert((depth, id));
+            self.pending.push((depth, id));
         }
     }
 
@@ -1042,15 +1201,32 @@ impl UiRenderState {
         let Some(active) = self.active.get(&id) else {
             return true;
         };
-        // Its parent resolved its declared lengths into its box and decided
+        // Asked where its parent asked it, which is what says whether the
-        // whether to draw it at all, so a change to either is the parent's
+        // question is still this widget's own: its parent resolved its
-        // to draw -- with the mark left on, so the parent draws it rather
+        // declared lengths into its box -- a bound of its own that the box
-        // than keeping it. So is a widget the parent asked twice: its
+        // falls outside is one of them -- and decided whether to draw it at
-        // layout rests on an answer this widget cannot give again alone.
+        // all, so a change to either is the parent's to draw, with the mark
-        let declared_changed = rsc.widgets().declared_lens(id) != active.declared;
+        // left on so the parent draws it rather than keeping it. So is a
+        // widget the parent asked twice: its layout rests on an answer this
+        // widget cannot give again alone. The root's parent is the window,
+        // which no draw made and no answer can move.
+        let at = match active.parent {
+            Some(parent) => self.placing_of(parent, self.active[&parent].region),
+            None => Placing::WINDOW,
+        };
+        let ask = at.ask(
+            rsc.widgets(),
+            &mut self.requests,
+            self.output_size,
+            id,
+            active.asked,
+        );
+        let active = &self.active[&id];
+        // Even an inactive bound changes what the parent must track about its offer.
+        let constraints_changed = ask.declared != active.declared || ask.bounds != active.bounds;
         let alignment_changed = rsc.widgets().alignment(id) != active.own_align;
         if let Some(parent) = active.parent
-            && (declared_changed
+            && (constraints_changed
                 || alignment_changed
                 || active.re_asked
                 || !active.drawn
@@ -1066,29 +1242,13 @@ impl UiRenderState {
         if !active.drawn {
             return true;
         }
-        // Nothing above the root resolved its rules or its alignment, so its
-        // box is its own to work out again against the output. Every other
-        // widget was given one.
-        let Some(parent) = active.parent else {
-            let (rel_base, region) = Self::root_layout(id, rsc.widgets());
-            let info = DrawInfo {
-                mask: active.parent_mask,
-                ..self.root_info(rel_base, region)
-            };
-            #[cfg(feature = "layout-diagnostics")]
-            diag::bump(Counter::LocalRedraws);
-            let old = self.remove(id, false, rsc);
-            self.draw_inner(id, info, old, rsc);
-            return true;
-        };
         let (was_answer, was_holds, was_place) = (active.answer, active.holds, active.placed);
-        // The question its parent asked, asked again: the same place of the
+        // The place the ask above came to: the same place of the box the
-        // box the parent was asked in, which is the box the parent's own
+        // parent was asked in, which is the box the parent's own draw ran in
-        // draw ran in and what its children's parts are of. Where the
+        // and what its children's parts are of. Where the parent's answer put
-        // parent's answer put its own drawing is not a question anybody
+        // its own drawing is not a question anybody asked, and nothing is
-        // asked, and nothing is asked in it here either.
+        // asked in it here either.
-        let parent_at = self.placing_of(parent, self.active[&parent].region);
+        let (rel_base, region) = (ask.rel_base, ask.region);
-        let (rel_base, region) = Self::ask_again(active, &parent_at, active.asked);
         let info = DrawInfo {
             layer: active.layer,
             parent: active.parent,
@@ -1098,8 +1258,11 @@ impl UiRenderState {
             mask: active.parent_mask,
             rel_base,
             region,
-            placed: active.asked,
+            placed: ask.place,
             asked: active.asked,
+            declared: ask.declared,
+            bounds: ask.bounds,
+            ask_holds: ask.holds,
             re_asked: false,
         };
         #[cfg(feature = "layout-diagnostics")]
@@ -1108,38 +1271,51 @@ impl UiRenderState {
         let old = self.remove(id, false, rsc);
         let drawn = self.draw_inner(id, info, old, rsc);
         let active = self.active.get_mut(&id).unwrap();
-        // A wider contract does not invalidate the guarantee the parent kept.
-        // Retain that guarantee so widening and narrowing back do not churn it.
-        if let Some(was) = was_answer
-            && drawn.answer.size == was.size
-            && drawn.answer.holds.covers(was.holds)
-        {
-            active.answer = was_answer;
-        }
         // Against the box it was asked in, which is what both contracts are
         // about. Where the answer put the drawing is shorter than that
         // wherever the widget reported less than it was offered.
-        if active.holds.covers(was_holds)
+        let (window, rel_base, region) = (self.output_size, active.rel_base, active.region);
-            && was_holds.contains(self.output_size, active.rel_base, active.region)
+        // A wider contract does not invalidate the guarantee the parent kept.
+        // Retain that guarantee so widening and narrowing back do not churn
+        // it -- but only where the narrower range still holds here: one this
+        // window is outside is refused by the parent's next ask, and refusing
+        // it throws away the drawing this one just made.
+        if let Some(was) = was_answer
+            && drawn.answer.size == was.size
+            && drawn.answer.holds.covers(was.holds)
+            && was.holds.contains(window, rel_base, region)
         {
+            active.answer = was_answer;
+        }
+        if active.holds.covers(was_holds) && was_holds.contains(window, rel_base, region) {
             active.holds = was_holds;
         }
-        if active.answer != was_answer || active.holds != was_holds {
+        let changed = active.answer != was_answer || active.holds != was_holds;
-            // The parent retains both the answer and the drawing's validity;
+        // Nothing above the root retained either, so there is nobody to tell
-            // even an unchanged size can narrow the range safe for a resize.
+        // and nowhere else the drawing has to go back to.
+        if let Some(parent) = active.parent {
+            match changed {
+                // The parent retains both the answer and the drawing's
+                // validity; even an unchanged size can narrow the range safe
+                // for a resize.
+                true => {
                     #[cfg(feature = "layout-diagnostics")]
                     {
                         diag::bump(Counter::SizeChanges);
                         diag::bump(Counter::ReaderEdges);
                     }
                     self.mark(parent, rsc.widgets_mut());
-        } else {
+                }
                 // The answer stands, so where the parent put it stands: the
                 // fresh drawing goes back there -- the same place, of the box
-            // the parent's answer chose rather than the one it was asked in.
+                // the parent's answer chose rather than the one it was asked
+                // in.
+                false => {
                     let at = self.placing_of(parent, self.active[&parent].placement);
                     self.place_in(id, &at, was_place, rsc);
                 }
+            }
+        }
         true
     }
 
@@ -1149,7 +1325,7 @@ impl UiRenderState {
     fn placing_of(&self, id: WidgetId, region: UiRegion) -> Placing {
         let active = &self.active[&id];
         Placing {
-            id,
+            id: Some(id),
             region,
             rel_base: active.rel_base,
             depth: active.depth,
@@ -1164,7 +1340,7 @@ impl Size {
     /// in. Both are lengths of the window, so the comparison is in its
     /// pixels. A share is a length only to whoever divides one, so it is not
     /// a claim about this box and cannot exceed it.
-    fn within_box(self, region: UiRegion, window: PxVec2, axis: Axis) -> bool {
+    fn within_box(&self, region: UiRegion, window: PxVec2, axis: Axis) -> bool {
         let len = self[axis];
         let window = window[axis];
         len.leftover != Weight::ZERO

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/refcount.rs

@@ -3,29 +3,18 @@ use std::sync::{
     atomic::{AtomicU32, Ordering},
 };
 
-#[derive(Debug)]
+/// How many handles to one thing there are, less one: a fresh counter is a
+/// single handle, which is what the derived default gives. Only
+/// [`crate::TextureHandle`] has any, a texture slot being shared by every
+/// widget drawing that picture.
+#[derive(Debug, Default)]
 pub struct RefCounter(Arc<AtomicU32>);
 
 impl RefCounter {
-    pub fn new() -> Self {
-        Self(Arc::new(0.into()))
-    }
-    pub fn refs(&self) -> u32 {
-        self.0.load(Ordering::Acquire)
-    }
     pub fn drop(&mut self) -> bool {
         let refs = self.0.fetch_sub(1, Ordering::Release);
         refs == 0
     }
-    pub fn quiet_clone(&self) -> Self {
-        Self(self.0.clone())
-    }
-}
-
-impl Default for RefCounter {
-    fn default() -> Self {
-        Self::new()
-    }
 }
 
 impl Clone for RefCounter {

core/src/widget/handle.rs

@@ -1,9 +1,6 @@
 use std::{marker::Unsize, ops::CoerceUnsized, sync::mpsc::Sender};
 
-use crate::{
+use crate::{UiRsc, Widget, util::SlotId};
-    UiRsc, Widget,
-    util::{RefCounter, SlotId},
-};
 
 pub type WidgetId = SlotId;
 
@@ -11,10 +8,12 @@ pub type WidgetId = SlotId;
 /// This is a strong handle that does not impl Clone, and when it is dropped,
 /// a signal is sent to the owning UI to clean up the resources.
 ///
+/// There is nothing to count: not being `Clone` is what makes one handle the
+/// only one, so the drop is the last one and the widget is one container's.
+///
 /// TODO: ergonomic clones when they get put in rust-analyzer & don't cause ICEs?
 pub struct StrongWidget<W: ?Sized = dyn Widget> {
     pub(super) id: WidgetId,
-    counter: RefCounter,
     send: Sender<WidgetId>,
     ty: *const W,
 }
@@ -37,7 +36,6 @@ impl<W: ?Sized> StrongWidget<W> {
     pub(crate) fn new(id: WidgetId, send: Sender<WidgetId>) -> Self {
         Self {
             id,
-            counter: RefCounter::new(),
             send,
             ty: null_ptr(),
         }
@@ -47,10 +45,6 @@ impl<W: ?Sized> StrongWidget<W> {
         self.id
     }
 
-    pub fn refs(&self) -> u32 {
-        self.counter.refs()
-    }
-
     pub fn weak(&self) -> WeakWidget<W> {
         let Self { ty, id, .. } = *self;
         WeakWidget { ty, id }
@@ -74,11 +68,9 @@ impl<W: ?Sized> WeakWidget<W> {
 
 impl<W: ?Sized> Drop for StrongWidget<W> {
     fn drop(&mut self) {
-        if self.counter.drop() {
         let _ = self.send.send(self.id);
     }
 }
-}
 
 pub trait WidgetIdFn<Rsc, W: ?Sized = dyn Widget>: FnOnce(&mut Rsc) -> WeakWidget<W> {}
 impl<Rsc, W: ?Sized, F: FnOnce(&mut Rsc) -> WeakWidget<W>> WidgetIdFn<Rsc, W> for F {}

