Keep the step 3/4 experiment as evidence

The worker's uncommitted attempt at evaluating children in parent-decided
boxes, preserved as it stood when it stopped: separate answer-only pixel
reads, per-child drawing contracts, provisional Fill asks in Span, and an
assertion that a placed drawing hold for its answer box. The suite passes
and every generated case stops on that assertion at Text. Not the
protocol; wip/one-ask is.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

core/src/fixed.rs

@@ -10,23 +10,30 @@ use std::{
 /// chain, and the same box summed from what its children asked for -- and has
 /// to decide whether the two are the same place. In floats they land a few
 /// bits apart, which is a defect wherever the answer changes what is drawn
-/// rather than where. Here adding and subtracting are exact and only a
-/// multiply or a conversion rounds, back onto the same steps, so two routes
-/// that come within half a step land on one number and everything downstream
-/// compares for equality instead of for nearness.
+/// rather than where. Here adding and subtracting are exact, a multiply
+/// drops to the step below, and a conversion between grids takes the nearest
+/// one, so two routes to one place land on one number and everything
+/// downstream compares for equality instead of for nearness.
 ///
 /// `SHIFT` is the number of fractional bits, which is what makes the steps
 /// divide a whole number: a power of two also converts to `f32` without
 /// rounding while the value fits in its mantissa.
+///
+/// Arithmetic wraps at the ends of the range, the way the `i32` underneath
+/// does. Saturating instead was measured at a twelfth of layout's
+/// instructions -- five per add against one -- to keep the ordering of
+/// coordinates two million pixels out, where nothing draws anyway. A value
+/// off the end is a defect either way; wrapping makes it an obvious one.
+/// Only [`Self::from_f32`] clamps, since a float has further to come from.
 #[repr(transparent)]
 #[derive(
     Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Default, bytemuck::Pod, bytemuck::Zeroable,
 )]
 pub struct Fixed<const SHIFT: u32>(i32);
 
-/// A length or a coordinate in pixels, to a sixty-fourth. Finer than anything
-/// a display can show, and exact in `f32` up to 262,144 px, which is what lets
-/// the same number reach the GPU.
+/// A length or a coordinate in pixels, in steps of `1/1024`. Finer than
+/// anything a display can show, and exact in `f32` up to 16,384 px, which is
+/// what lets the same number reach the GPU.
 pub type Px = Fixed<PX_SHIFT>;
 
 /// How many bits of a pixel a [`Px`] keeps. One place, because [`PxVec2`]
@@ -56,8 +63,8 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
     /// The gap between neighbouring values, which is also how far apart two
     /// numbers can be and still mean the same place.
     pub const STEP: Self = Self(1);
-    /// Also what stands in for an unbounded end, since arithmetic saturates
-    /// here rather than wrapping past it.
+    /// Also what stands in for an unbounded end: compared against, never
+    /// added to, since arithmetic wraps past it.
     pub const MIN: Self = Self(i32::MIN);
     pub const MAX: Self = Self(i32::MAX);
 
@@ -77,12 +84,13 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
     }
 
     pub const fn from_int(v: i32) -> Self {
-        Self(v.saturating_mul(Self::one().0))
+        Self(v.wrapping_mul(Self::one().0))
     }
 
-    /// Rounds to the nearest step, and saturates rather than wrapping. A NaN
-    /// has no nearest step and becomes zero, which is a caller's mistake
-    /// rather than a value worth carrying.
+    /// Rounds to the nearest step, and clamps to the ends of the grid rather
+    /// than wrapping: this is where a number from outside arrives, and a float
+    /// has the range to be anywhere. A NaN has no nearest step and becomes
+    /// zero, which is a caller's mistake rather than a value worth carrying.
     ///
     /// Half-away is written out rather than called through `f32::round`,
     /// which is not `const`: a layout constant has to stay a constant.
@@ -101,6 +109,18 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
         })
     }
 
+    /// The first step at or above `v`, where [`Self::from_f32`] takes the
+    /// nearest one and is below it half the time. For a bound that has to
+    /// admit the value it came from: a measurement rounded down is a bound
+    /// that leaves out the thing it was measured from.
+    pub const fn ceil_from_f32(v: f32) -> Self {
+        let nearest = Self::from_f32(v);
+        match nearest.to_f32() < v {
+            true => nearest.next_up(),
+            false => nearest,
+        }
+    }
+
     /// From a number as it is written in source -- `16`, `1.5` -- which is
     /// the other place a value enters the grid.
     pub fn from_num(v: impl UiNum) -> Self {
@@ -114,43 +134,41 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
     /// 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 => narrow((self.0 as i64) << (TO - SHIFT)),
-            false => narrow(shift_round(self.0 as i64, SHIFT - TO)),
+            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.saturating_add(rhs.0))
+        Self(self.0.wrapping_add(rhs.0))
     }
 
     pub const fn sub(self, rhs: Self) -> Self {
-        Self(self.0.saturating_sub(rhs.0))
+        Self(self.0.wrapping_sub(rhs.0))
     }
 
     pub const fn neg(self) -> Self {
-        Self(self.0.saturating_neg())
+        Self(self.0.wrapping_neg())
     }
 
     /// Scaled by a number on any grid, which is how a length takes a fraction
     /// of itself and keeps being a length: the product is measured in the
     /// receiver's steps.
+    ///
+    /// Dropped to the step below rather than taken to the nearest one
+    /// (Bryan, 2026-09-16), which costs a share a thousandth of a pixel of
+    /// its row -- less than an even number of pixels draws. Toward negative
+    /// infinity on both sides of zero, since that is a shift and nothing
+    /// else: a value and its negation therefore land different distances
+    /// from where they came, so a flipped span can sit a step from its
+    /// mirror image.
     pub const fn mul<const BY: u32>(self, by: Fixed<BY>) -> Self {
-        Self(narrow(shift_round(self.0 as i64 * by.0 as i64, BY)))
-    }
-
-    /// A part of a span that is often nothing: no part of nothing is
-    /// nothing, for the cost of a comparison rather than a widening
-    /// multiply and a rounding.
-    pub const fn scaled<const BY: u32>(self, by: Fixed<BY>) -> Self {
-        match self.0 == 0 {
-            true => self,
-            false => self.mul(by),
-        }
+        Self(((self.0 as i64 * by.0 as i64) >> BY) as i32)
     }
 
     /// Repeated a whole number of times, which no grid rounds.
     pub const fn mul_int(self, by: i32) -> Self {
-        Self(narrow(self.0 as i64 * by as i64))
+        Self(self.0.wrapping_mul(by))
     }
 
     /// Divided into a whole number of parts, rounded to the nearest step.
@@ -159,12 +177,13 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
         if by == 0 {
             return Self::ZERO;
         }
-        Self(narrow(div_round(self.0 as i64, by as i64)))
+        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 saturates so that a
-    /// release build lays out something absurd rather than dying.
+    /// 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 {
@@ -173,7 +192,7 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
                 false => Self::MAX,
             };
         }
-        Self(narrow(div_round((self.0 as i64) << BY, by.0 as i64)))
+        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
@@ -183,13 +202,13 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
         if den.0 == 0 {
             return Self::ZERO;
         }
-        Self(narrow(div_round((num.0 as i64) << SHIFT, den.0 as i64)))
+        Self(div_round((num.0 as i64) << SHIFT, den.0 as i64) as i32)
     }
 
     /// `from` and `to` a fraction of the way apart, the fraction being the
     /// receiver -- the argument order [`crate::util::LerpUtil`] already uses.
     pub const fn lerp<const OF: u32>(self, from: Fixed<OF>, to: Fixed<OF>) -> Fixed<OF> {
-        from.add(to.sub(from).scaled(self))
+        from.add(to.sub(from).mul(self))
     }
 
     pub const fn min(self, other: Self) -> Self {
@@ -207,7 +226,7 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
     }
 
     pub const fn abs(self) -> Self {
-        Self(self.0.saturating_abs())
+        Self(self.0.wrapping_abs())
     }
 
     pub const fn clamp(self, lo: Self, hi: Self) -> Self {
@@ -219,11 +238,11 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
     /// boundary. The step is the whole gap, so there is nothing to exclude
     /// between this and the boundary itself.
     pub const fn next_up(self) -> Self {
-        Self(self.0.saturating_add(1))
+        Self(self.0.wrapping_add(1))
     }
 
     pub const fn next_down(self) -> Self {
-        Self(self.0.saturating_sub(1))
+        Self(self.0.wrapping_sub(1))
     }
 }
 