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 +1,609 @@
+use crate::{
+    ActiveData, Axis, Bound, LayoutLen, Len, Px, Rel, StrongWidget, UiNum, Weight, WidgetId,
+    Widgets, util::HashMap,
+};
+use std::cmp::Ordering;
+
+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)
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum Op {
+    Sum,
+    Min,
+    Max,
+}
+
+/// One operand of a [`Node`]: a length, or another node. Node numbers are an
+/// arena's own, so an operand says nothing about which arena it came from --
+/// [`RequestedLen`] is the form that does, and the only one that leaves one.
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum Operand {
+    Linear(LayoutLen),
+    Node(u32),
+}
+
+impl Operand {
+    /// The length itself, where no comparison is waiting on an allocation.
+    fn linear(&self) -> Option<LayoutLen> {
+        match *self {
+            Self::Linear(len) => Some(len),
+            Self::Node(_) => None,
+        }
+    }
+}
+
+/// One sum or comparison, with its two operands. Whether anything under it
+/// divides leftover space is carried on the node rather than walked for,
+/// because every caller of one asks.
+#[derive(Clone, Copy, Debug, PartialEq)]
+struct Node {
+    op: Op,
+    a: Operand,
+    b: Operand,
+    leftover: bool,
+}
+
+/// The nodes of one expression, numbered from zero, and the folding that
+/// happens as each is added. There are two kinds of owner and one kind of
+/// arena: a rule's [`SizeRequest`] holds a small one for as long as the rule
+/// lasts, and [`RequestArena`] holds the layout pass's.
+#[derive(Clone, Debug, Default, PartialEq)]
+struct Nodes(Vec<Node>);
+
+impl Nodes {
+    fn node(&self, index: u32) -> Node {
+        self.0[index as usize]
+    }
+
+    fn leftover(&self, at: Operand) -> bool {
+        match at {
+            Operand::Linear(len) => len.leftover > Weight::ZERO,
+            Operand::Node(index) => self.node(index).leftover,
+        }
+    }
+
+    /// `a op b`, which is a node only where the answer needs one. Two
+    /// lengths that keep their order whatever the room comes to are already
+    /// decided, and so are two operands that are the same thing.
+    fn combine(&mut self, op: Op, a: Operand, b: Operand) -> Operand {
+        if let (Some(x), Some(y)) = (a.linear(), b.linear()) {
+            if matches!(op, Op::Sum) {
+                return Operand::Linear(x + y);
+            }
+            if let Some(order) = independent_order(x, y) {
+                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 = u32::try_from(self.0.len()).expect("more nodes than one arena can number");
+        let leftover = self.leftover(a) || self.leftover(b);
+        self.0.push(Node { op, a, b, leftover });
+        Operand::Node(index)
+    }
+
+    /// Copies `at` and everything under it out of `from`, with every length
+    /// it holds passed through `resolve`. Folded again on the way in, since
+    /// resolving a fraction can settle a comparison that was open before it.
+    fn graft(
+        &mut self,
+        from: &Self,
+        at: Operand,
+        resolve: impl Copy + Fn(LayoutLen) -> LayoutLen,
+    ) -> Operand {
+        let index = match at {
+            Operand::Linear(len) => return Operand::Linear(resolve(len)),
+            Operand::Node(index) => index,
+        };
+        let Node { op, a, b, .. } = from.node(index);
+        let a = self.graft(from, a, resolve);
+        let b = self.graft(from, b, resolve);
+        self.combine(op, a, b)
+    }
+
+    fn write(&self, f: &mut std::fmt::Formatter<'_>, at: Operand) -> std::fmt::Result {
+        let index = match at {
+            Operand::Linear(len) => return write!(f, "{len}"),
+            Operand::Node(index) => index,
+        };
+        let Node { op, a, b, .. } = self.node(index);
+        write!(
+            f,
+            "{}(",
+            match op {
+                Op::Sum => "sum",
+                Op::Min => "min",
+                Op::Max => "max",
+            }
+        )?;
+        self.write(f, a)?;
+        write!(f, ", ")?;
+        self.write(f, b)?;
+        write!(f, ")")
+    }
+}
+
+/// A size request before a container has divided its leftover space.
+/// Comparisons keep both operands until the share is known.
+///
+/// An expression is the same nodes the layout pass allocates, in an arena of
+/// its own: importing one copies those nodes into the pass's arena, so there
+/// is no second shape to keep in step and one place where folding is decided.
+#[derive(Clone, PartialEq)]
+pub enum SizeRequest {
+    Linear(LayoutLen),
+    /// Behind a pointer, because a plain length is what nearly every rule
+    /// holds and an expression should cost those rules nothing.
+    Expr(Box<Expr>),
+}
+
+/// An expression's own arena, and which of its nodes is the whole of it.
+/// `root` is a node number rather than an operand, so an expression that
+/// folded all the way down to a length cannot be written as one: that is a
+/// [`SizeRequest::Linear`].
+#[derive(Clone, Debug, PartialEq)]
+pub struct Expr {
+    nodes: Nodes,
+    root: u32,
+}
+
+impl SizeRequest {
+    pub fn min(self, other: impl Into<Self>) -> Self {
+        self.join(Op::Min, other.into())
+    }
+
+    pub fn max(self, other: impl Into<Self>) -> Self {
+        self.join(Op::Max, other.into())
+    }
+
+    pub fn clamp(self, min: impl Into<Self>, max: impl Into<Self>) -> Self {
+        self.max(min).min(max)
+    }
+
+    /// `self op other`, keeping `self`'s arena and copying `other`'s into
+    /// it, so the two sets of node numbers become one. Two requests that are
+    /// the same request compare to the same thing whatever the room is, which
+    /// is a comparison worth not building -- but adding something to itself
+    /// is twice it.
+    fn join(self, op: Op, other: Self) -> Self {
+        if self == other && !matches!(op, Op::Sum) {
+            return self;
+        }
+        let (mut nodes, a) = match self {
+            Self::Linear(len) => (Nodes::default(), Operand::Linear(len)),
+            Self::Expr(expr) => {
+                let Expr { nodes, root } = *expr;
+                (nodes, Operand::Node(root))
+            }
+        };
+        let b = match other {
+            Self::Linear(len) => Operand::Linear(len),
+            Self::Expr(expr) => nodes.graft(&expr.nodes, Operand::Node(expr.root), |len| len),
+        };
+        match nodes.combine(op, a, b) {
+            Operand::Linear(len) => Self::Linear(len),
+            Operand::Node(root) => Self::Expr(Box::new(Expr { nodes, root })),
+        }
+    }
+}
+
+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 std::ops::Add for SizeRequest {
+    type Output = Self;
+
+    fn add(self, other: Self) -> Self {
+        self.join(Op::Sum, other)
+    }
+}
+
+impl std::fmt::Display for SizeRequest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Linear(len) => write!(f, "{len}"),
+            Self::Expr(expr) => expr.nodes.write(f, Operand::Node(expr.root)),
+        }
+    }
+}
+
+/// The same, since an arena printed as a struct is not a tree anyone can
+/// write out again, which is what a request is printed for.
+impl std::fmt::Debug for SizeRequest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        std::fmt::Display::fmt(self, f)
+    }
+}
+
+/// 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: u32,
+        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(Default)]
+pub(crate) struct RequestArena {
+    nodes: Nodes,
+    epoch: u64,
+}
+
+impl RequestArena {
+    pub(crate) fn reset(&mut self) {
+        self.nodes.0.clear();
+        self.epoch = self
+            .epoch
+            .checked_add(1)
+            .expect("layout generation exhausted");
+    }
+
+    /// An operand of this pass's arena as the handle that leaves it. The
+    /// epoch says which pass numbered the node, so a handle a widget kept
+    /// past its pass is caught rather than answering about whatever node
+    /// took its place.
+    fn handle(&self, at: Operand) -> RequestedLen {
+        RequestedLen(match at {
+            Operand::Linear(len) => RequestValue::Linear(len),
+            Operand::Node(index) => RequestValue::Deferred {
+                index,
+                epoch: self.epoch,
+                leftover: self.nodes.leftover(at),
+            },
+        })
+    }
+
+    /// The other direction, checked once where a handle comes back in rather
+    /// than again at every level of the walk it starts.
+    fn operand(&self, request: RequestedLen) -> Operand {
+        match request.0 {
+            RequestValue::Linear(len) => Operand::Linear(len),
+            RequestValue::Deferred { index, epoch, .. } => {
+                assert_eq!(epoch, self.epoch, "request retained beyond its layout pass");
+                Operand::Node(index)
+            }
+        }
+    }
+
+    /// A rule's expression in this pass's arena, with its fractions resolved
+    /// against the rel base the widget is being asked with.
+    pub(crate) fn import(&mut self, request: &SizeRequest, base: Len) -> RequestedLen {
+        let at = match request {
+            SizeRequest::Linear(len) => Operand::Linear(len.within_len(base)),
+            SizeRequest::Expr(expr) => {
+                self.nodes
+                    .graft(&expr.nodes, Operand::Node(expr.root), |len| {
+                        len.within_len(base)
+                    })
+            }
+        };
+        self.handle(at)
+    }
+
+    pub(crate) fn bounded(&mut self, request: RequestedLen, bound: Bound) -> RequestedLen {
+        let request = match bound.min {
+            Some(min) => self.combine(Op::Max, request, min.into()),
+            None => request,
+        };
+        match bound.max {
+            Some(max) => self.combine(Op::Min, request, max.into()),
+            None => request,
+        }
+    }
+
+    fn combine(&mut self, op: Op, a: RequestedLen, b: RequestedLen) -> RequestedLen {
+        let (a, b) = (self.operand(a), self.operand(b));
+        let at = self.nodes.combine(op, a, b);
+        self.handle(at)
+    }
+
+    pub(crate) fn minimum(&self, request: RequestedLen, window: Px) -> Px {
+        let at = self.operand(request);
+        Px::from_raw(self.segment(at, Ratio::ZERO, window).fixed as i32)
+    }
+
+    fn segment(&self, of: Operand, at: Ratio, window: Px) -> Segment {
+        let index = match of {
+            Operand::Linear(len) => {
+                debug_assert!(
+                    len.leftover >= Weight::ZERO,
+                    "a leftover weight cannot be negative"
+                );
+                return Segment {
+                    fixed: i64::from(len.without_leftover().to_px(window).raw()),
+                    weight: i64::from(len.leftover.raw()),
+                    end: None,
+                };
+            }
+            Operand::Node(index) => index,
+        };
+        let Node { op, a, b, .. } = self.nodes.node(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(self.operand(*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(self.operand(*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];
+        let request = match &rule.request {
+            Some(request) => self.arena.import(request, self.rel_base),
+            None => {
+                let widget = self.widgets.get_dyn(child.id())?;
+                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 {
+        self.arena.bounded(request, bound.within_len(self.rel_base))
+    }
+
+    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

@@ -1,58 +1,154 @@
 use crate::util::impl_axis_index;
-use crate::{Axis, LayoutLen, Len};
+use crate::{LayoutLen, Len, Rel, SizeRequest, Weight};
 
-/// What a widget's length on one axis is, as a rule its parent applies where
+/// A preferred length and independent bounds on one axis. Without a
-/// it draws it rather than an answer the widget gives about itself.
+/// request, the widget's drawing supplies the preferred length.
-///
+#[derive(Debug, Clone, PartialEq, Default)]
-/// A rule and a drawn size are not two opinions to reconcile: a rule wins on
+pub struct SizeRule {
-/// the axis it names, and the `Size` returned by `draw` answers only the axes
+    pub request: Option<SizeRequest>,
-/// with no rule. That is what lets a span divide its space around a length
+    pub bound: Bound,
-/// nobody has drawn yet, and it is why a rule lives beside the widget rather
-/// than inside it -- the widget under the rule never has to know about it.
-#[derive(Debug, Clone, Copy, PartialEq, Default)]
-pub enum SizeRule {
-    /// Whatever the widget reports from drawing.
-    #[default]
-    Free,
-    /// This length, whatever the widget reports.
-    Exact(LayoutLen),
 }
 
 impl SizeRule {
-    /// The length this rule gives without the widget being drawn, if it can
+    pub const FREE: Self = Self {
-    /// give one.
+        request: None,
-    pub fn declared(&self) -> Option<Len> {
+        bound: Bound::ANY,
-        self.exact().and_then(LayoutLen::declared)
+    };
+
+    pub fn min(min: Len) -> Self {
+        Self::bounded(Bound {
+            min: Some(min),
+            max: None,
+        })
     }
 
-    /// The length this rule gives outright, whatever the widget reports --
+    pub fn max(max: Len) -> Self {
-    /// which makes the widget's answer on that axis moot. A share counts: it
+        Self::bounded(Bound {
-    /// is a length the widget's parent still has to divide, so it is exact
+            min: None,
-    /// here and resolved there, unlike `declared`, which is only the ones
+            max: Some(max),
-    /// that give a box directly.
+        })
+    }
+
+    pub fn clamp(min: Len, max: Len) -> Self {
+        Self::bounded(Bound {
+            min: Some(min),
+            max: Some(max),
+        })
+    }
+
+    /// A bound and no preferred length, so whatever the widget draws is held
+    /// to it.
+    fn bounded(bound: Bound) -> Self {
+        Self {
+            request: None,
+            bound,
+        }
+    }
+
+    pub fn has_fraction(&self) -> bool {
+        self.exact().is_some_and(|len| len.rel != Rel::ZERO) || self.bound.has_fraction()
+    }
+
+    /// A linear preferred length, before applying the independent bounds.
     pub fn exact(&self) -> Option<LayoutLen> {
-        match self {
+        match self.request {
-            Self::Free => None,
+            Some(SizeRequest::Linear(len)) => Some(len),
-            Self::Exact(len) => Some(*len),
+            _ => None,
+        }
+    }
+
+    /// Requests whose final length needs allocation, including a linear
+    /// share constrained by an independent bound.
+    pub(crate) fn deferred(&self) -> Option<&SizeRequest> {
+        let request = self.request.as_ref()?;
+        match request {
+            SizeRequest::Linear(len)
+                if len.leftover == Weight::ZERO || self.bound == Bound::ANY =>
+            {
+                None
+            }
+            _ => Some(request),
         }
     }
 }
 
+/// A floor, a cap, or both, independent of the preferred length.
+/// Fractions use the incoming rel base. An allocated slot keeps the
+/// allocator's base for bounds, even when the slot narrows the widget's own.
+///
+/// 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,
+    };
+}
+
+impl_axis_index!(Bounds => Bound);
+
 impl From<LayoutLen> for SizeRule {
     fn from(len: LayoutLen) -> Self {
-        Self::Exact(len)
+        SizeRequest::from(len).into()
+    }
+}
+
+impl From<SizeRequest> for SizeRule {
+    fn from(request: SizeRequest) -> Self {
+        Self {
+            request: Some(request),
+            bound: Bound::ANY,
+        }
     }
 }
 
 impl From<Option<LayoutLen>> for SizeRule {
     fn from(len: Option<LayoutLen>) -> Self {
-        len.map_or(Self::Free, Self::Exact)
+        len.map_or(Self::FREE, Self::from)
     }
 }
 
 /// 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,
@@ -60,12 +156,10 @@ pub struct SizeRules {
 
 impl_axis_index!(SizeRules => SizeRule);
 
-/// What a widget's box is on each axis where something says so outright,
+/// Box lengths chosen by the ask: a declaration, a natural-size hint, or
-/// before it is drawn: a rule beside it, or a hint it gives about itself.
+/// a binding bound. These are window lengths, already resolved against the
-/// Whoever draws the widget resolves these against its rel base.
+/// incoming rel base. Moving a drawing preserves them instead of resolving
-///
+/// a fraction against its destination a second time.
-/// 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>,
@@ -74,13 +168,6 @@ pub struct Declared {
 
 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,
+    Axis, AxisAlign, Bound, IdLike, Len, RegionAlign, SizeRequest, SizeRule, SizeRules,
-    WidgetData, WidgetId,
+    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 {
+    pub fn size_rules(&self, id: impl IdLike) -> &SizeRules {
-        self.data(id).unwrap().size
+        &self.data(id).unwrap().size
     }
 
     /// Sets one axis's rule. The widget is marked rather than its parent
@@ -137,6 +145,42 @@ impl Widgets {
         self.needs_redraw.insert(id);
     }
 
+    /// Changes the preferred length, leaving the bounds beside it alone.
+    pub fn set_len(&mut self, id: impl IdLike, axis: Axis, len: impl Into<SizeRequest>) {
+        let id = id.id();
+        let request = Some(len.into());
+        let rule = &mut self.data_mut(id).unwrap().size[axis];
+        if rule.request == request {
+            return;
+        }
+        rule.request = request;
+        self.needs_redraw.insert(id);
+    }
+
+    /// Puts a floor under this widget's length on one axis, keeping a cap it
+    /// already had and the preferred length beside it.
+    pub fn set_min_len(&mut self, id: impl IdLike, axis: Axis, min: Len) {
+        self.edit_bound(id.id(), axis, |bound| bound.min = Some(min));
+    }
+
+    /// Puts a cap over it, keeping a floor it already had.
+    pub fn set_max_len(&mut self, id: impl IdLike, axis: Axis, max: Len) {
+        self.edit_bound(id.id(), axis, |bound| bound.max = Some(max));
+    }
+
+    /// Edits one axis's bound where it sits, rather than reading the whole
+    /// rule out and writing it back: an expression beside the bound is not
+    /// this edit's business, and copying it to move one end would be the
+    /// only thing here that ever copies one.
+    fn edit_bound(&mut self, id: WidgetId, axis: Axis, edit: impl FnOnce(&mut Bound)) {
+        let bound = &mut self.data_mut(id).unwrap().size[axis].bound;
+        let before = *bound;
+        edit(bound);
+        if *bound != before {
+            self.needs_redraw.insert(id);
+        }
+    }
+
     /// Where this widget sits in a box longer than the length it takes.
     pub fn alignment(&self, id: impl IdLike) -> RegionAlign {
         self.data(id).unwrap().align
@@ -154,7 +198,7 @@ impl Widgets {
         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 +232,6 @@ 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 "$mode" >/dev/null
+# The extent `replay-touch` positions against, so a script's coordinates are
-# The extent `replay-touch` positions against, so a script's coordinates
+# the output's own pixels. Set beside every mode change, since a gesture
-# are the output's own pixels.
+# scaled against a mode the output no longer has lands somewhere else and
-out_w=${mode%x*}
+# still looks like a run that worked.
-out_h=${mode#*x}; out_h=${out_h%@*}
+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
+            // The two regions are on the grid and the pointer is not, so the
-            // it is compared against is on the grid.
+            // step between them is taken there and the pointer keeps the
-            let pos = (PxVec2::from_f32(ctx.data.pos) + container_pos - id_pos).to_f32();
+            // 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() {
+        let (output, suboptimal) = match self.surface.get_current_texture() {
-            CurrentSurfaceTexture::Success(texture) => texture,
+            CurrentSurfaceTexture::Success(texture) => (texture, false),
-            CurrentSurfaceTexture::Suboptimal(texture) => {
+            // Used for this frame, and the swapchain rebuilt after it has
-                self.surface.configure(&self.device, &self.config);
+            // been presented: configuring the surface while a texture it
-                texture
+            // 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/harness.rs

@@ -164,11 +164,11 @@ impl Harness {
         self.render.resize(size, self.rsc.widgets_mut());
     }
 
-    /// Changes a length rule after the fact, the way `.width()` sets one.
+    /// Changes a length rule after the fact, the way `.width()` sets one --
+    /// which leaves a bound beside it alone, where writing the whole rule
+    /// would drop it and pass the case for the wrong reason.
     pub fn set_len(&mut self, id: impl IdLike, axis: Axis, len: impl Into<LayoutLen>) {
-        self.rsc
+        self.rsc.widgets_mut().set_len(id, axis, len.into());
-            .widgets_mut()
-            .set_size_rule(id, axis, SizeRule::Exact(len.into()));
     }
 
     /// Sets the root and lays it out, so a pointer event has something to hit.

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.
@@ -176,9 +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>,
+    pub size: Option<SizeRules>,
-    /// The alignment it carries, under the same one-offer rule.
+    /// The alignment it carries, under the same one-offer rule. An axis left
-    pub align: Option<Aligns>,
+    /// 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.
@@ -195,6 +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 {
@@ -294,7 +292,7 @@ impl Plan {
                 align: None,
                 ..self.clone()
             }),
-            self.size.map(|_| Plan {
+            self.size.as_ref().map(|_| Plan {
                 size: None,
                 ..self.clone()
             }),
@@ -344,6 +342,19 @@ 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 {
+                rules[axis].bound = Bound::ANY;
+            }
+        });
+    }
+
     /// The same tree with `edits` applied, by the indices the generator would
     /// have used for them.
     ///
@@ -375,7 +386,7 @@ 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;
             }
@@ -448,9 +459,11 @@ 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,
             }),
@@ -632,9 +645,10 @@ 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;
@@ -643,15 +657,47 @@ impl Sow<'_> {
         })
     }
 
-    fn len(&mut self) -> Option<LayoutLen> {
+    fn len(&mut self) -> LayoutLen {
-        match self.rng.below(4) {
+        LayoutLen::px(20.0 + self.rng.below(180) as f32)
-            0 => Some(LayoutLen::px(20.0 + self.rng.below(180) as f32)),
+    }
-            1 => Some(LayoutLen::LEFTOVER),
+
-            _ => None,
+    /// A length of a box rather than a length of the window, which is what a
+    /// bound is.
+    ///
+    /// Pixels, because a fraction is resolved against the rel base the widget
+    /// was asked with and `deferred_generated.rs` is where that is varied:
+    /// its relative-bound corpus rewrites these as fractions, so growing them
+    /// that way here would buy overlap and move every box in the cold dump.
+    /// Two depth-5 trees once disagreed warm against cold with fractions in
+    /// them, which is why this was written as pixels; they were named by
+    /// seed, and seeds stopped naming those trees when the leaves grew
+    /// images. Re-measured 2026-09-20: 600 depth-5 trees over all sixteen
+    /// cases agree with every bound here a fraction.
+    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(Len::px(a.min(b).to_f32()), 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),
@@ -661,21 +707,27 @@ 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],
+            true => Align {
-            false => [x, y],
+                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.
@@ -773,10 +825,6 @@ 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);
@@ -796,6 +844,7 @@ 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)
@@ -804,23 +853,25 @@ 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 {
+        if let Some(lens) = plan.size.clone() {
-            self.rsc
+            self.rsc.ui_mut().widgets.set_size_rules(id, lens.x, lens.y);
-                .ui_mut()
-                .widgets
-                .set_size_rules(id, lens[0], lens[1]);
             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) {
+            for axis in Axis::BOTH {
-                widgets.set_alignment(id, axis, align.unwrap_or_default());
+                widgets.set_alignment(id, axis, resolved[axis]);
             }
             self.tree.aligned.push(id);
         }
@@ -831,6 +882,20 @@ 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),
@@ -839,6 +904,7 @@ 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);
@@ -915,6 +981,10 @@ 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

@@ -16,6 +16,15 @@ impl Widget for Image {
     }
 }
 
+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 }
+    }
+}
+
 pub fn image<State: UiRsc>(image: impl LoadableImage) -> impl WidgetFn<State, Image> {
     let image = image.get_image().expect("Failed to load image");
     move |state| Image {

src/widget/mask.rs

@@ -8,11 +8,14 @@ impl Widget for Masked {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         painter.set_mask(UiRegion::FULL);
         painter.widget(&self.inner);
-        // What it occupies is its box, on both axes, for the reason `Scroll`
+        // What it occupies is its box, on both axes, because it clips what is
-        // reports the same: it clips what is inside to that box, so it can
+        // inside to that box: it can neither take less of one nor honestly ask
-        // neither take less of one nor honestly ask for more. Passing the
+        // for more. Passing the inner size up instead asks to be placed at a
-        // inner size up instead asks to be placed at a length it does not
+        // length it does not draw, and the framework would place the drawing it
-        // draw, and the framework would place the drawing it clipped away.
+        // clipped away. `Scroll` reports its box too, for a reason of its own:
+        // it is a viewport whose content is positioned by a move rather than
+        // clipped, since masking is a capability a caller opts into by putting
+        // one of these around it.
         Size::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/offset.rs

@@ -6,6 +6,10 @@ 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 {
         painter
             .widget_at(&self.inner, UiRegion::FULL.offset(self.amt))

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
@@ -22,11 +26,11 @@ impl Widget for Pad {
         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
             },
         }
@@ -57,6 +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

@@ -24,36 +24,37 @@ impl Widget for Scroll {
             self.amt = self.content_len - self.container_len;
         }
         self.update_amt();
-        let align = painter.alignment()[self.axis];
+        // Reading the box in pixels above holds this drawing to that one
-        // Content of a fixed length that fits sits at the start of any box it
+        // length, so these two say where it holds more widely.
-        // 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
+        // Content of a fixed length that fits is handed the whole box below,
-        // the drawing holds for that length alone. One scrolled part way sits
+        // and nothing here reads the box again, so every longer box gives the
-        // where it is until the box shrinks past what is left of it. Kept to
+        // same drawing: it holds from the length the content needs upwards,
-        // the end, it moves with every length.
+        // 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 && align == AxisAlign::NEG {
+        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
+        // Content that fills the viewport is the viewport, and is handed back
-        // sits is this widget's own alignment -- the same property that would
+        // as it came -- it has nothing to scroll through, so the clamp above
-        // have placed the whole scroll in a box longer than it.
+        // has already put `amt` at zero. Writing the same box as its own
-        let slack = (self.container_len - self.content_len).max(Px::ZERO);
+        // length in pixels is the same box in another form, and the two do
-        let anchor = slack.mul(align.rel());
+        // not round alike: a part centred in `rel 1` lands a step from one
-        // Content that fills the viewport and has not been scrolled is the
+        // centred in `px 900`, since halving a difference is not halving each
-        // viewport, and is handed back as it came. Writing the same box as
+        // part of it.
-        // its own length in pixels is the same box in another form, and the
+        let content = match self.content_len > self.container_len {
-        // two do not round alike: a part centred in `rel 1` lands a step from
-        // one centred in `px 900`, since halving a difference is not halving
-        // each part of it.
-        let moved = anchor != Px::ZERO || self.amt != Px::ZERO;
-        let content = match moved || self.content_len != self.container_len {
             true => {
-                let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
+                let start = Len::from_parts(Rel::ZERO, -self.amt);
                 UiSpan::new(start, start.offset(self.content_len)).shifted_desc()
             }
             false => PlaceDescAxis::WHOLE,