@@ -260,6 +279,8 @@ pub(crate) const fn div_toward(num: i64, den: i64, up: bool) -> i64 {
     }
 }
 
+/// Clamped to the ends, unlike a [`Fixed`]'s own arithmetic: a range of box
+/// lengths that runs past `i32` really is unbounded.
 pub(crate) const fn narrow(v: i64) -> i32 {
     if v > i32::MAX as i64 {
         return i32::MAX;
@@ -363,6 +384,12 @@ impl<const SHIFT: u32> FixedVec2<SHIFT> {
         Self::new(Fixed::from_f32(v.x), Fixed::from_f32(v.y))
     }
 
+    /// The first step at or above each part, for a measurement reported as a
+    /// box: what it occupies is not less than what was measured.
+    pub fn ceil_from_f32(v: Vec2) -> Self {
+        Self::new(Fixed::ceil_from_f32(v.x), Fixed::ceil_from_f32(v.y))
+    }
+
     pub fn to_f32(self) -> Vec2 {
         Vec2::new(self.x.to_f32(), self.y.to_f32())
     }
@@ -454,29 +481,54 @@ mod tests {
         assert_eq!(Px::from_int(100) * Rel::ZERO, Px::ZERO);
     }
 
+    /// Toward negative infinity on both sides of zero, which is what makes
+    /// it a shift rather than a shift and a sign branch -- and what makes a
+    /// value and its negation land different distances from where they came,
+    /// so a flipped span can sit a step from its mirror image.
     #[test]
-    fn halves_round_away_from_zero_either_side() {
+    fn a_multiply_drops_to_the_step_below_on_both_sides_of_zero() {
         // A step and a half of one, which has no step of its own.
         let step_and_a_half = Rel::from_f32(1.5).div_int(Px::ONE.raw());
-        assert_eq!(Px::ONE * step_and_a_half, Px::from_raw(2));
+        assert_eq!(Px::ONE * step_and_a_half, Px::from_raw(1));
         assert_eq!(Px::ONE.neg() * step_and_a_half, Px::from_raw(-2));
     }
 
+    /// A division rounds to the nearest step, so it cannot put back the
+    /// steps a truncating multiply dropped: a round trip comes back short,
+    /// never long, and by the few steps the two operations gave up.
     #[test]
-    fn dividing_by_a_fraction_undoes_multiplying_by_it() {
+    fn dividing_by_a_fraction_cannot_undo_a_truncating_multiply() {
         let third = Rel::ONE / Rel::from_int(3);
         let len = Px::from_int(300);
-        assert_eq!(len * third / third, len);
+        let back = len * third / third;
+        assert!(back <= len, "{back:?} is longer than {len:?}");
+        assert!(len - back <= Px::from_raw(3), "{back:?} against {len:?}");
         assert_eq!(Px::from_int(100) / Rel::from_f32(0.5), Px::from_int(200));
     }
 
+    /// The bound a greedy line break needs: the width it was measured at is
+    /// not on the grid, and the narrowest box the break still holds for is
+    /// the step at or above it, never the one below.
     #[test]
-    fn arithmetic_saturates_rather_than_wrapping() {
-        assert_eq!(Px::MAX + Px::ONE, Px::MAX);
-        assert_eq!(Px::MIN - Px::ONE, Px::MIN);
+    fn a_ceiling_never_lands_below_the_number_it_came_from() {
+        let step = 1.0 / (1 << PX_SHIFT) as f32;
+        for n in 0..64 {
+            let v = 189.0 + n as f32 * step / 3.0;
+            let up = Px::ceil_from_f32(v);
+            assert!(up.to_f32() >= v, "{up:?} is below {v}");
+            assert!(
+                up.to_f32() - v < step,
+                "{up:?} is more than a step above {v}"
+            );
+        }
+        // An exact step is its own ceiling.
+        assert_eq!(Px::ceil_from_f32(189.5), Px::from_f32(189.5));
+    }
+
+    #[test]
+    fn a_number_from_outside_is_clamped_to_the_grid() {
         assert_eq!(Px::from_f32(1e12), Px::MAX);
         assert_eq!(Px::from_f32(-1e12), Px::MIN);
-        assert_eq!(Px::from_int(i32::MAX), Px::MAX);
     }
 
     #[test]

core/src/layout_diagnostics.rs

@@ -54,10 +54,13 @@ pub(crate) enum Counter {
     TextShapes,
     TextBreaks,
     GlyphPlacements,
+    OutsidePinnedLen,
+    OutsideFrame,
+    OutsideExtent,
 }
 
 impl Counter {
-    const COUNT: usize = Self::GlyphPlacements as usize + 1;
+    const COUNT: usize = Self::OutsideExtent as usize + 1;
 
     const NAMES: [&'static str; Self::COUNT] = [
         "updates",
@@ -89,6 +92,9 @@ impl Counter {
         "text shapes",
         "text line breaks",
         "glyph placements",
+        "reuse outside: the length it was pinned to",
+        "reuse outside: a frame length",
+        "reuse outside: an extent length",
     ];
 }
 

core/src/orientation/align.rs

@@ -84,8 +84,7 @@ pub struct RegionAlign {
 }
 
 impl RegionAlign {
-    /// Both axes at the near edge. What a container passes as an override for
-    /// a child it is going to position itself.
+    /// 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,

core/src/orientation/axis.rs

@@ -7,6 +7,17 @@ pub enum Axis {
     Y,
 }
 
+impl Axis {
+    /// A per-axis pair with `aligned` on this axis and `ortho` on the other,
+    /// which is what `from_axis` does for a vector.
+    pub fn pair<T>(self, aligned: T, ortho: T) -> [T; 2] {
+        match self {
+            Self::X => [aligned, ortho],
+            Self::Y => [ortho, aligned],
+        }
+    }
+}
+
 impl std::ops::Not for Axis {
     type Output = Self;
 

core/src/orientation/len.rs

@@ -125,6 +125,13 @@ impl Size {
             Axis::Y => self.y,
         }
     }
+
+    pub fn axis_mut(&mut self, axis: Axis) -> &mut LayoutLen {
+        match axis {
+            Axis::X => &mut self.x,
+            Axis::Y => &mut self.y,
+        }
+    }
 }
 
 impl LayoutLen {
@@ -151,6 +158,18 @@ impl LayoutLen {
         Len::from_parts(self.rel.add(share), self.px)
     }
 
+    /// This length, given as a part of a box `len` long, as a part of the
+    /// box `len` is itself a part of. The share is untouched: it is a claim
+    /// on whoever divides the room, not a fraction of anything.
+    pub const fn within_len(self, len: Len) -> Self {
+        let part = Len::from_parts(self.rel, self.px).within_len(len);
+        Self {
+            px: part.px,
+            rel: part.rel,
+            leftover: self.leftover,
+        }
+    }
+
     pub fn px(px: impl UiNum) -> Self {
         Self {
             px: Px::from_num(px),

core/src/orientation/pos.rs

@@ -219,7 +219,7 @@ impl Len {
         }
     }
 
-    pub fn within_len(&self, len: Len) -> Self {
+    pub const fn within_len(&self, len: Len) -> Self {
         self.within(&UiSpan {
             start: Len::ZERO,
             end: len,
@@ -282,26 +282,12 @@ impl UiSpan {
         self.end += offset;
     }
 