src/widget/position/span.rs

@@ -8,69 +8,75 @@ pub struct Span {
 }
 
 impl Widget for Span {
+    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)
+    }
+
     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);
-        // A length for every child before their final slots are chosen: from
+        self.collect(painter, row, lens, true);
-        // a hint where one says, and from drawing otherwise. The rel base passes
+        let gaps = self.gaps();
-        // through unchanged, so `rel(0.5)` is half the area this span was
+        let fixed = lens
-        // given whatever else is in it and wherever this child sits among
+            .iter()
-        // them; what a drawn child is asked in is the room left from the
+            .try_fold(Len::from_parts(Rel::ZERO, gaps), |sum, len| {
-        // cursor, because a text has to wrap at the width actually there.
+                Some(sum + len.linear()?.without_leftover())
-        let mut cursor = Len::ZERO;
+            });
-        let mut lens = Vec::with_capacity(self.children.len());
+        if let Some(fixed) = fixed
-        for child in &self.children {
+            && lens.iter().any(|len| len.has_leftover())
-            let len = match painter.size_hint(child, axis) {
+            && !painter.longer_than(row, fixed, axis)
-                Some(len) => len,
+        {
-                None => {
+            // With no share to assign, intrinsic drawings keep the remaining
-                    // Across itself the child sits where its own alignment
+            // offer, including overflow. Their answer is only moved into a slot.
-                    // says, in the whole of the row: a span is what contains
+            self.collect(painter, row, lens, false);
-                    // its children there, and nothing divides that axis.
-                    let room = self.slot(row, cursor, row).shifted_desc().on_axis(axis);
-                    painter.widget_at(child, room).len(axis)
         }
-            };
+        let nonlinear = lens.iter().any(|len| len.linear().is_none());
-            cursor += len.without_leftover();
+        if nonlinear {
-            cursor.px += self.gap;
+            painter.allocate(lens, row - Len::from_parts(Rel::ZERO, gaps), axis, values);
-            lens.push(len);
         }
-
+        let allocated = nonlinear.then(|| &values[..]);
-        let gaps = self
+        let total = match allocated {
-            .gap
+            Some(allocated) => LayoutLen {
-            .mul_int(self.children.len().saturating_sub(1) as i32);
+                px: allocated.iter().fold(gaps, |sum, len| sum + *len),
-        let total = lens.iter().fold(
+                ..LayoutLen::ZERO
+            },
+            None => lens.iter().fold(
                 LayoutLen {
                     px: gaps,
                     ..LayoutLen::ZERO
                 },
-            |sum, len| sum + *len,
+                |sum, len| sum + len.linear().unwrap(),
-        );
+            ),
-
-        // What is left for the shares to divide: the row less everything
-        // fixed, as a length of the rel base rather than a number of pixels.
-        let room = row - total.without_leftover();
-        // Whether anything is left over is a question in pixels: `rel(0.5)`
-        // beside 300 px is full at 600 and overfull at 400. Asked of `room`
-        // itself, and answered back through the same expression, so the
-        // boundary is the drawing's own and not a second way of finding it:
-        // the three cases a rounded division needed -- the fixed parts
-        // growing slower than the box, faster, or exactly with it -- are the
-        // sign of `room.rel`, which `through` already reads. What the
-        // generated oracle checks is the consequence, since which children
-        // exist at all turns on this.
-        let any_leftover = total.leftover > Weight::ZERO;
-        let has_room = any_leftover && painter.to_px(room, axis) > Px::ZERO;
-        if any_leftover {
-            let holds = match has_room {
-                true => Holds::from(Px::STEP..=Px::MAX),
-                false => Holds::from(Px::MIN..=Px::ZERO),
         };
-            painter.window_holds(axis, holds.through(room));
+        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
@@ -93,17 +99,34 @@ impl Widget for Span {
             false => fixed,
             true => fixed + room.scale(Rel::ratio(taken, total.leftover)),
         };
-        for (child, &len) in self.children.iter().zip(&lens) {
+        for (index, (child, request)) in self.children.iter().zip(lens.iter()).enumerate() {
-            // A child asking for nothing but a part of what is left over,
+            // An allocated row already has a length for every child; without
-            // when nothing is, is not drawn at all. One that also asked for
+            // one the request is the length and the room is divided here.
-            // pixels or a fraction keeps those and overflows.
+            // Either way a child asking for nothing but a part of what is
-            if len.is_only_leftover() && !has_room {
+            // 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 = reached(fixed, taken);
-            if len.leftover > Weight::ZERO && has_room {
+            if shares {
                 taken += len.leftover;
             }
             fixed += len.without_leftover();
@@ -116,15 +139,15 @@ impl Widget for Span {
             // fixed child's slot is its own answer, so a drawing made in the
             // room is put there as it is, and one not made yet is made here.
             let slot = self.slot(row, from, to);
-            let mut place = slot.shifted_desc().fills().on_axis(axis);
+            let mut place = slot.shifted_desc().allocated().on_axis(axis);
-            if len.leftover > Weight::ZERO && has_room {
+            if shares {
                 place = place.rel_base(axis, slot.len());
             }
             let used = painter.place_at(child, place).len(!axis);
             if shrinks {
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.
-                // A scalable child therefore makes Children scalable too;
+                // A scalable child therefore makes the span scalable too;
                 // only fixed children are compared with one another.
                 if !used.is_px() {
                     ortho = LayoutLen::LEFTOVER;
@@ -135,13 +158,15 @@ impl Widget for Span {
             fixed.px += self.gap;
         }
 
-        // Carried whole rather than collapsed to one share: a span that sizes
+        // Where nothing was allocated the weight is carried whole rather
-        // from its children does not resolve `leftover`, it passes the weight up,
+        // than collapsed to one share, so nesting spans divides the same
-        // so nesting spans divides the same space rather than re-dividing a
+        // space rather than re-dividing a share of it: four `leftover(1)`
-        // share of it. Four `leftover(1)` children under two spans under one span
+        // children under two spans under one span get a quarter each, which
-        // get a quarter each, which collapsing to `leftover(1)` per level does
+        // one share per level does not give. Resolution happens at the
-        // not give. Resolution happens at the nearest ancestor with a length,
+        // nearest ancestor with a length, and the root always has one --
-        // 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),
@@ -151,6 +176,43 @@ impl Widget for Span {
 }
 
 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

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,

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 undo && let Some((old, selection)) = self.text.history.pop() {
-            if let Some((old, selection)) = self.text.history.pop() {
             self.set(&old);
             self.text.selection = selection;
             self.clamp_selection_to_layout();
-            }
         } else if self.text.view.buf.text() != old.0 {
             self.text.history.push(old);
         }

src/widget/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)] {
+            for axis in Axis::BOTH {
-                if let Some(align) = align {
+                if let Some(align) = align[axis] {
                     widgets.set_alignment(id, axis, align);
                 }
             }
@@ -53,32 +53,64 @@ widget_trait! {
         move |state| {
             let id = self.add(state);
             let widgets = &mut state.ui_mut().widgets;
-            widgets.set_size_rule(id, Axis::X, SizeRule::Exact(size.x));
+            widgets.set_len(id, Axis::X, size.x);
-            widgets.set_size_rule(id, Axis::Y, SizeRule::Exact(size.y));
+            widgets.set_len(id, Axis::Y, size.y);
             id
         }
     }
 
-    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
+            state.ui_mut().widgets.set_len(id, Axis::X, len);
-                .ui_mut()
-                .widgets
-                .set_size_rule(id, Axis::X, SizeRule::Exact(len));
             id
         }
     }
 
-    fn height(self, len: impl Into<LayoutLen>) -> impl WidgetIdFn<Rsc, WL::Widget> {
+    /// Sets a floor on this widget's offered width and reported width.
+    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
+            state.ui_mut().widgets.set_min_len(id, Axis::X, len);
-                .ui_mut()
+            id
-                .widgets
+        }
-                .set_size_rule(id, Axis::Y, SizeRule::Exact(len));
+    }
+
+    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
+        }
+    }
+
+    /// Caps this widget's offered width and reported width.
+    fn max_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_max_len(id, Axis::X, len);
+            id
+        }
+    }
+
+    fn max_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_max_len(id, Axis::Y, len);
+            id
+        }
+    }
+
+    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_len(id, Axis::Y, len);
             id
         }
     }
@@ -134,9 +166,9 @@ widget_trait! {
         |state| self.add(state)
     }
 
-    // Named for the type it makes rather than as `wrapped`, which would read
+    /// This widget in a [`Wrapper`], which is how it gets a second length or
-    // as the text setting. `widget_trait!` takes no attributes, so what it is
+    /// alignment beside the one it already carries. Named for the type it
-    // for is on `Wrapper` itself.
+    /// makes rather than as `wrapped`, which would read as the text setting.
     fn wrapper(self) -> impl WidgetFn<Rsc, Wrapper> {
         |state| Wrapper {
             inner: Some(self.add_strong(state)),

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 +1,124 @@
+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 rule in [
+        SizeRule::FREE,
+        leftover(1).clamp(20, 80).into(),
+        SizeRule::clamp(20.into(), 80.into()),
+    ] {
+        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);
+            h.rsc.widgets_mut().set_size_rule(a, Axis::X, rule.clone());
+            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!("rule={rule:?}: {allocations} allocations over 100 resize frames");
+        assert_eq!(allocations, 0);
+    }
+}
+
+/// A text drawn again at the width it already has places no glyphs and shapes
+/// nothing, so the frame costs nothing at all -- which is what the shaping
+/// cache is for, and a copy of the attrs made to ask it undid for any text
+/// naming its font family.
+///
+/// Only that case: a text drawn at a width it has not seen places its glyphs,
+/// and placing them allocates a list to hold them.
+#[test]
+fn redrawing_a_text_at_one_width_allocates_nothing() {
+    let mut h = Harness::new((600, 200));
+    let mut col = Span::empty(Dir::DOWN);
+    let mut texts = Vec::new();
+    for _ in 0..8 {
+        let text =
+            wtext("wrapping shapes one source into as many lines as the box leaves room for")
+                .size(16)
+                // Named rather than generic, because a named one is the family
+                // that costs an allocation to copy.
+                .family(Family::Named("sans-serif".into()))
+                .wrap(true)
+                .add_strong(&mut h.rsc);
+        texts.push(text.id());
+        col.push(text);
+    }
+    let root = col.add(&mut h.rsc);
+    h.set_root(root);
+    let redraw = |h: &mut Harness| {
+        for &id in &texts {
+            h.rsc.widgets_mut().mark_for_redraw(id);
+        }
+        h.frame();
+    };
+    for _ in 0..8 {
+        redraw(&mut h);
+    }
+    COUNT.set(Some(0));
+    for _ in 0..100 {
+        redraw(&mut h);
+    }
+    let allocations = COUNT.replace(None).unwrap();
+    println!("text: {allocations} allocations over 100 redraws of 8 texts");
+    assert_eq!(allocations, 0);
+}

tests/bounds_cost.rs

@@ -0,0 +1,88 @@
+//! CPU comparison of a bounds attribute against the `MaxSize` wrapper it
+//! replaced, using the same builder calls and the same geometry. The wrapper
+//! is gone from this tree, so its side of the comparison is run by checking
+//! out a commit that still has it: the fixture is written to build the same
+//! way at both.
+//!
+//!     MODE=cap FRAMES=2000 cargo test --release --test bounds_cost \
+//!       -- --ignored --nocapture
+//!
+//! `MODE` is `plain`, `exact` or `cap`; `REDRAW=1` marks every widget for
+//! redraw each frame; `FRAMES` is how many resize frames to measure. Use
+//! repeated `perf stat -e instructions:u` runs on the executable directly.
+//! Process totals include the cold frame, so compare identical modes and
+//! frame counts. Wall time on this machine is not a stable comparison.
+
+mod rig;
+
+use iris::{harness::Harness, prelude::*};
+use rig::env;
+
+/// The two widths the loop alternates. The cap of 80 binds at 300 and does
+/// not at 100, so the measured frames cross it in both directions.
+const WIDTHS: [i32; 2] = [100, 300];
+
+#[test]
+#[ignore = "instruction-count measurement"]
+fn bounds_cost() {
+    let mode = env("MODE", "cap".to_string());
+    let redraw = env("REDRAW", 0_u8) != 0;
+    let frames = env("FRAMES", 2000_usize);
+    let mut h = Harness::new((300, 512));
+    let mut column = Span::empty(Dir::DOWN);
+    let mut leaves = Vec::new();
+    for _ in 0..128 {
+        let first = match mode.as_str() {
+            "plain" => rect(Color::RED).add_strong(&mut h.rsc).any(),
+            "exact" => rect(Color::RED).width(40).add_strong(&mut h.rsc).any(),
+            "cap" => rect(Color::RED).max_width(80).add_strong(&mut h.rsc).any(),
+            _ => panic!("unknown MODE {mode}"),
+        };
+        leaves.push(first.id());
+        let second = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let row = Span {
+            children: vec![first, second],
+            dir: Dir::RIGHT,
+            gap: Px::ZERO,
+        };
+        column.push(row.height(4).add_strong(&mut h.rsc));
+    }
+    h.set_root(column);
+    let ids: Vec<_> = h.render.active.keys().copied().collect();
+    println!(
+        "mode={mode}, widgets={}, rule_bytes={}",
+        ids.len(),
+        std::mem::size_of::<SizeRule>()
+    );
+    // That the fixture measures what it says is checked on both sides of the
+    // crossing here rather than inside the measured loop, which is what the
+    // other rigs do. Per frame it measured only 0.65% of the total (5.780B
+    // against 5.743B instructions at MODE=cap, FRAMES=2000), but none of it
+    // is the layout the number is about.
+    for width in WIDTHS {
+        h.resize((width, 512));
+        h.frame();
+        let expected = match mode.as_str() {
+            "plain" => width / 2,
+            "exact" => 40,
+            "cap" => (width / 2).min(80),
+            _ => unreachable!("the mode was checked while building"),
+        };
+        for id in &leaves {
+            assert_eq!(h.region(id).unwrap().size().x, Px::from_int(expected));
+        }
+        println!(
+            "width {width}: first leaf {}",
+            h.region(&leaves[0]).unwrap()
+        );
+    }
+    for frame in 0..frames {
+        if redraw {
+            for &id in &ids {
+                h.rsc.widgets_mut().mark_for_redraw(id);
+            }
+        }
+        h.resize((WIDTHS[frame % WIDTHS.len()], 512));
+        h.frame();
+    }
+}

tests/cases/deferred.rs

@@ -0,0 +1,421 @@
+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.widgets_mut().set_max_len(a, Axis::X, 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_comparison_between_two_comparisons_keeps_both_of_them() {
+    // Joining two expressions is the one path that copies a request's nodes
+    // into another's arena; the floor puts a node under the copied one, so
+    // its operands have to be renumbered as they land.
+    let capped = leftover(1).min(px(40));
+    let floored = leftover(2).min(px(70)).max(px(10));
+    let mut h = Harness::new((90, 100));
+    let both = rect(Color::RED).width(capped.max(floored)).add(&mut h.rsc);
+    let rest = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((both, rest).span(Dir::RIGHT));
+    // Under either cap, so the doubled share is the longer of the two.
+    assert_corners!(h, both, (0, 0), (60, 100));
+    assert_corners!(h, rest, (60, 0), (90, 100));
+    h.resize((300, 100));
+    h.frame();
+    // Over both caps, so the comparison is between 40 and 70.
+    assert_corners!(h, both, (0, 0), (70, 100));
+    assert_corners!(h, rest, (70, 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(Len::rel(0.25), 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(Len::rel(0.25), 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));
+}
+
+#[test]
+fn an_intrinsic_share_cap_uses_the_allocators_fractional_base() {
+    let mut h = Harness::new((300, 100));
+    let head = rect(Color::BLUE).width(30).add(&mut h.rsc);
+    let bounded = rect(Color::RED).max_width(Len::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));
+}

tests/cases/determinism.rs

@@ -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().mark_for_redraw(wide);
-            h.rsc.widgets_mut().get_dyn_mut(narrow);
+            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::*;
 
@@ -219,6 +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));
@@ -626,7 +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));
 
@@ -694,7 +824,7 @@ fn row_under_fractions(kid: Option<LayoutLen>, gap: f32, box_w: f32) -> (Vec<Px>
         if let Some(len) = kid {
             h.rsc
                 .widgets_mut()
-                .set_size_rule(r.id(), Axis::X, SizeRule::Exact(len));
+                .set_size_rule(r.id(), Axis::X, SizeRule::from(len));
         }
         ids.push(r.id());
         kids.push(r.add_strong(&mut h.rsc));
@@ -858,3 +988,341 @@ fn a_collapsed_share_keeps_the_gaps_before_the_next_slot() {
         }
     }
 }
+
+/// 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 holds the length a widget reports as well as narrowing the box it
+/// is offered, and the two are not the same question. Here two 200-wide rects
+/// fill a row in a 250 window, so a cap of 300 leaves the box alone and only
+/// cuts what the row reports -- which the window then centres, past both its
+/// edges -- while a floor raises the report and the rects stay where the 250
+/// box put them.
+#[test]
+fn a_bound_holds_what_a_widget_answers() {
+    let bounded_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, row, left)
+    };
+    let (h, row, left) = bounded_row(SizeRule::max(Len::px(300.0)));
+    assert_eq!(
+        h.region(&row).unwrap().size().x,
+        Px::from_int(300),
+        "the cap, not the 400 drawn"
+    );
+    assert_eq!(
+        h.region(&left).unwrap().size().x,
+        Px::from_int(200),
+        "the box the children were given"
+    );
+    assert_corners!(h, row, (-25, 0), (275, 200));
+
+    let (h, row, _) = bounded_row(SizeRule::min(Len::px(600.0)));
+    assert_eq!(
+        h.region(&row).unwrap().size().x,
+        Px::from_int(600),
+        "the floor, not the 400 drawn"
+    );
+
+    let (h, row, _) = bounded_row(SizeRule::FREE);
+    assert_eq!(
+        h.region(&row).unwrap().size().x,
+        Px::from_int(400),
+        "what it drew"
+    );
+}
+
+/// A cap narrows the box the widget is asked in, whether a declaration of its
+/// own decides that box or the allocator divides a share into it. The cap is
+/// an attribute of the widget rather than something wrapped around it, which
+/// is what the id assertions say.
+#[test]
+fn a_cap_attribute_narrows_the_widgets_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);
+    assert_eq!(fills.id(), capped.id());
+    h.set_root(capped);
+    assert_eq!(h.region(&fills).unwrap().size().x, Px::from_int(300));
+
+    // A share with nothing beside it: the cap is composed into the request
+    // and the allocator answers with it rather than the whole 400.
+    let mut h = Harness::new((400, 200));
+    let share = rect(Color::RED).width(leftover(1)).add(&mut h.rsc);
+    let capped = share.max_width(300).add(&mut h.rsc);
+    assert_eq!(share.id(), capped.id());
+    h.set_root(capped);
+    assert_eq!(h.region(&share).unwrap().size().x, Px::from_int(300));
+}
+
+#[test]
+fn a_cap_attribute_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));
+}
+
+struct Offered {
+    seen: Rc<Cell<PxVec2>>,
+    answer: Size,
+}
+
+impl Widget for Offered {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.seen.set(painter.px_size());
+        painter.primitive(RectPrimitive::color(Color::RED));
+        self.answer
+    }
+}
+
+#[test]
+fn bounds_constrain_the_offer_without_replacing_an_intrinsic_answer() {
+    for axis in Axis::BOTH {
+        for node in [false, true] {
+            let mut h = Harness::new((400, 400));
+            let seen = Rc::new(Cell::new(PxVec2::ZERO));
+            let probe = h.rsc.widgets_mut().add_strong(Offered {
+                seen: seen.clone(),
+                answer: Size::px(Vec2::new(40.0, 40.0)),
+            });
+            let id = probe.id();
+            h.rsc.widgets_mut().set_region_node(id, node);
+            h.rsc.widgets_mut().set_max_len(id, axis, 100.into());
+            h.state.root = Some(probe);
+            h.frame();
+            assert_eq!(seen.get()[axis], Px::from_int(100));
+            assert_eq!(h.region(&id).unwrap().size()[axis], Px::from_int(40));
+            h.rsc.widgets_mut().set_min_len(id, axis, 60.into());
+            h.frame();
+            assert_eq!(h.region(&id).unwrap().size()[axis], Px::from_int(60));
+            h.resize((50, 50));
+            h.frame();
+            assert_eq!(seen.get()[axis], Px::from_int(60));
+            h.rsc.widgets_mut().set_size_rule(id, axis, SizeRule::FREE);
+            h.frame();
+            assert_eq!(seen.get()[axis], Px::from_int(50));
+            assert_eq!(h.region(&id).unwrap().size()[axis], Px::from_int(40));
+        }
+    }
+}
+
+#[test]
+fn a_cap_attribute_is_the_scroll_viewport() {
+    for node in [false, true] {
+        let mut h = Harness::new((400, 400));
+        let content = rect(Color::RED).height(400).add(&mut h.rsc);
+        let inner = content.add_strong(&mut h.rsc);
+        let scroll = Scroll::new(inner, Axis::Y).max_height(100).add(&mut h.rsc);
+        h.rsc.widgets_mut().set_region_node(scroll, node);
+        h.set_root(scroll);
+        assert_corners!(h, scroll, (0, 150), (400, 250));
+        assert_corners!(h, content, (0, -150), (400, 250));
+        h.rsc.widgets_mut().get_mut(&scroll).unwrap().scroll(1000.0);
+        h.frame();
+        assert_corners!(h, content, (0, 150), (400, 550));
+        h.resize((400, 80));
+        h.frame();
+        assert_corners!(h, scroll, (0, 0), (400, 80));
+        assert_corners!(h, content, (0, 0), (400, 400));
+    }
+}
+
+#[test]
+fn a_cap_attribute_wraps_text_before_it_answers() {
+    let mut h = Harness::new((400, 500));
+    let text = wtext("one two three four five six seven eight nine ten")
+        .size(16)
+        .wrap(true)
+        .max_width(80)
+        .align(Align::TOP_LEFT)
+        .add(&mut h.rsc);
+    h.set_root(text);
+    let capped = h.region(&text).unwrap().size();
+    assert!(capped.x <= Px::from_int(80));
+    assert!(capped.y > Px::from_int(30));
+    h.rsc
+        .widgets_mut()
+        .set_size_rule(text, Axis::X, SizeRule::FREE);
+    h.frame();
+    let free = h.region(&text).unwrap().size();
+    assert!(free.x > capped.x);
+    assert!(free.y < capped.y);
+}
+
+#[test]
+fn dimensions_and_bounds_are_independent_attributes_in_either_order() {
+    for bounds_first in [false, true] {
+        let mut h = Harness::new((400, 400));
+        let probe = rect(Color::RED).add(&mut h.rsc);
+        let bounded = if bounds_first {
+            probe
+                .max_width(80)
+                .min_height(60)
+                .width(120)
+                .height(40)
+                .add(&mut h.rsc)
+        } else {
+            probe
+                .width(120)
+                .height(40)
+                .max_width(80)
+                .min_height(60)
+                .add(&mut h.rsc)
+        };
+        assert_eq!(probe.id(), bounded.id());
+        h.set_root(bounded);
+        assert_eq!(
+            h.region(&probe).unwrap().size(),
+            PxVec2::from_f32((80, 60).into())
+        );
+        h.rsc.widgets_mut().set_len(probe, Axis::X, 50);
+        h.rsc.widgets_mut().set_len(probe, Axis::Y, 100);
+        h.frame();
+        assert_eq!(
+            h.region(&probe).unwrap().size(),
+            PxVec2::from_f32((50, 100).into())
+        );
+    }
+}
+
+#[test]
+fn changing_a_share_cap_replaces_it_without_losing_the_share() {
+    let mut h = Harness::new((300, 100));
+    let capped = rect(Color::RED)
+        .max_width(80)
+        .width(leftover(1))
+        .add(&mut h.rsc);
+    let sibling = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((capped, sibling).span(Dir::RIGHT));
+    assert_corners!(h, capped, (0, 0), (80, 100));
+    assert_corners!(h, sibling, (80, 0), (300, 100));
+    h.rsc.widgets_mut().set_max_len(capped, Axis::X, 160.into());
+    h.frame();
+    assert_corners!(h, capped, (0, 0), (150, 100));
+    assert_corners!(h, sibling, (150, 0), (300, 100));
+}
+
+// Reduced from seed 104 at depth 5: a widget widening its own window
+// contract must not erase the bound's crossing at a quarter-window of 173.
+#[test]
+fn a_widgets_window_contract_cannot_widen_its_bounds_contract() {
+    fn tree(h: &mut Harness) -> WidgetId {
+        let content = rect(Color::RED)
+            .width(137)
+            .min_height(194)
+            .add_strong(&mut h.rsc);
+        let scroll = Scroll::new(content, Axis::X).add_strong(&mut h.rsc);
+        let probe = rect(Color::RED).add_strong(&mut h.rsc);
+        let id = probe.id();
+        let wide = rect(Color::GREEN).add_strong(&mut h.rsc);
+        let narrow = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let branch = iris::random::Branch {
+            probe,
+            wide,
+            narrow,
+            threshold: 459.0,
+        }
+        .min_width(94)
+        .max_width(173)
+        .add_strong(&mut h.rsc);
+        let stack = Stack {
+            children: vec![scroll, branch],
+            size: StackSize::Child(0),
+        }
+        .add_strong(&mut h.rsc);
+        let mut children: Vec<StrongWidget> = (0..3)
+            .map(|_| rect(Color::RED).add_strong(&mut h.rsc).any())
+            .collect();
+        children.push(stack);
+        h.set_root(
+            Span {
+                children,
+                dir: Dir::RIGHT,
+                gap: Px::ZERO,
+            }
+            .height(rel(1)),
+        );
+        id
+    }
+    let mut warm = Harness::new((1920, 1200));
+    let probe = tree(&mut warm);
+    warm.resize((640, 900));
+    warm.frame();
+    assert_corners!(warm, probe, (480, 0), (640, 40));
+    let mut cold = Harness::new((640, 900));
+    let other = tree(&mut cold);
+    assert_eq!(warm.region(&probe), cold.region(&other));
+}
+
+#[test]
+fn a_fixed_declaration_keeps_its_cap_when_the_row_has_no_leftover() {
+    let mut h = Harness::new((100, 100));
+    let fixed = rect(Color::RED).width(200).max_width(100).add(&mut h.rsc);
+    let share = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((fixed, share).span(Dir::RIGHT));
+    assert_corners!(h, fixed, (0, 0), (100, 100));
+}
+
+#[test]
+fn a_new_bound_reaches_the_parent_even_when_the_current_answer_is_unchanged() {
+    let mut h = Harness::new((400, 100));
+    let leaf = rect(Color::RED).add(&mut h.rsc);
+    let inner = leaf.add_strong(&mut h.rsc);
+    let root = Scroll::new(inner, Axis::Y)
+        .pad(Padding::uniform(0))
+        .width(154)
+        .height(100)
+        .add(&mut h.rsc);
+    h.set_root(root);
+    assert_eq!(h.region(&leaf).unwrap().size().x, Px::from_int(154));
+    h.rsc.widgets_mut().set_min_len(leaf, Axis::X, 140.into());
+    h.rsc.widgets_mut().set_len(root, Axis::X, 78);
+    h.frame();
+    assert_corners!(h, leaf, (130, 0), (270, 100));
+}