-    /// The whole of the box it sits in: a span that composes to nothing and
-    /// a parent that changes nothing.
-    pub const fn is_full(&self) -> bool {
-        self.start.rel.raw() == Rel::ZERO.raw()
-            && self.start.px.raw() == Px::ZERO.raw()
-            && self.end.rel.raw() == Rel::ONE.raw()
-            && self.end.px.raw() == Px::ZERO.raw()
-    }
-
+    /// Composing a box through the one it sits in, and the hottest line in
+    /// layout. It used to skip the multiplies where a span was the whole of
+    /// its parent or the parent the whole of its own; both come out of the
+    /// multiply unchanged anyway, and the body those comparisons cost was
+    /// what kept the inliner from taking this at all.
     pub const fn within(&self, parent: &Self) -> Self {
-        // A part that is the whole box is the box, and a box composed through
-        // the whole of its parent is itself. Both are exact -- multiplying by
-        // one rounds to what it started as -- and both are common enough to
-        // be worth four comparisons rather than four multiplies to find out.
-        if self.is_full() {
-            return *parent;
-        }
-        if parent.is_full() {
-            return *self;
-        }
         Self {
             start: self.start.within(parent),
             end: self.end.within(parent),

core/src/primitive/text.rs

@@ -107,13 +107,6 @@ impl Default for TextAttrs {
     }
 }
 
-/// How far below the longest line a width may fall and still be answered by
-/// the break in hand. A parent that offers a child the length it reported
-/// composes that length back through the box chain, so the two differ in the
-/// last bits -- and at exactly the longest line, that decides whether a line
-/// fits. Sub-pixel, so no break it admits is one a reader could see.
-const BREAK_EPSILON_PX: f32 = 0.05;
-
 /// Keeps text and its corresponding layout from getting out of sync.
 pub struct TextBuffer {
     text: String,
@@ -183,6 +176,23 @@ impl TextBuffer {
         self.layout_key.as_ref()?.max_width
     }
 
+    /// Widths covered by the current line breaks, including a wider shaping
+    /// retained when a later draw requested a narrower box.
+    pub fn width_holds(&self) -> crate::Holds {
+        let Some(width) = self.wrap_width() else {
+            return crate::Holds::ANY;
+        };
+        let width = Px::from_f32(width);
+        let soft_wrapped = self.layout.lines().any(|line| {
+            matches!(
+                line.break_reason(),
+                parley::layout::BreakReason::Regular | parley::layout::BreakReason::Emergency
+            )
+        });
+        let upper = if soft_wrapped { width } else { Px::MAX };
+        crate::Holds::from(Px::ceil_from_f32(self.layout.width()).min(width)..=upper)
+    }
+
     pub fn size(&self) -> Vec2 {
         Vec2::new(self.layout.width(), self.layout.height())
     }
@@ -200,15 +210,19 @@ impl TextBuffer {
         // 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 a chance to
-        // disagree with itself -- which is what happens when a parent offers
-        // a child the length that child just reported, and the two land
-        // either side of a float.
+        // 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 + BREAK_EPSILON_PX >= self.layout.width()
+            && want >= self.layout.width()
         {
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::TextShapeHits);

core/src/render/mod.rs

@@ -106,7 +106,8 @@ impl UiRenderNode {
             self.active.push(i);
             for change in draws.apply_free() {
                 if let Some(inst) = ui_render.active.get_mut(&change.id) {
-                    for h in &mut inst.primitives {
+                    for primitive in &mut inst.primitives {
+                        let h = &mut primitive.handle;
                         if h.layer == i && h.kind == change.kind && h.inst_idx == change.old {
                             h.inst_idx = change.new;
                             break;

core/src/render/shader/prelude.wgsl

@@ -35,6 +35,10 @@ struct MoveOffset {
 // belongs to the pixel above it. Flooring the product instead drops a pixel
 // wherever a fraction divides a window exactly: a fifth of 1920 comes out of
 // `REL_STEP` as 383.99998, and five tabs each lose their last column.
+//
+// Taken over the whole coordinate, fraction and pixels summed, since a floor
+// does not distribute over a sum: floored apart, a half of one and a half of
+// the other lose the pixel the two together make.
 fn snap_floor(v: vec2<f32>) -> vec2<f32> {
     return floor(v + PX_STEP * 0.5);
 }
@@ -147,8 +151,8 @@ fn vs_main(
     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) + snap_floor(top_left_px);
-    let bot_right = snap_floor(bot_right_rel * window.dim) + snap_floor(bot_right_px);
+    let top_left = snap_floor(top_left_rel * window.dim + top_left_px);
+    let bot_right = snap_floor(bot_right_rel * window.dim + bot_right_px);
     let size = bot_right - top_left;
 
     let uv = vec2<f32>(
@@ -179,8 +183,8 @@ fn masked(in: VertexOutput, color: vec4<f32>) -> vec4<f32> {
     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) + snap_floor(tl_px);
-    let bot_right = snap_floor(br * window.dim) + snap_floor(br_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

@@ -1,6 +1,6 @@
 use crate::{
-    Holds, LayerId, LayoutLen, MaskIdx, MoveIdx, PrimitiveHandle, RegionAlign, Size, TextureHandle,
-    UiRegion, WidgetId,
+    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Place, RegionAlign, RetainedPrimitive, Size,
+    TextureHandle, UiRegion, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -9,20 +9,36 @@ use crate::{
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
-    /// The box its drawing is in, in `parent_move`'s coordinates.
-    pub region: UiRegion,
-    /// The box its parent first asked about it in, as a part of the box the
-    /// parent was itself asked in. Any later box it was given was decided
-    /// knowing its answer, so this is where a question about it is asked
-    /// again -- and it is kept relative so that it follows the parent's.
-    pub offer: UiRegion,
-    /// What it answered there: the size and what that held for.
-    pub answer: (Size, [Holds; 2]),
-    /// What the widget said it used of its box, the last time it drew.
+    /// Its frame in `parent_move`'s coordinates: what a fraction it declares
+    /// or reports is a fraction of, composed. Everything it draws sits inside
+    /// this by way of `extent`.
+    pub frame_abs: UiRegion,
+    /// Where its drawing goes, in the frame's own coordinates.
+    pub extent: UiRegion,
+    /// That frame in its parent's frame coordinates, before composition:
+    /// forwarded whole by a transparent container, narrowed by a declared
+    /// length. Its length is the same on every ask, which is what
+    /// a local redraw relies on to ask its parent's own question again.
+    pub frame: UiRegion,
+    /// What of its parent's extent the drawing was given, and what it was
+    /// given at the parent's first ask of it -- the question a cold layout
+    /// asks. A part is a length from the extent's start, so an extent that
+    /// moved re-places every child by re-adding that start.
+    pub place: [Place; 2],
+    pub offer_place: [Place; 2],
+    /// The box that ask gave it, in its frame's coordinates. Kept rather
+    /// than worked out again from where its parent's own box is now: a
+    /// parent drawn again in the box its own answer chose gives its children
+    /// boxes it never measured anything in, and the measurement this widget
+    /// answered is the one its parent's layout was built on.
+    pub offer_part: UiRegion,
+    /// The measured answer and its dependencies. A hint-only dependency or
+    /// a widget first encountered during placement has no measurement yet.
+    pub answer: Option<(Size, LayoutHolds)>,
+    /// What the widget said it used of its frame, the last time it drew.
     pub size: Size,
-    /// The pixel lengths of `region`, per axis, that its drawing and `size`
-    /// hold for.
-    pub holds: [Holds; 2],
+    /// The frame and extent reads that this drawing holds for.
+    pub holds: LayoutHolds,
     pub drawn: bool,
     pub parent: Option<WidgetId>,
     /// How far down the tree it was drawn, the root being 1. Carried down a
@@ -30,35 +46,51 @@ pub struct ActiveData {
     /// widget a frame visits and cannot drift while one is being drawn.
     pub depth: usize,
     pub textures: Vec<TextureHandle>,
-    pub primitives: Vec<PrimitiveHandle>,
+    /// Its primitives, each keeping the box it was written in -- in this
+    /// widget's extent coordinates, which is what a move recomposes from.
+    pub primitives: Vec<RetainedPrimitive>,
+    pub mask_region: Option<UiRegion>,
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
     /// The movable region its primitives are positioned through: its own when
     /// opted in, otherwise the nearest ancestor's.
     pub move_idx: MoveIdx,
-    /// The declared lengths whoever drew this widget resolved into its box.
+    /// The declared lengths whoever drew this widget resolved into its frame.
     /// A change to one moves a box this widget cannot fix by drawing again,
     /// and comparing them is what says so.
     pub declared: [Option<LayoutLen>; 2],
-    /// The alignment its parent asked it with. A local redraw repeats that
-    /// question, including an override chosen by a container.
-    pub align: RegionAlign,
-    /// Whether that alignment was the parent's override rather than the
-    /// widget's own property.
-    pub align_override: bool,
-    /// Its own alignment when it was last drawn. A change to the property is
-    /// found against this even when its parent overrode the alignment.
+    /// Its alignment when it was last drawn, which a change to the property
+    /// is found against.
     pub own_align: RegionAlign,
-    /// The movable region whose coordinates `region` uses.
+    /// The movable region whose coordinates `frame_abs` uses.
     pub parent_move: MoveIdx,
+    /// The mask its drawing is clipped to: one it set itself, or the one it
+    /// inherited from whoever drew it.
     pub mask: MaskIdx,
+    /// That inherited one. The two differ exactly where the widget set a
+    /// mask of its own, which is the one it owns and the one a move rewrites
+    /// -- and the one a redraw of it must not be handed back, since setting
+    /// a mask asserts there is none.
+    pub parent_mask: MaskIdx,
     pub layer: LayerId,
 }
 
 impl ActiveData {
-    /// Whether its drawing and size hold for a box of these pixel lengths.
-    pub fn holds_at(&self, px: crate::PxVec2) -> bool {
-        self.holds[0].contains(px.x) && self.holds[1].contains(px.y)
+    /// What it answered when its parent measured it, where it has been
+    /// measured at all. Not `size`, which is what its last drawing reported:
+    /// a drawing made in the box that answer chose is answering a different
+    /// question.
+    pub fn measured(&self) -> Option<Size> {
+        self.answer.map(|(size, _)| size)
+    }
+
+    /// Whether what it answered still stands for a frame of these pixel
+    /// lengths. The answer was given in the box its parent first asked
+    /// about, which is what it is checked against -- `holds` on the record
+    /// is about the box the answer then chose.
+    pub fn answers_at(&self, px: crate::PxVec2, part: UiRegion) -> bool {
+        self.answer
+            .is_some_and(|(_, holds)| holds.contains(px, part))
     }
 }

core/src/ui/holds.rs

@@ -10,9 +10,9 @@ use std::ops::RangeInclusive;
 ///
 /// The ends are lengths on the grid rather than floats with a tolerance
 /// around them: a box offered back at the length a widget reported comes back
-/// as the same number, so a range means what it says. What widening there is
-/// belongs to [`Self::through`], which has a rounding to undo, and is derived
-/// from that rounding rather than chosen.
+/// as the same number, so a range means what it says. The one place a range
+/// is wider than the length it came from is [`Self::through`], and what it is
+/// wider by is the floor that inverting a fraction undoes.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Holds {
     pub lo: Px,
@@ -41,32 +41,32 @@ impl Holds {
     }
 
     /// What a box has to be for a part of it, `len` of the box long, to stay
-    /// in this range. A part with no relative extent is a fixed length: it
-    /// was drawn at that length and any box keeps it there.
+    /// in this range: the exact preimage of `px + floor(rel * box)`, which is
+    /// the one way a box in pixels is reached. A part with no relative extent
+    /// is a fixed length -- it was drawn at that length and any box keeps it
+    /// there.
     ///
-    /// The way in is `px + rel * box` taken to the nearest step, so a part
-    /// of exactly `lo` came from anything within half a step of it and the
-    /// answer is an interval even where this range is one length. Inverting
-    /// the length alone instead gives a point that need not even contain the
-    /// box the part was drawn in, which is a range excluding the drawing it
-    /// was made for.
+    /// The answer is an interval even where this range is a single length,
+    /// because the multiply on the way in drops to the step below and many
+    /// boxes therefore give one length. That is a floor rather than an
+    /// allowance: inverting it is two divisions and nothing else, and the
+    /// whole of a box maps back to itself.
     pub const fn through(self, len: Len) -> Self {
+        if self.lo.raw() == Px::MIN.raw() && self.hi.raw() == Px::MAX.raw() {
+            return Self::ANY;
+        }
         let rel = len.rel.raw() as i64;
         if rel == 0 {
             return Self::ANY;
         }
-        // Three half steps either side -- one for the rounding on the way
-        // in, two for the difference between a length composed down the
-        // chain and the same length measured against the window -- and half
-        // of what a `Rel` counts in, to divide by the fraction. Exact until
-        // the division takes it back to the grid.
         let px = len.px.raw() as i64;
-        let half_rel = REL_SHIFT - 1;
-        let lo = ((self.lo.raw() as i64 - px) * 2 - 3) << half_rel;
-        let hi = ((self.hi.raw() as i64 - px) * 2 + 3) << half_rel;
-        // Dividing by a negative turns the ends around, so which end each
-        // bound comes from is decided before dividing rather than by taking
-        // the min and max of four divisions.
+        // `floor(rel * box) >= lo - px` is `rel * box >= (lo - px) << REL`, and
+        // `floor(rel * box) <= hi - px` is `rel * box < (hi - px + 1) << REL`.
+        let lo = (self.lo.raw() as i64 - px) << REL_SHIFT;
+        let hi = (((self.hi.raw() as i64 - px) + 1) << REL_SHIFT) - 1;
+        // Dividing by a negative fraction turns the ends around, so which
+        // bound each comes from is decided before dividing rather than by
+        // taking the min and max of four divisions.
         match rel > 0 {
             true => Self::raws(div_toward(lo, rel, true), div_toward(hi, rel, false)),
             false => Self::raws(div_toward(hi, rel, true), div_toward(lo, rel, false)),
@@ -95,6 +95,16 @@ mod tests {
     use super::*;
     use crate::Rel;
 
+    #[test]
+    fn an_unrestricted_range_stays_unrestricted_through_any_length() {
+        for rel in [-2.0, -0.5, 0.0, 0.5, 1.0, 2.0] {
+            for px in [-8, 0, 8] {
+                let len = Len::from_parts(Rel::from_f32(rel), Px::from_int(px));
+                assert_eq!(Holds::ANY.through(len), Holds::ANY);
+            }
+        }
+    }
+
     #[test]
     fn through_reverses_a_range_for_a_negative_fraction() {
         // `10 - box / 2` is between 20 and 40 for boxes from -60 to -20.
@@ -116,6 +126,48 @@ mod tests {
         }
     }
 
+    /// A widget handed the whole of its parent's box, with or without pixels
+    /// taken off it, has no fraction to invert: multiplying by one is exact
+    /// and taking the pixels off again is too, so the box maps back to
+    /// itself. Allowing for anything here compounded a step a level down a
+    /// chain of widgets each taking the whole of its parent.
+    #[test]
+    fn the_whole_of_a_box_maps_back_to_itself() {
+        let at = Px::from_int(956);
+        assert_eq!(Holds::at(at).through(Len::FULL), Holds::at(at));
+        let less_eight = Len::from_parts(Rel::ONE, Px::from_int(-8));
+        assert_eq!(
+            Holds::at(at).through(less_eight),
+            Holds::at(at + Px::from_int(8))
+        );
+    }
+
+    /// The range is the exact preimage at both ends, so a box one step
+    /// outside it really does give a length outside this range. What a wider
+    /// range costs is a drawing reused where it does not hold.
+    #[test]
+    fn a_box_one_step_outside_the_range_is_outside_it() {
+        let part = Len::from_parts(Rel::from_f32(1.0 / 3.0), Px::from_int(-146));
+        let at = Px::from_int(300);
+        let holds = Holds::at(at).through(part);
+        for inside in [holds.lo, holds.hi] {
+            assert_eq!(part.to_px(inside), at, "{inside:?} left out of {holds:?}");
+        }
+        for outside in [holds.lo.next_down(), holds.hi.next_up()] {
+            assert_ne!(part.to_px(outside), at, "{outside:?} admitted by {holds:?}");
+        }
+    }
+
+    /// A truncating multiply only ever drops, so the step it needs allowing
+    /// for on the way in belongs at the top of the range and not the bottom.
+    #[test]
+    fn a_fraction_widens_further_up_than_down() {
+        let half = Len::from_parts(Rel::from_f32(0.5), Px::ZERO);
+        let holds = Holds::at(Px::from_int(100)).through(half);
+        let box_len = Px::from_int(200);
+        assert!(holds.hi - box_len > box_len - holds.lo, "{holds:?}");
+    }
+
     #[test]
     fn a_boundary_the_next_step_along_does_not_admit_it() {
         let boundary = Px::from_int(10);