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::from(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
+/// No simplification is larger, which is the half of "the shrinker stops" a
-/// fixed point instead of circling. A shrinker that can return to a tree it
+/// widget count can see. Most are not smaller either -- a dropped alignment
-/// has already tried does not stop.
+/// 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

@@ -45,7 +45,6 @@ fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>
 
 struct Layered {
     children: [StrongWidget<Rect>; 2],
-    _revision: usize,
 }
 
 impl Widget for Layered {
@@ -65,14 +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 {
+    let root = Layered { children }.add(&mut h.rsc);
-        children,
-        _revision: 0,
-    }
-    .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());
@@ -181,9 +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
+    // Marked with nothing about it changed, and it reports the same size
-    // widget returns the same size, so the parent has nothing to lay out.
+    // either way, so the parent has nothing to lay out.
-    let _ = h.rsc.widgets_mut().get_dyn_mut(first.id());
+    h.rsc.widgets_mut().mark_for_redraw(first.id());
     h.frame();
 
     assert_eq!(draws.get(), settled + 1);
@@ -198,7 +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));
@@ -660,7 +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));
 }
@@ -774,6 +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
@@ -919,15 +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| {
+        assert_eq!(
-            let active = &h.render.active[&id];
+            mask_bounds(&warm, warm.render.active[&leaf.id()].mask),
-            let mask = &h.rsc.ui().masks[active.mask.idx()];
+            mask_bounds(&cold, cold.render.active[&other.id()].mask)
-            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()));
     }
 }
 
@@ -994,7 +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()));
@@ -1184,15 +1185,10 @@ fn padding_and_stack_boxes_follow_the_region_without_drawing_again() {
                 assert_eq!(warm.region(&a), cold.region(&b));
                 assert_eq!(primitive_bounds(&warm, a), primitive_bounds(&cold, b));
             }
-            let mask = |h: &Harness, id: WidgetId| {
+            assert_eq!(
-                let active = &h.render.active[&id];
+                mask_bounds(&warm, warm.render.active[&leaf.id()].mask),
-                let mask = &h.rsc.ui().masks[active.mask.idx()];
+                mask_bounds(&cold, cold.render.active[&other.id()].mask)
-                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()));
         }
     }
 }
@@ -1374,20 +1370,14 @@ fn a_redrawn_mask_keeps_reused_primitives_clipped_when_it_moves() {
         h.set_root((first, masked).span(Dir::DOWN));
         let mask = h.render.active[&masked.id()].mask;
         let settled = draws.get();
-        h.rsc.widgets_mut().get_dyn_mut(masked.id());
+        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();
-        let clip = h.rsc.ui().masks[mask.idx()];
+        assert_eq!(mask_bounds(&h, mask), h.region(&masked).unwrap());
-        let clip = h
-            .render
-            .moves
-            .resolve(clip.move_idx, clip.region)
-            .to_px(h.render.output_size());
-        assert_eq!(clip, h.region(&masked).unwrap());
         assert_corners!(h, inner, (0, 10), (400, 200));
     }
 }
@@ -1480,14 +1470,17 @@ fn a_redrawn_subtree_is_not_undrawn_by_the_parent_it_left() {
 
 /// 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.
+/// 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
     }
@@ -1511,6 +1504,7 @@ fn widening_what_a_drawing_holds_for_does_not_relay_out_the_parent() {
     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));
@@ -1533,3 +1527,66 @@ fn widening_what_a_drawing_holds_for_does_not_relay_out_the_parent() {
         "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"
+    );
+}
+
+/// A container that measures a child by drawing it, takes that drawing back,
+/// and then places the child, which `place_at` answers by asking again: taking
+/// a child back takes it out of the child list, and the note on the child
+/// saying it is in there has to go with it, or the placement re-expresses a
+/// drawing that no longer exists.
+#[test]
+fn a_child_taken_back_and_placed_again_is_asked_again() {
+    struct Retake(StrongWidget);
+
+    impl Widget for Retake {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget(&self.0);
+            painter.undraw(&self.0);
+            painter.place_at(&self.0, PlaceDesc::WHOLE).size()
+        }
+    }
+
+    let mut h = Harness::new((600, 200));
+    let child = rect(Color::RED).add_strong(&mut h.rsc);
+    let id = child.id();
+    let root = Retake(child).add(&mut h.rsc);
+    h.set_root(root);
+    h.frame();
+
+    assert_corners!(h, id, (0, 0), (600, 200));
+}

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));
@@ -127,6 +127,11 @@ fn wrapping_content_beside_a_fixed_length_is_stable_warm_and_cold() {
 /// parent would place the part it cut off, and the framework would put a
 /// drawing longer than its box somewhere. `Masked` is the second of these
 /// after `Scroll`, and the assertion in `draw_at` is what says so.
+// What it checks is a debug assertion, which a release build does not compile
+// -- and a `should_panic` test of one fails there rather than passing
+// vacuously, so it is not built either. Every measurement rig here is run in
+// release, so `cargo test --release` has to pass.
+#[cfg(debug_assertions)]
 #[test]
 #[should_panic = "clips to"]
 fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
@@ -142,8 +147,8 @@ 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);
-    h.frame();
 }
 
 /// Content that fits sits in the viewport, not in a box of the window's
@@ -161,3 +166,26 @@ fn content_that_fits_is_placed_in_the_viewport_and_not_in_the_window() {
     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 wrapped = wtext("Wrapping shapes")
-    let sized = wrapped.width(76).add(&mut h.rsc);
+        .size(16)
-    let aligned = sized;
+        .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![
+    vec![plain.id(), wrapped.id(), stack.id(), root.id()]
-        plain.id(),
-        wrapped.id(),
-        sized.id(),
-        aligned.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();
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-    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"));
 }
 
-/// 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 inner = (text,).span(Dir::RIGHT).sized((189, 176)).add(&mut h.rsc);
-    let sized = inner.sized((189, 176)).add(&mut h.rsc);
     let filler = rect(Color::RED).add(&mut h.rsc);
-    let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
+    let root = (filler, inner).span(Dir::RIGHT).add(&mut h.rsc);
     h.state.root = Some(root.add_strong(&mut h.rsc));
-    vec![
+    vec![text.id(), inner.id(), filler.id(), root.id()]
-        text.id(),
-        aligned.id(),
-        inner.id(),
-        sized.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();
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-    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"));
 }
 
-/// 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));
+    h.state.root = Some(span.add_strong(&mut h.rsc));
-    (
+    (vec![wrapped.id(), plain.id(), span.id()], span)
-        vec![wrapped.id(), plain.id(), span.id(), aligned.id()],
-        span_handle,
-    )
 }
 
 #[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();
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-    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"));
 }
 
-/// 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 = rect(Color::RED).width(87).add(&mut h.rsc);
-    let fixed = block.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();
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-    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"));
 }
 
 /// 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();
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-    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"));
 }
 
 /// 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();
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-    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"));
 }
 
 /// Ten widgets, of the shape `tests/shrink.rs` reduces the oracle's seed 220
@@ -720,14 +684,7 @@ 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();
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
-    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"));
 }
 
 const PARAGRAPH: &str = "Wrapping shapes one source into as many lines as the \

tests/chain_cost.rs

@@ -8,19 +8,22 @@
 //! submitted and waited on, so this is the GPU's cost and not the recording
 //! loop's -- which is what `draw_cost.rs` measures instead.
 //!
-//! The instances are two pixels wide so that vertex work dominates; a chain
+//! Two fixtures, because the walk happens in both stages. `chain_cost_by_depth`
-//! walk that does not show up against small quads will not show up against
+//! draws instances two pixels wide so that vertex work dominates; a walk that
-//! anything.
+//! does not show up against small quads will not show up against anything.
-//!
+//! `mask_cost_by_depth` draws one screenful through a mask instead, which is
-//! The instance is leaked deliberately, for the reason `draw_cost.rs` gives.
+//! where a walk in the fragment stage would show and nowhere else.
 
 use iris::prelude::*;
 use iris_core::{
-    Len, MaskIdx, MoveIdx, PrimitiveInst, RectPrimitive, UiData, UiRegion, UiRenderNode,
+    Len, Mask, MaskIdx, MoveIdx, PrimitiveInst, RectPrimitive, UiData, UiRegion, UiRenderNode,
     UiRenderState, UiSpan,
 };
 use wgpu::{Color as GpuColor, *};
 
+#[path = "gpu/mod.rs"]
+mod gpu;
+
 const SIZE: u32 = 1024;
 const INSTANCES: usize = 200_000;
 const FRAMES: u32 = 20;
@@ -29,18 +32,7 @@ const FRAMES: u32 = 20;
 const BATCHES: u32 = 8;
 
 fn gpu() -> Option<(Device, Queue, f32)> {
-    let all = Instance::new(InstanceDescriptor::new_without_display_handle());
+    let adapter = gpu::adapter()?;
-    let instance = match pollster::block_on(all.request_adapter(&RequestAdapterOptions::default()))
-    {
-        Ok(_) => all,
-        Err(_) => Instance::new(InstanceDescriptor {
-            backends: Backends::GL,
-            ..InstanceDescriptor::new_without_display_handle()
-        }),
-    };
-    let instance: &'static Instance = Box::leak(Box::new(instance));
-    let adapter =
-        pollster::block_on(instance.request_adapter(&RequestAdapterOptions::default())).ok()?;
     if !adapter.features().contains(Features::TIMESTAMP_QUERY) {
         println!("no timestamp queries on {:?}", adapter.get_info().name);
         return None;
@@ -55,22 +47,17 @@ fn gpu() -> Option<(Device, Queue, f32)> {
     Some((device, queue, period))
 }
 
-fn config(format: TextureFormat) -> SurfaceConfiguration {
+/// Which stage the fill puts the work in: many small quads, where a walk per
-    SurfaceConfiguration {
+/// vertex is what shows, or one screenful of masked rows, where a walk per
-        usage: TextureUsages::RENDER_ATTACHMENT,
+/// fragment would.
-        format,
+#[derive(Clone, Copy)]
-        color_space: SurfaceColorSpace::Auto,
+enum Fixture {
-        width: SIZE,
+    Quads,
-        height: SIZE,
+    Masked,
-        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) {
+fn fill(ui: &mut UiData, render: &mut UiRenderState, depth: usize, fixture: Fixture) {
     let kind = ui.primitives.kind::<RectPrimitive>();
     let id = ui.widgets.add_strong(Rect::new(UiColor::WHITE)).id();
 
@@ -80,20 +67,51 @@ fn fill(ui: &mut UiData, render: &mut UiRenderState, depth: usize) {
     }
 
     let px = |v: f32| Len::px(v);
-    for i in 0..INSTANCES {
+    let rows = SIZE as usize;
-        let x = (i % (SIZE as usize / 2)) as f32 * 2.0;
+    let mask_idx = match fixture {
-        let y = (i / (SIZE as usize / 2)) as f32;
+        Fixture::Quads => MaskIdx::NONE,
+        // Its own chain as long as the instances', since a viewport sits as
+        // deep in the tree as the content it clips.
+        Fixture::Masked => {
+            let idx = ui.masks.push(Mask {
+                region: UiRegion::FULL,
+                move_idx: slot,
+            });
+            // Nothing frees it here, but the owner's reference is what a real
+            // one is kept alive by.
+            ui.masks.push_ref(idx);
+            idx
+        }
+    };
+    let instances = match fixture {
+        Fixture::Quads => INSTANCES,
+        Fixture::Masked => rows,
+    };
+    for i in 0..instances {
+        let region = match fixture {
+            Fixture::Quads => {
+                let x = (i % (rows / 2)) as f32 * 2.0;
+                let y = (i / (rows / 2)) as f32;
+                UiRegion::new(
+                    UiSpan::new(px(x), px(x + 2.0)),
+                    UiSpan::new(px(y), px(y + 1.0)),
+                )
+            }
+            // A full row each, so one screenful of fragments goes through the
+            // mask and the vertex stage is four corners per row.
+            Fixture::Masked => UiRegion::new(
+                UiSpan::new(px(0.0), px(SIZE as f32)),
+                UiSpan::new(px(i as f32), px(i as f32 + 1.0)),
+            ),
+        };
         render.layers.write(
             0,
             PrimitiveInst {
                 kind,
                 id,
                 primitive: RectPrimitive::color(UiColor::WHITE),
-                region: UiRegion::new(
+                region,
-                    UiSpan::new(px(x), px(x + 2.0)),
+                mask_idx,
-                    UiSpan::new(px(y), px(y + 1.0)),
-                ),
-                mask_idx: MaskIdx::NONE,
                 move_idx: slot,
             },
         );
@@ -101,28 +119,15 @@ 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 {
+fn pass_cost(device: &Device, queue: &Queue, period: f32, depth: usize, fixture: Fixture) -> 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);
+    fill(&mut ui, &mut render, depth, fixture);
     node.update(device, queue, &mut ui, &mut render);
 