core/src/ui/layout_holds.rs

@@ -0,0 +1,63 @@
+use crate::{Axis, Holds, Len, PxVec2, UiRegion};
+
+const AXES: [Axis; 2] = [Axis::X, Axis::Y];
+
+/// What one evaluation of a widget depends on: the pixel lengths of its
+/// frame and of its own box that its drawing and its answer hold for, and
+/// the symbolic length of its own box where it read one.
+///
+/// The symbolic length is a pin rather than a range: a container places its
+/// children as lengths of its frame measured from where its own box starts,
+/// so what it draws turns on that box's length and on nothing about where it
+/// is. It reaches the parent only where the box it pinned is the parent's
+/// own; anywhere else the parent chose that length itself, and a widget
+/// pinned this way is checked when it is re-placed.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct LayoutHolds {
+    pub frame: [Holds; 2],
+    pub extent: [Holds; 2],
+    pub extent_len: [Option<Len>; 2],
+}
+
+impl LayoutHolds {
+    pub const ANY: Self = Self {
+        frame: [Holds::ANY; 2],
+        extent: [Holds::ANY; 2],
+        extent_len: [None; 2],
+    };
+
+    pub fn and(self, other: Self) -> Self {
+        let mut result = Self::ANY;
+        for n in 0..2 {
+            result.frame[n] = self.frame[n].and(other.frame[n]);
+            result.extent[n] = self.extent[n].and(other.extent[n]);
+            debug_assert!(
+                self.extent_len[n].is_none()
+                    || other.extent_len[n].is_none()
+                    || self.extent_len[n] == other.extent_len[n]
+            );
+            result.extent_len[n] = self.extent_len[n].or(other.extent_len[n]);
+        }
+        result
+    }
+
+    pub fn covers(self, other: Self) -> bool {
+        (0..2).all(|n| {
+            self.frame[n].lo <= other.frame[n].lo
+                && self.frame[n].hi >= other.frame[n].hi
+                && self.extent[n].lo <= other.extent[n].lo
+                && self.extent[n].hi >= other.extent[n].hi
+                && self.extent_len[n].is_none_or(|len| other.extent_len[n] == Some(len))
+        })
+    }
+
+    pub fn contains(self, px: PxVec2, extent: UiRegion) -> bool {
+        AXES.into_iter().all(|axis| {
+            let n = axis as usize;
+            let len = extent.axis(axis).len();
+            self.frame[n].contains(px.axis(axis))
+                && self.extent[n].contains(len.to_px(px.axis(axis)))
+                && self.extent_len[n].is_none_or(|pinned| pinned == len)
+        })
+    }
+}

core/src/ui/mod.rs

@@ -11,12 +11,16 @@ pub const CHAIN_LIMIT: u32 = 64;
 
 mod active;
 mod holds;
+mod layout_holds;
 mod painter;
+mod place;
 mod render_state;
 
 pub use active::*;
 pub use holds::*;
+pub use layout_holds::*;
 pub use painter::{Painter, PrimitiveLike};
+pub use place::*;
 pub use render_state::*;
 
 #[derive(Default)]
@@ -68,17 +72,24 @@ impl Moves {
         }
     }
 
-    /// Composes a region held in `idx`'s coordinates down the chain, which is
-    /// the same walk the vertex shader does.
+    /// The same walk the vertex shader does, in the same `Len` the shader is
+    /// handed, for asking where a drawing will actually land -- hit testing,
+    /// and nothing layout decides on. Layout threads its lengths down the
+    /// draw instead, so no box it compares is composed back up this chain.
     pub fn resolve(&self, idx: MoveIdx, local: UiRegion) -> UiRegion {
         let mut region = local;
+        self.walk(idx, |entry| region = region.within(entry));
+        region
+    }
+
+    fn walk(&self, idx: MoveIdx, mut step: impl FnMut(&UiRegion)) {
         let mut at = idx;
         for _ in 0..CHAIN_LIMIT {
             if at == MoveIdx::NONE {
-                return region;
+                return;
             }
-            let entry = self.arena[at.idx()];
-            region = region.within(&entry.region);
+            let entry = &self.arena[at.idx()];
+            step(&entry.region);
             at = entry.parent;
         }
         debug_assert!(
@@ -86,7 +97,6 @@ impl Moves {
             "a move chain longer than {CHAIN_LIMIT} resolves to the wrong place, \
              and the shader stops at the same depth"
         );
-        region
     }
 
     /// How many slots a region in `idx` is composed through, which is what

core/src/ui/painter.rs

@@ -1,12 +1,12 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Holds, LayoutLen, Len, Px, PxVec2, RegionAlign, Rel, RenderedText, Size, StrongWidget,
-    TextAttrs, TextBuffer, TextData, TextureHandle, UiRegion, UiRenderState, UiRsc, UiVec2, Weight,
-    WidgetId, Widgets,
+    Axis, Holds, LayoutHolds, LayoutLen, Len, Part, Place, Px, PxVec2, RegionAlign, RenderedText,
+    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
+    UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
     render::{
-        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveHandle, PrimitiveInst,
-        PrimitiveKind, TexturePrimitive,
+        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
+        TexturePrimitive,
     },
     ui::render_state::DrawInfo,
 };
@@ -17,27 +17,57 @@ pub struct Painter<'a> {
     pub(super) state: &'a mut UiRenderState,
     pub(super) rsc: &'a mut dyn UiRsc,
 
-    /// This widget's box, in the coordinates of `move_idx`.
-    pub(super) region: UiRegion,
+    /// What a fraction this widget declares or reports is a fraction of, in
+    /// the coordinates of `move_idx`: forwarded from its parent unchanged
+    /// through a span, a stack or a scroll, and narrowed only by what was
+    /// decided above it -- a declared length, or the root. Its length
+    /// is the same on every ask of the widget, which is what keeps a fraction
+    /// under it from being resolved twice.
+    pub(super) frame: UiRegion,
+    /// Where this widget's drawing goes, in the frame's own coordinates.
+    /// Everything it writes is in these coordinates, and its children are
+    /// placed as parts of it.
+    pub(super) extent: UiRegion,
+    /// The extent's symbolic length where this draw read it, which makes the
+    /// drawing one that holds for that length alone -- the way reading a
+    /// length in pixels makes it hold for that number of pixels.
+    pub(super) extent_len: [Option<Len>; 2],
+    /// Symbolic box lengths read only to compute the answer. A container can
+    /// replace the provisional drawings used for that answer with drawings
+    /// in decided boxes, so this contract is independent of the final one.
+    pub(super) answer_extent_len: [Option<Len>; 2],
+    /// The frame in pixels, which its children's frames are a length of:
+    /// threaded down rather than composed back up the chain, so every length
+    /// in layout is one multiply from its parent's and [`Holds::through`]
+    /// inverts exactly that.
+    pub(super) px: PxVec2,
     pub(super) mask: MaskIdx,
     pub(super) textures: Vec<TextureHandle>,
-    pub(super) primitives: Vec<PrimitiveHandle>,
+    pub(super) primitives: Vec<RetainedPrimitive>,
+    pub(super) mask_region: Option<UiRegion>,
+    /// Only children whose answers were read constrain this widget's answer.
+    pub(super) answer_under: LayoutHolds,
     pub(super) children: Vec<WidgetId>,
-    /// The children asked about so far, so the first box each was asked in
+    /// The children asked about so far, so the first place each was asked in
     /// is the one recorded as its offer.
     pub(super) offered: Vec<WidgetId>,
-    /// The box this widget was first asked about in, in pixels.
-    pub(super) offered_px: PxVec2,
-    /// Whether this draw is in that box, which makes the questions it asks
-    /// the ones a cold layout asks and their answers the ones to keep.
+    /// Whether this draw is at the place its parent first asked about, which
+    /// makes the questions it asks the ones a cold layout asks and their
+    /// answers the ones to keep.
     pub(super) at_offer: bool,
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
-    /// What this draw itself read of its box in pixels, per axis: every
+    /// What this draw itself read of its frame in pixels, per axis: every
     /// length until it reads one, then that one, unless it says otherwise.
-    pub(super) own: [Holds; 2],
-    /// What the children it asked about and drew keep it to.
-    pub(super) under: [Holds; 2],
+    pub(super) frame_own: [Holds; 2],
+    /// The same for its own box.
+    pub(super) extent_own: [Holds; 2],
+    /// Pixel-box dependencies used only to compute the answer. These do not
+    /// constrain a retained drawing placed inside that answer.
+    pub(super) answer_extent_own: [Holds; 2],
+    /// The final drawing kept for each child. Asking one child again replaces
+    /// its provisional drawing and therefore replaces this contract too.
+    pub(super) under: Vec<(WidgetId, LayoutHolds)>,
     /// The movable region this widget's primitives are positioned through:
     /// its own when opted in, otherwise the nearest ancestor's.
     pub(super) move_idx: MoveIdx,
@@ -57,6 +87,24 @@ impl<'a> Painter<'a> {
 
     /// Takes the kind, for a caller writing many of one primitive.
     fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: UiRegion) {
+        self.write_resolved(kind, primitive, region, self.resolve(region));
+    }
+
+    /// A box in this widget's extent coordinates, composed into the
+    /// coordinates its move slot is in: through the extent, then through the
+    /// frame the extent is a part of. The same two steps a recomposition
+    /// replays, so a moved drawing lands where a cold one does.
+    fn resolve(&self, region: UiRegion) -> UiRegion {
+        region.within(&self.extent).within(&self.frame)
+    }
+
+    fn write_resolved<P: Primitive>(
+        &mut self,
+        kind: PrimitiveKind<P>,
+        primitive: P,
+        region: UiRegion,
+        resolved: UiRegion,
+    ) {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::PrimitiveWrites);
         let h = self.state.layers.write(
@@ -65,15 +113,15 @@ impl<'a> Painter<'a> {
                 kind,
                 id: self.id,
                 primitive,
-                region,
+                region: resolved,
                 mask_idx: self.mask,
                 move_idx: self.move_idx,
             },
         );
-        self.push_primitive(h);
+        self.push_primitive(RetainedPrimitive { handle: h, region });
     }
 
-    fn push_primitive(&mut self, h: PrimitiveHandle) {
+    fn push_primitive(&mut self, h: RetainedPrimitive) {
         if self.mask != MaskIdx::NONE {
             // TODO: I have no clue if this works at all :joy:
             self.rsc.ui_mut().masks.push_ref(self.mask);
@@ -81,90 +129,70 @@ impl<'a> Painter<'a> {
         self.primitives.push(h);
     }
 
-    /// Writes a primitive to be rendered
+    /// Writes a primitive over the whole of this widget's own box.
     pub fn primitive(&mut self, primitive: impl PrimitiveLike) {
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, self.region)
+        self.primitive_at(primitive, UiRegion::FULL)
     }
 
+    /// Writes a primitive in a part of this widget's own box, in that box's
+    /// coordinates.
     pub fn primitive_within(&mut self, primitive: impl PrimitiveLike, region: UiRegion) {
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, region.within(&self.region));
+        self.primitive_at(primitive, region);
     }
 
+    /// Sets a mask, in this widget's own box's coordinates.
     pub fn set_mask(&mut self, region: UiRegion) {
+        self.mask_region = Some(region);
         assert!(self.mask == MaskIdx::NONE);
+        let resolved = self.resolve(region);
+        let move_idx = self.move_idx;
         self.mask = self.rsc.ui_mut().masks.push(Mask {
-            region,
-            move_idx: self.move_idx,
+            region: resolved,
+            move_idx,
         });
     }
 
-    /// Draws a widget within this widget's region.
+    /// Draws a widget in the whole of this widget's own box, with the frame
+    /// forwarded unchanged: what a container that is only a wrapper around
+    /// one child wants, and what every transparent container passes for the
+    /// frame.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_within(id, UiRegion::FULL)
+        self.widget_at(id, UiRegion::FULL, [Place::Within(Part::All); 2])
     }
 
-    /// What a widget's rules declare its lengths to be, which whoever draws
-    /// it resolves into its box. 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>) -> [Option<LayoutLen>; 2] {
-        declared_lens(self.rsc.widgets(), id.id())
-    }
-
-    /// Takes back a child that was drawn only to find out how long it is.
-    /// Its drawing is dropped and it is not one of this widget's children
-    /// this frame; what it answered is still something this widget asked.
-    pub fn undraw<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
-        self.children.retain(|child| *child != id.id());
-        self.state.undraw_rec(id.id(), self.rsc);
-    }
-
-    /// Draws a widget somewhere within this one. `region` is in this widget's
-    /// own coordinates, and the child's declared lengths are still to be
-    /// taken from it. Where the child's drawing sits inside what it is given
-    /// is the child's alignment, applied where the child is drawn, so a
-    /// container positions a child either by handing it a box of exactly its
-    /// length or by leaving it room and letting its alignment decide.
-    pub fn widget_within<'s, W: ?Sized>(
+    /// Draws a child, saying what its fractions are of and where its drawing
+    /// goes.
+    ///
+    /// `frame` is that reference, in this widget's own frame coordinates:
+    /// [`UiRegion::FULL`] forwards this widget's frame, which is what a
+    /// container that only divides room passes, so a fraction under it means
+    /// the same wherever it sits and however deeply it is nested. Narrowing
+    /// it is for what is decided from above, and a declared length narrows
+    /// it here.
+    ///
+    /// `place` is where the drawing goes, per axis, as a part of this
+    /// widget's extent: see [`Place`]. A narrowed frame is its own extent,
+    /// since the narrowing is what said where the drawing goes.
+    pub fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
-        region: UiRegion,
-    ) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, region, None)
-    }
-
-    /// Draws a widget with an alignment chosen by its container rather than
-    /// the widget's property. Containers use this when the box they hand down
-    /// already expresses the size they report around the child.
-    pub fn widget_aligned<'s, W: ?Sized>(
-        &'s mut self,
-        id: &'s StrongWidget<W>,
-        region: UiRegion,
-        align: RegionAlign,
-    ) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, region, Some(align))
-    }
-
-    fn widget_at<'s, W: ?Sized>(
-        &'s mut self,
-        id: &'s StrongWidget<W>,
-        region: UiRegion,
-        align_override: Option<RegionAlign>,
+        frame: UiRegion,
+        place: [Place; 2],
     ) -> DrawResult<'s, 'a, W> {
         let region_node = self.rsc.widgets().is_region_node(id.id());
         let declared = self.declared_lens(id);
-        let align = align_override.unwrap_or_else(|| self.rsc.widgets().alignment(id.id()));
-        // Composing `FULL` through a box is not quite the identity in f32,
-        // so a child with nothing declared keeps the box it would have had.
-        let local = match declared.iter().any(Option::is_some) {
-            true => declared_box(region, declared, align),
-            false => region,
-        };
+        let align = self.rsc.widgets().alignment(id.id());
+        let (local, extent) = frame_and_extent(
+            frame,
+            part_of(self.extent, place),
+            narrowed_by(declared, frame),
+            align,
+        );
         let within = match local == UiRegion::FULL {
-            true => self.region,
-            false => local.within(&self.region),
+            true => self.frame,
+            false => local.within(&self.frame),
         };
         #[cfg(feature = "layout-diagnostics")]
         if region_node {
@@ -176,17 +204,20 @@ impl<'a> Painter<'a> {
             self.children.push(id.id());
         }
         let first_ask = self.offer(id.id());
-        let offer = match first_ask {
-            true => local,
-            false => self.state.active.get(&id.id()).map_or(local, |a| a.offer),
+        let offer_place = if first_ask {
+            place
+        } else {
+            self.state
+                .active
+                .get(&id.id())
+                .map_or(place, |a| a.offer_place)
         };
-        let answers_offer = self.at_offer && local == offer;
-        // The answer and what it holds for, both about the box asked in. The
-        // child's record may say something else once its drawing has been
+        let px = local.size().to_px(self.px);
+        // The answer and what it holds for, both about the place asked in.
+        // The child's record may say something else once its drawing has been
         // placed: a drawing made again in its placed box holds for that box.
-        let (size, holds) = self.state.draw_inner(
+        let (size, answer_holds, holds) = self.state.draw_inner(
             id.id(),
-            within,
             DrawInfo {
                 layer: self.layer,
                 parent: Some(self.id),
@@ -194,28 +225,47 @@ impl<'a> Painter<'a> {
                 parent_move: self.move_idx,
                 region_node,
                 mask: self.mask,
-                offer,
-                offered_px: self.px_within_offer(offer),
-                align: align_override,
+                frame: local,
+                frame_abs: within,
+                part: extent,
+                place,
+                offer_place,
+                px,
             },
             None,
             self.rsc,
         );
-        if answers_offer {
-            self.state.active.get_mut(&id.id()).unwrap().answer = (size, holds);
-        }
-        // Whatever the child's answer holds for keeps this one to the boxes
-        // that give the child a length inside it.
-        for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
-            *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
+        let compose = |holds| in_parent(holds, local, extent, place, declared);
+        let holds = compose(holds);
+        match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
+            Some((_, kept)) => *kept = holds,
+            None => self.under.push((id.id(), holds)),
         }
         DrawResult {
             child: id,
             painter: self,
-            size,
+            size: in_parent_frame(size, local.size(), declared),
+            answer_holds: compose(answer_holds),
         }
     }
 