-    let target = device.create_texture(&TextureDescriptor {
+    let target = gpu::target(device, format, SIZE, false);
-        label: Some("chain cost"),
-        size: Extent3d {
-            width: SIZE,
-            height: SIZE,
-            depth_or_array_layers: 1,
-        },
-        mip_level_count: 1,
-        sample_count: 1,
-        dimension: TextureDimension::D2,
-        format,
-        usage: TextureUsages::RENDER_ATTACHMENT,
-        view_formats: &[],
-    });
     let view = target.create_view(&TextureViewDescriptor::default());
 
     let queries = device.create_query_set(&QuerySetDescriptor {
@@ -202,17 +207,15 @@ fn pass_cost(device: &Device, queue: &Queue, period: f32, depth: usize) -> f64 {
     best
 }
 
-#[test]
+fn by_depth(fixture: Fixture, instances: usize) {
-#[ignore = "measurement, not a check"]
-fn chain_cost_by_depth() {
     let Some((device, queue, period)) = gpu() else {
         println!("no gpu with timestamps; nothing measured");
         return;
     };
-    println!("{INSTANCES} instances, {SIZE}x{SIZE}, best of {BATCHES} batches");
+    println!("{instances} instances, {SIZE}x{SIZE}, best of {BATCHES} batches");
     let mut base = None;
     for depth in [1, 2, 4, 8, 16, 32, 64] {
-        let ns = pass_cost(&device, &queue, period, depth);
+        let ns = pass_cost(&device, &queue, period, depth, fixture);
         let base = *base.get_or_insert(ns);
         println!(
             "depth {depth:>3}: {:>9.1} us   {:+6.1}% against depth 1",
@@ -221,3 +224,19 @@ fn chain_cost_by_depth() {
         );
     }
 }
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn chain_cost_by_depth() {
+    by_depth(Fixture::Quads, INSTANCES);
+}
+
+/// One screenful of rows, every one clipped by a mask whose own chain is that
+/// deep. What this says that the quads cannot is whether a mask costs the walk
+/// once per instance or once per fragment: at a screenful of fragments per
+/// chain, the second is the difference between these two tables.
+#[test]
+#[ignore = "measurement, not a check"]
+fn mask_cost_by_depth() {
+    by_depth(Fixture::Masked, SIZE as usize);
+}

tests/children_cost.rs

@@ -0,0 +1,59 @@
+//! What one container's draw costs against the number of children it has.
+//!
+//!     cargo test --release --test children_cost -- --ignored --nocapture
+//!
+//! Every other rig here varies depth, the window, or what changed between
+//! frames; this one varies width, which is the dimension a container's own
+//! per-child bookkeeping is counted in. A list of rows is the shape that gets
+//! wide -- a transcript, a file tree -- and a cost per child that is not flat
+//! down this table is a cost paid twice for every child added.
+//!
+//! Wall time rather than instructions, because what is being told apart here
+//! is a factor rather than a few percent, and the table says which it is: a
+//! flat right-hand column is linear and a rising one is not.
+
+mod rig;
+
+use iris::harness::Harness;
+use iris::prelude::*;
+use rig::env;
+use std::time::Instant;
+
+/// A column of leaves each with a length of its own, so the span asks every
+/// one of them and reads what each answered.
+fn build(h: &mut Harness, children: usize) -> WidgetId {
+    let mut col = Span::empty(Dir::DOWN);
+    for _ in 0..children {
+        col.push(
+            rect(Color::RED)
+                .height(LayoutLen::px(4.0))
+                .add_strong(&mut h.rsc),
+        );
+    }
+    let root = col.add(&mut h.rsc);
+    h.set_root(root);
+    root.id()
+}
+
+#[test]
+#[ignore = "measurement, not a check"]
+fn draw_cost_by_children() {
+    let frames = env("FRAMES", 40_usize);
+    println!("{frames} full redraws of one span, per child in the last column");
+    for children in [100_usize, 200, 400, 800, 1600] {
+        // Tall enough that no child is collapsed for want of room.
+        let mut h = Harness::new((600.0, children as f32 * 8.0));
+        let root = build(&mut h, children);
+        h.frame();
+        let start = Instant::now();
+        for _ in 0..frames {
+            h.rsc.widgets_mut().mark_for_redraw(root);
+            h.frame();
+        }
+        let ms = start.elapsed().as_secs_f64() * 1000.0 / frames as f64;
+        println!(
+            "children {children:>5}: {ms:>8.3} ms per redraw, {:>7.4} ms each",
+            ms / children as f64
+        );
+    }
+}

tests/deferred_generated.rs

@@ -0,0 +1,85 @@
+mod rig;
+#[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 depth = rig::env("IRIS_DEFERRED_DEPTH", 4_usize);
+    let seeds = rig::seeds("IRIS_DEFERRED_SEED", "IRIS_DEFERRED_SEEDS", 20);
+    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(),
+                        // Both sides an expression, the one shape that
+                        // copies a request's nodes into another's.
+                        3 => leftover(1).min(px(40)).max(leftover(2).min(px(70))).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 {
+                    let bound = &mut rules[axis].bound;
+                    if bound.min.is_some() {
+                        bound.min = Some(Len::rel(0.25));
+                    }
+                    if bound.max.is_some() {
+                        bound.max = Some(Len::rel(0.75));
+                    }
+                }
+            }
+        });
+    });
+}
+
+#[test]
+fn preferred_requests_with_independent_bounds_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].request = Some(match index % 4 {
+                        0 => leftover(1).into(),
+                        1 => rel(0.5).into(),
+                        2 => px(80).into(),
+                        _ => (leftover(1) + px(30)).min(leftover(2)),
+                    });
+                }
+            }
+        });
+    });
+}

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
+    let adapter = gpu::adapter()?;
-    // 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()?;
     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

@@ -11,11 +11,13 @@
 //! `IRIS_GENERATED_SEED`, `IRIS_GENERATED_SEEDS` and `IRIS_GENERATED_DEPTH`
 //! select what the long run covers.
 
+mod rig;
 #[path = "scenario/mod.rs"]
 mod scenario;
 
 use iris::random::{Edits, Plan, plan};
-use scenario::{ALL, Case, diverges, env, over_seeds};
+use rig::env;
+use scenario::{ALL, Case, diverges, over_seeds};
 
 /// How deep the generator branches. The generator widens two to four ways per
 /// level, so depth is exponential in width and a deep narrow tree is not
@@ -25,10 +27,14 @@ fn depth() -> usize {
     env("IRIS_GENERATED_DEPTH", 4)
 }
 
-/// The seeds the ordinary tests take. Seven that have never failed; 86,
+/// The seeds the ordinary tests take: a corpus rather than a set of
-/// which a `Scroll` fixed point once settled differently on; and 20, which
+/// regression cases, since a seed names a tree only for as long as the
-/// caught a locally redrawn widget being placed twice in the box its parent
+/// generator draws the same things in the same order. Adding images to the
-/// had already placed it in.
+/// 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) {
@@ -104,16 +110,10 @@ 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")
+    let seeds = rig::seeds("IRIS_GENERATED_SEED", "IRIS_GENERATED_SEEDS", 100);
-        .ok()
-        .and_then(|v| v.parse().ok())
-    {
-        Some(seed) => vec![seed],
-        None => (1..=env("IRIS_GENERATED_SEEDS", 100_u64)).collect(),
-    };
     over_seeds(seeds, |seed| {
         let grown = plan(seed, depth, &Edits::default());
         for case in ALL {

tests/gpu/mod.rs

@@ -0,0 +1,66 @@
+//! The adapter, the surface configuration and the target the GPU rigs share,
+//! so no two of them can probe for a device or make a target in 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![],
+    }
+}
+
+/// A square colour target to draw a pass into. `copy` adds the usage a rig
+/// that reads the pixels back needs; one that only times the pass does not.
+// This module is compiled into each rig target separately, so a helper the
+// ones that make no target of their own do not call is dead code there.
+#[allow(dead_code)]
+pub fn target(device: &Device, format: TextureFormat, size: u32, copy: bool) -> Texture {
+    device.create_texture(&TextureDescriptor {
+        label: Some("gpu rig target"),
+        size: Extent3d {
+            width: size,
+            height: size,
+            depth_or_array_layers: 1,
+        },
+        mip_level_count: 1,
+        sample_count: 1,
+        dimension: TextureDimension::D2,
+        format,
+        usage: match copy {
+            true => TextureUsages::RENDER_ATTACHMENT | TextureUsages::COPY_SRC,
+            false => TextureUsages::RENDER_ATTACHMENT,
+        },
+        view_formats: &[],
+    })
+}

tests/layout_diagnostics.rs

@@ -13,15 +13,43 @@
 //!
 //! `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.
+
+mod rig;
 
 use iris::harness::Harness;
 use iris::prelude::*;
-use iris::random::{Edits, Tree, grow};
+use iris::random::{Edits, Tree, build, plan};
+use rig::env;
 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 +65,8 @@ 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());
+    harness.rsc.widgets_mut().mark_for_redraw(root.id());
-    let _ = harness.rsc.widgets_mut().get_dyn_mut(leaf.id());
+    harness.rsc.widgets_mut().mark_for_redraw(leaf.id());
     harness.frame();
 
     let report = diagnostics::take();
@@ -69,13 +97,6 @@ fn a_selected_widget_retains_its_layout_events() {
     diagnostics::clear_traced_widgets();
 }
 
-fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
-    std::env::var(name)
-        .ok()
-        .and_then(|value| value.parse().ok())
-        .unwrap_or(fallback)
-}
-
 #[cfg(feature = "layout-diagnostics")]
 fn trace_selected(tree: &Tree) {
     let Ok(value) = std::env::var("IRIS_TRACE_INDEX") else {
@@ -101,9 +122,17 @@ 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 +214,7 @@ 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 +224,6 @@ 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 +231,7 @@ 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 +246,7 @@ 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);
             }
         });
     }
@@ -232,7 +260,7 @@ fn layout_cost() {
             harness
                 .rsc
                 .widgets_mut()
-                .set_size_rule(sized, Axis::X, SizeRule::Exact(len));
+                .set_size_rule(sized, Axis::X, SizeRule::from(len));
         });
     }
 
@@ -251,6 +279,21 @@ 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

@@ -8,17 +8,14 @@
 //!
 //! 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.
+//! 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.
+
+mod rig;
 
 use iris::harness::Harness;
-use iris::random::{Edits, grow};
+use iris::random::{Edits, build, plan};
-
+use rig::env;
-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"]
@@ -28,7 +25,11 @@ fn every_cold_layout_is_printed() {
     let mut out = String::new();
     for seed in 1..=seeds {
         let mut harness = Harness::new((1920.0, 1200.0));
-        let (root, tree) = grow(&mut harness.rsc, seed, depth, &Edits::default());
+        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() {

tests/mask_clip.rs

@@ -0,0 +1,162 @@
+//! Which pixels a mask lets through, read back off the GPU.
+//!
+//!     cargo test --release --test mask_clip -- --ignored --nocapture
+//!
+//! Ignored because it needs a device, which not every machine running the
+//! suite has -- and a deliberate run on one without fails rather than passing
+//! with nothing checked. Nothing else here sees a mask at all: `iris::harness`
+//! draws no pixels, and a mask's rectangle is resolved through its own move
+//! chain in the shader, so the CPU's idea of it is not what clips anything.
+
+use iris::prelude::*;
+use iris_core::{
+    Len, Mask, MoveIdx, PrimitiveInst, RectPrimitive, UiData, UiRegion, UiRenderNode,
+    UiRenderState, UiSpan,
+};
+use wgpu::{Color as GpuColor, *};
+
+#[path = "gpu/mod.rs"]
+mod gpu;
+
+const SIZE: u32 = 256;
+
+/// The mask is a box inside a move chain two links long and the drawing
+/// overflows it on both axes, so what comes back is the mask's own rectangle
+/// composed through that chain -- and a clip resolved through the wrong one,
+/// or not composed at all, lands somewhere else.
+#[test]
+#[ignore = "needs a gpu"]
+fn a_mask_clips_its_own_box_composed_through_its_chain() {
+    let adapter = gpu::adapter().expect("no adapter to draw with");
+    println!("adapter: {:?}", adapter.get_info().name);
+    let (device, queue) = pollster::block_on(adapter.request_device(&DeviceDescriptor::default()))
+        .expect("no device on that adapter");
+    let format = TextureFormat::Bgra8Unorm;
+    let mut node = UiRenderNode::new(&device, &gpu::config(format, SIZE));
+    let mut ui = UiData::default();
+    let mut render = UiRenderState::new();
+    let kind = ui.primitives.kind::<RectPrimitive>();
+    let id = ui.widgets.add_strong(Rect::new(UiColor::WHITE)).id();
+    let px = Len::px;
+
+    let outer = render.moves.push(MoveIdx::NONE, UiRegion::FULL);
+    let shift = (16.0, 24.0);
+    let inner = render.moves.push(
+        outer,
+        UiRegion::new(
+            UiSpan::new(px(shift.0), px(shift.0) + Len::FULL),
+            UiSpan::new(px(shift.1), px(shift.1) + Len::FULL),
+        ),
+    );
+    let clip = (20.0, 30.0, 120.0, 90.0);
+    let mask = ui.masks.push(Mask {
+        region: UiRegion::new(
+            UiSpan::new(px(clip.0), px(clip.2)),
+            UiSpan::new(px(clip.1), px(clip.3)),
+        ),
+        move_idx: inner,
+    });
+    // The owner's reference, which is what keeps a real one alive.
+    ui.masks.push_ref(mask);
+    render.layers.write(
+        0,
+        PrimitiveInst {
+            kind,
+            id,
+            primitive: RectPrimitive::color(UiColor::WHITE),
+            region: UiRegion::new(
+                UiSpan::new(px(0.0), px(200.0)),
+                UiSpan::new(px(0.0), px(200.0)),
+            ),
+            mask_idx: mask,
+            move_idx: inner,
+        },
+    );
+    node.update(&device, &queue, &mut ui, &mut render);
+
+    let target = gpu::target(&device, format, SIZE, true);
+    let view = target.create_view(&TextureViewDescriptor::default());
+    let row = SIZE * 4;
+    let readback = device.create_buffer(&BufferDescriptor {
+        label: Some("mask clip"),
+        size: (row * SIZE) as u64,
+        usage: BufferUsages::MAP_READ | BufferUsages::COPY_DST,
+        mapped_at_creation: false,
+    });
+    let mut encoder = device.create_command_encoder(&CommandEncoderDescriptor::default());
+    {
+        let pass = &mut encoder.begin_render_pass(&RenderPassDescriptor {
+            label: None,
+            color_attachments: &[Some(RenderPassColorAttachment {
+                view: &view,
+                resolve_target: None,
+                ops: Operations {
+                    load: LoadOp::Clear(GpuColor::BLACK),
+                    store: StoreOp::Store,
+                },
+                depth_slice: None,
+            })],
+            depth_stencil_attachment: None,
+            timestamp_writes: None,
+            occlusion_query_set: None,
+            multiview_mask: None,
+        });
+        node.draw(pass);
+    }
+    let whole = Extent3d {
+        width: SIZE,
+        height: SIZE,
+        depth_or_array_layers: 1,
+    };
+    encoder.copy_texture_to_buffer(
+        TexelCopyTextureInfo {
+            texture: &target,
+            mip_level: 0,
+            origin: Origin3d::ZERO,
+            aspect: TextureAspect::All,
+        },
+        TexelCopyBufferInfo {
+            buffer: &readback,
+            layout: TexelCopyBufferLayout {
+                offset: 0,
+                bytes_per_row: Some(row),
+                rows_per_image: Some(SIZE),
+            },
+        },
+        whole,
+    );
+    queue.submit(Some(encoder.finish()));
+    let slice = readback.slice(..);
+    slice.map_async(MapMode::Read, |_| {});
+    device
+        .poll(PollType::Wait {
+            submission_index: None,
+            timeout: None,
+        })
+        .expect("the pass did not finish");
+    let pixels = slice.get_mapped_range().expect("the target did not map");
+
+    let mut lit = 0;
+    let mut bounds: Option<(u32, u32, u32, u32)> = None;
+    for y in 0..SIZE {
+        for x in 0..SIZE {
+            if pixels[(y * row + x * 4) as usize] == 0 {
+                continue;
+            }
+            lit += 1;
+            let (x0, y0, x1, y1) = bounds.unwrap_or((x, y, x, y));
+            bounds = Some((x0.min(x), y0.min(y), x1.max(x), y1.max(y)));
+        }
+    }
+    // The clip shifted by the chain. Its far edge is exclusive: a fragment
+    // exactly on it is the first one outside.
+    let want = (
+        (clip.0 + shift.0) as u32,
+        (clip.1 + shift.1) as u32,
+        (clip.2 + shift.0) as u32 - 1,
+        (clip.3 + shift.1) as u32 - 1,
+    );
+    assert_eq!(bounds, Some(want), "{lit} pixels through the mask");
+    let (x0, y0, x1, y1) = want;
+    assert_eq!(lit, (x1 - x0 + 1) * (y1 - y0 + 1), "the clip has a hole");
+}

tests/revision_cost.rs

@@ -1,22 +1,28 @@
-//! What a resize frame costs and what it holds, on a tree the revision before
+//! Text-layout workloads with stable paragraphs for comparisons across revisions.
-//! #16 also builds.
+//! 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
-//! Deliberately written in the API subset `43ce8c7` and this branch share, so
+//! provide the same workload across the full PR.
-//! the same source can be dropped into an old worktree and measured there:
-//! that is the only like-for-like comparison with the code the retained
-//! layout replaced. The random tree cannot carry one, because the generator
-//! itself changed with the work.
 //!
 //!     ROWS=40 FRAMES=500 cargo test --release --test revision_cost \
 //!       -- --ignored --nocapture resize_cost
+//!     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
+//! `text_updates_cost` selects idle, repaint, edit, or scroll with `PHASE`.
-//! `perf stat -e instructions:u` on the test binary directly.
+//! 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.
+
+mod rig;
 
 use iris::harness::Harness;
 use iris::prelude::*;
+use rig::env;
 use std::time::Instant;
 
 /// xorshift64, so one seed is one set of paragraphs on any machine.
@@ -78,13 +84,6 @@ fn words(rng: &mut Rng, least: usize, most: usize) -> String {
 
 const OUTPUT: (f32, f32) = (900.0, 1200.0);
 
-fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
-    std::env::var(name)
-        .ok()
-        .and_then(|value| value.parse().ok())
-        .unwrap_or(fallback)
-}
-
 /// A row of a fixed-width rect beside a column of one wrapping and one
 /// overflowing text: the shape that makes a container measure a child in a
 /// box it will not keep.
@@ -137,9 +136,10 @@ fn resize_cost() {
         println!("paragraph {at}: {:?}", h.region(id));
     }
 
-    // Two widths in turn is the friendly case for anything that remembers an
+    // The sweep cycles 256 widths, avoiding the two-width cache-friendly case.
-    // answer, so `SWEEP=1` never repeats one -- a drag rather than a toggle.
     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 +151,11 @@ 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 +199,73 @@ 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/rig/mod.rs

@@ -0,0 +1,25 @@
+//! What the rigs need and none of them should spell its own way. Every
+//! fuzzer and measurement here is run by hand with its parameters in the
+//! environment, so one reader is shared rather than copied into each target.
+
+/// A rig's parameter from the environment, or its default. A switch is
+/// `env("NAME", 0_u8) != 0`, so `NAME=1` turns it on.
+pub 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)
+}
+
+/// The seeds a scan runs: the one `one` names on its own, or `1..=` the
+/// count `many` gives. One seed replaces the range rather than narrowing
+/// it, which is how a tree a scan failed on is run again by itself.
+// This module is compiled into each rig target separately, so a helper the
+// measurement rigs have no seeds to choose is dead code in those builds.
+#[allow(dead_code)]
+pub fn seeds(one: &str, many: &str, count: u64) -> Vec<u64> {
+    match std::env::var(one).ok().and_then(|seed| seed.parse().ok()) {
+        Some(seed) => vec![seed],
+        None => (1..=env(many, count)).collect(),
+    }
+}

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
@@ -30,13 +30,6 @@ pub fn over_seeds(seeds: Vec<u64>, run: impl Fn(u64) + Sync) {
     });
 }
 
-pub fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
-    std::env::var(name)
-        .ok()
-        .and_then(|v| v.parse().ok())
-        .unwrap_or(fallback)
-}
-
 /// The window a tree is grown in, and the one a resize takes it to.
 const OUTER: (f32, f32) = (1920.0, 1200.0);
 const INNER: (f32, f32) = (640.0, 900.0);
@@ -190,35 +183,48 @@ impl Case {
 
 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> {
+/// A length in pixels, or a cap over one: a rule that reads the box it is
-    Some(LayoutLen::px(20.0 + rng.below(180) as f32))
+/// 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 {
+fn resize_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> SizeRules {
-    let lens = [a_len(rng), a_len(rng)];
+    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) {
+    let taken = RegionAlign::from(align);
-        warm.rsc
+    for axis in Axis::BOTH {
-            .widgets_mut()
+        warm.rsc.widgets_mut().set_alignment(id, axis, taken[axis]);
-            .set_alignment(id, axis, align.unwrap_or_default());
     }
     align
 }
@@ -344,9 +350,21 @@ fn change(case: Case, warm: &mut Harness, tree: &mut Tree, plan: &Plan, rng: &mu
 /// 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() {
+    // The tree a failure names is written out again from what it printed, so
-        Some(len) => format!("{len}"),
+    // every part of a rule prints: the preferred length and the bounds are
-        None => "-".into(),
+    // independent, and a bound lumped into "no rule" could not be rebuilt.
+    let rule = |r: &SizeRule| {
+        let mut out = r
+            .request
+            .as_ref()
+            .map_or_else(String::new, ToString::to_string);
+        if let Some(min) = r.bound.min {
+            out += &format!(">{}", LayoutLen::from(min));
+        }
+        if let Some(max) = r.bound.max {
+            out += &format!("<{}", LayoutLen::from(max));
+        }
+        if out.is_empty() { "-".into() } else { out }
     };
     let align = h.rsc.widgets().alignment(id);
     let side = |a: AxisAlign| {
@@ -363,8 +381,8 @@ 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));
+        out += &format!("[x:{},y:{}]", rule(&rules.x), rule(&rules.y));
     }
     if align != RegionAlign::default() {
         out += &format!("@{},{}", side(align.x), side(align.y));

tests/shrink.rs

@@ -17,11 +17,13 @@
 //! It is a fuzzer: run it once the ordinary tests pass, and turn what it
 //! finds into a test of its own rather than leaving a seed as the record.
 
+mod rig;
 #[path = "scenario/mod.rs"]
 mod scenario;
 
 use iris::random::{Edits, Plan, plan};
-use scenario::{ALL, Case, diverges, env, over_seeds};
+use rig::env;
+use scenario::{ALL, Case, diverges, over_seeds};
 
 /// Takes the first simplification that still fails, until none does. The
 /// simplifications come biggest first, so this walks down rather than
@@ -58,13 +60,7 @@ fn cases() -> Vec<Case> {
 fn no_grown_tree_lays_out_differently_warm_than_cold() {
     let depth: usize = env("SHRINK_DEPTH", 5);
     let cases = cases();
-    let seeds: Vec<u64> = match std::env::var("SHRINK_SEED")
+    let seeds = rig::seeds("SHRINK_SEED", "SHRINK_SEEDS", 400);
-        .ok()
-        .and_then(|v| v.parse().ok())
-    {
-        Some(seed) => vec![seed],
-        None => (1..=env("SHRINK_SEEDS", 400_u64)).collect(),
-    };
     let count = seeds.len();
 
     over_seeds(seeds, |seed| {

tests/suite.rs

@@ -8,6 +8,8 @@
 //! The rigs stay their own targets: `shrink` and `generated` are fuzzers run
 //! on their own, and the `*_cost` and `*_diagnostics` ones are measurements.
 
+#[path = "cases/deferred.rs"]
+mod deferred;
 #[path = "cases/determinism.rs"]
 mod determinism;
 #[path = "cases/drift.rs"]

tests/trace_unsettled.rs

@@ -1,5 +1,5 @@
-//! Traces the six-widget tree in `unsettled.rs`, to see what box its text is
+//! Traces the four-widget trees in `unsettled.rs`, to see what box their text
-//! actually drawn in on a first frame against a settled one.
+//! 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 wrapped = wtext("Wrapping shapes")
-    let sized = wrapped.width(76).add(&mut h.rsc);
+        .size(16)
-    let aligned = sized;
+        .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![
+    vec![plain.id(), wrapped.id(), stack.id(), root.id()]
-        plain.id(),
-        wrapped.id(),
-        sized.id(),
-        aligned.id(),
-        stack.id(),
-        root.id(),
-    ]
 }
 
 fn dump(label: &str, report: &diag::Report, text: WidgetId) {
@@ -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 inner = (text,).span(Dir::RIGHT).sized((189, 176)).add(&mut h.rsc);
-    let sized = inner.sized((189, 176)).add(&mut h.rsc);
     let filler = rect(Color::RED).add(&mut h.rsc);
-    let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
+    let root = (filler, inner).span(Dir::RIGHT).add(&mut h.rsc);
     h.state.root = Some(root.add_strong(&mut h.rsc));
-    vec![
+    vec![text.id(), inner.id(), filler.id(), root.id()]
-        text.id(),
-        aligned.id(),
-        inner.id(),
-        sized.id(),
-        filler.id(),
-        root.id(),
-    ]
 }
 
 #[test]