+    /// Takes back a child that was drawn only to find out how long it is.
+    /// Its drawing is dropped and it is not one of this widget's children
+    /// this frame; what it answered is still something this widget asked.
+    pub fn undraw<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
+        self.children.retain(|child| *child != id.id());
+        self.under.retain(|(child, _)| *child != id.id());
+        self.state.undraw_rec(id.id(), self.rsc);
+    }
+
+    /// What a widget's rules declare its lengths to be, which whoever draws
+    /// it resolves into its frame. 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>) -> [Option<LayoutLen>; 2] {
+        declared_lens(self.rsc.widgets(), id.id())
+    }
+
     /// What a child says its length is without being drawn, if it can say.
     /// Asking counts as reading its size.
     pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
@@ -244,46 +294,11 @@ impl<'a> Painter<'a> {
         }
     }
 
-    /// A child's length in the box it is about to be offered, if it can be
-    /// had without drawing it: from its hint, or from a drawing it already
-    /// has that holds for that box.
-    pub fn known_len<W: ?Sized>(
-        &mut self,
-        child: &StrongWidget<W>,
-        axis: Axis,
-        region: UiRegion,
-    ) -> Option<LayoutLen> {
-        let declared = self.declared_lens(child);
-        let align = self.rsc.widgets().alignment(child.id());
-        let local = declared_box(region, declared, align);
-        let within = local.within(&self.region);
-        let first_ask = self.offer(child.id());
-        if first_ask && let Some(active) = self.state.active.get_mut(&child.id()) {
-            active.offer = local;
-        }
-        if let Some(hint) = self.size_hint(child, axis) {
-            return Some(hint);
-        }
-        let px = self.state.px_of(self.move_idx, within);
-        let (size, holds) =
-            self.state
-                .retained_size(child.id(), px, self.move_idx, self.rsc.widgets())?;
-        #[cfg(feature = "layout-diagnostics")]
-        diag::bump(Counter::RetainedSizeHits);
-        self.depend_on(child);
-        if first_ask {
-            let active = self.state.active.get_mut(&child.id()).unwrap();
-            active.answer = (size, holds);
-        }
-        for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
-            *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
-        }
-        Some(size.axis(axis))
-    }
-
     /// Whether this is the first box a child is asked about in during a draw
-    /// that is itself in the box it was asked in -- the question a cold
-    /// layout asks, whose answer is the one to keep.
+    /// that is itself the one its parent measured -- the question a cold
+    /// layout asks, whose answer is the one to keep. A drawing made again in
+    /// a box chosen from an answer asks about that box instead, and what it
+    /// hears back is not a measurement of anything.
     fn offer(&mut self, child: WidgetId) -> bool {
         if !self.at_offer || self.offered.contains(&child) {
             return false;
@@ -292,15 +307,6 @@ impl<'a> Painter<'a> {
         true
     }
 
-    /// The pixel size of a part of the box this widget was asked in.
-    fn px_within_offer(&self, local: UiRegion) -> PxVec2 {
-        let size = local.size();
-        PxVec2::new(
-            size.x.to_px(self.offered_px.x),
-            size.y.to_px(self.offered_px.y),
-        )
-    }
-
     fn depend_on<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
         if !self.size_deps.contains(&child.id()) {
             self.size_deps.push(child.id());
@@ -319,11 +325,15 @@ impl<'a> Painter<'a> {
         ui.text.render(buffer, attrs, width)
     }
 
+    /// Writes glyphs in the selected frame or extent coordinates.
     // TODO: merge the text methods into the primitive ones.
     pub fn glyphs(&mut self, text: &RenderedText, origin: UiRegion) {
+        // Glyph offsets and sizes are pixels, which compose additively.
+        // Only the shared origin needs composing through the extent.
+        let resolved = self.resolve(origin);
         let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {
-            let mut region = origin;
+            let place = |mut region: UiRegion| {
                 region.x.end = region.x.start;
                 region.y.end = region.y.start;
                 let mut region = region.offset(UiVec2::from_px(glyph.offset));
@@ -333,7 +343,9 @@ impl<'a> Painter<'a> {
                 );
                 region.x.end = region.x.start.offset(size.x);
                 region.y.end = region.y.start.offset(size.y);
-            self.write(
+                region
+            };
+            self.write_resolved(
                 kind,
                 GlyphPrimitive {
                     uv_min: glyph.entry.uv_min,
@@ -342,15 +354,44 @@ impl<'a> Painter<'a> {
                     color: text.color,
                     flags: glyph.entry.flags(),
                 },
-                region,
+                place(origin),
+                place(resolved),
             );
         }
     }
 
-    /// This widget's box, in the coordinates its own primitives are written
-    /// in -- so a region composed `within` it may be drawn directly.
-    pub fn region(&self) -> UiRegion {
-        self.region
+    /// The symbolic length of this widget's own box along one axis, in the
+    /// lengths of its frame that it places its children in. Reading it pins
+    /// the drawing to that length -- and to nothing about where the box
+    /// starts, which is what lets a container move without being drawn
+    /// again. One axis at a time, because a container that divides one axis
+    /// holds for any length of the other.
+    pub fn extent_len(&mut self, axis: Axis) -> Len {
+        let len = self.extent.axis(axis).len();
+        self.extent_len[axis as usize] = Some(len);
+        self.answer_extent_len[axis as usize] = Some(len);
+        len
+    }
+
+    /// The symbolic length used to compute this widget's answer, where the
+    /// final drawing itself is rebuilt without depending on that length.
+    pub fn answer_extent_len(&mut self, axis: Axis) -> Len {
+        let len = self.extent.axis(axis).len();
+        self.answer_extent_len[axis as usize] = Some(len);
+        len
+    }
+
+    /// Says that the final drawing uses a symbolic length already read for
+    /// the answer.
+    pub fn drawing_uses_extent_len(&mut self, axis: Axis, len: Len) {
+        debug_assert_eq!(self.extent.axis(axis).len(), len);
+        self.extent_len[axis as usize] = Some(len);
+    }
+
+    /// A part of this widget's box, expressed in its frame coordinates so it
+    /// can be used as a child frame decided here.
+    pub fn extent_part(&self, axis: Axis, part: Part) -> UiSpan {
+        part.of(*self.extent.axis(axis))
     }
 
     /// Where this widget sits in a box longer than the length it takes. A
@@ -378,49 +419,83 @@ impl<'a> Painter<'a> {
             .is_some()
     }
 
-    /// The part of this widget's box that something of `size` takes, at the
-    /// near edge. A container that reports one child's size gives every child
-    /// this, so what it draws is inside what it says it occupies.
-    pub fn box_of(&self, size: Size) -> UiRegion {
-        placed_box(UiRegion::FULL, size, RegionAlign::NEAR, [None; 2])
-    }
-
-    /// This widget's box in pixels. Reading it makes the drawing one that
-    /// holds for this box only, until `holds` says how far it goes.
+    /// This widget's own box in pixels. Reading it makes the drawing one
+    /// that holds for this box only, until `holds` says how far it goes.
     pub fn px_size(&mut self) -> PxVec2 {
-        let px = self.state.px_of(self.move_idx, self.region);
-        for (own, len) in self.own.iter_mut().zip([px.x, px.y]) {
-            if *own == Holds::ANY {
-                *own = Holds::at(len);
-            }
-        }
-        px
+        PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
     }
 
-    /// One axis of this widget's box in pixels. Prefer this to
+    /// This widget's own box in pixels, used only to compute its answer.
+    pub fn answer_px_size(&mut self) -> PxVec2 {
+        PxVec2::new(
+            self.answer_px_len(Axis::X),
+            self.answer_px_len(Axis::Y),
+        )
+    }
+
+    /// One axis of this widget's own box in pixels. Prefer this to
     /// [`Self::px_size`] when the other axis cannot affect the drawing.
     pub fn px_len(&mut self, axis: Axis) -> Px {
-        let len = self.state.px_of(self.move_idx, self.region).axis(axis);
-        let own = &mut self.own[axis as usize];
+        let part = self.extent.axis(axis).len();
+        let len = part.to_px(self.px.axis(axis));
+        let own = &mut self.extent_own[axis as usize];
         if *own == Holds::ANY {
             *own = Holds::at(len);
         }
         len
     }
 
-    /// The lengths of this widget's box on `axis` that what it is drawing
+    /// One pixel length used only to compute this widget's answer. The final
+    /// drawing may be retained when that answer is placed in another box.
+    pub fn answer_px_len(&mut self, axis: Axis) -> Px {
+        let len = self.extent.axis(axis).len().to_px(self.px.axis(axis));
+        let own = &mut self.answer_extent_own[axis as usize];
+        if *own == Holds::ANY {
+            *own = Holds::at(len);
+        }
+        len
+    }
+
+    /// The lengths of this widget's own box on `axis` that what it is drawing
     /// holds for -- the same primitives, in the same fractions and offsets
-    /// of the box, and the same reported size. A widget that read its
-    /// length in pixels holds for that one alone until it says otherwise.
+    /// of the box, and the same reported size. A widget that read its length
+    /// in pixels holds for that one alone until it says otherwise.
     pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+        let part = self.extent.axis(axis).len();
         let holds = holds.into();
         debug_assert!(
-            holds.contains(self.state.px_of(self.move_idx, self.region).axis(axis)),
+            holds.contains(part.to_px(self.px.axis(axis))),
             "'{}' ({:?}) says its drawing holds for lengths that leave out its own box",
             self.label(),
             self.id
         );
-        self.own[axis as usize] = holds;
+        self.extent_own[axis as usize] = holds;
+    }
+
+    /// One axis of this widget's frame in pixels -- what a fraction of its
+    /// area resolves against, and so what a container divides among its
+    /// children. Its own box is a part of this one.
+    pub fn frame_px_len(&mut self, axis: Axis) -> Px {
+        let len = self.px.axis(axis);
+        let own = &mut self.frame_own[axis as usize];
+        if *own == Holds::ANY {
+            *own = Holds::at(len);
+        }
+        len
+    }
+
+    /// [`Self::holds`] stated about the frame rather than about this
+    /// widget's own box, for a container whose drawing turns on what its
+    /// fractions are of rather than on the part of it it took.
+    pub fn frame_holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+        let holds = holds.into();
+        debug_assert!(
+            holds.contains(self.px.axis(axis)),
+            "'{}' ({:?}) says its drawing holds for lengths that leave out its frame",
+            self.label(),
+            self.id
+        );
+        self.frame_own[axis as usize] = holds;
     }
 
     pub fn text_data(&mut self) -> &mut TextData {
@@ -463,6 +538,7 @@ pub struct DrawResult<'p, 'a, W: ?Sized> {
     painter: &'p mut Painter<'a>,
     child: &'p StrongWidget<W>,
     size: Size,
+    answer_holds: LayoutHolds,
 }
 
 impl<W: ?Sized> DrawResult<'_, '_, W> {
@@ -473,6 +549,7 @@ impl<W: ?Sized> DrawResult<'_, '_, W> {
             diag::size_read(self.child.id(), self.painter.id, self.size);
         }
         self.painter.depend_on(self.child);
+        self.painter.answer_under = self.painter.answer_under.and(self.answer_holds);
         self.size
     }
 
@@ -506,6 +583,72 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
+/// What a child depends on, said about the boxes the widget that drew it
+/// has rather than the ones the child was given.
+///
+/// `frame` is the child's frame in this widget's frame coordinates and
+/// `extent` the box it was given, in the child's own frame coordinates. Both
+/// reach it as one length, so what it holds for maps back through that
+/// length exactly -- and where the box it was given is this widget's own,
+/// what it says about that box is what this widget can say about its own.
+pub(crate) fn in_parent(
+    holds: LayoutHolds,
+    frame: UiRegion,
+    extent: UiRegion,
+    place: [Place; 2],
+    declared: [Option<LayoutLen>; 2],
+) -> LayoutHolds {
+    let mut result = LayoutHolds::ANY;
+    for axis in AXES {
+        let n = axis as usize;
+        let frame_len = frame.axis(axis).len();
+        result.frame[n] = holds.frame[n].through(frame_len);
+        match (place[n].part(), declared[n]) {
+            // Its box is this widget's own, or a part of it in that box's
+            // own lengths: so what it holds for is a range on this widget's
+            // own box, which is what lets that box move without a redraw. A
+            // length it pinned is this widget's length wherever the part is
+            // the whole of it, and pins the same way.
+            (Part::All, None) if *frame.axis(axis) == UiSpan::FULL => {
+                result.extent[n] = holds.extent[n];
+                result.extent_len[n] = holds.extent_len[n];
+            }
+            // Its box is a part of this widget's own box, in that box's own
+            // lengths, so what it holds for maps back through that part into
+            // a range on this widget's box.
+            (Part::Of(span), None) => {
+                result.extent[n] = holds.extent[n].through(span.len());
+            }
+            // Its box is a part of this widget's frame: a length of the
+            // frame is all that reaches it, so what it holds for is a range
+            // on the frame and none of it on this widget's own box.
+            _ => {
+                result.frame[n] = result.frame[n].and(
+                    holds.extent[n]
+                        .through(extent.axis(axis).len())
+                        .through(frame_len),
+                );
+            }
+        }
+    }
+    result
+}
+
+/// A child's answer as lengths of the parent's own region. A widget reports
+/// a fraction of its own region, and `of` is that region as a length of this
+/// one. Pixels come through untouched, being that many pixels wherever they
+/// end up. A declared axis is already the parent's: it resolved the rule in
+/// its own region, and the rule is what the report says.
+fn in_parent_frame(size: Size, of: UiVec2, declared: [Option<LayoutLen>; 2]) -> Size {
+    let mut size = size;
+    for (axis, declared) in AXES.into_iter().zip(declared) {
+        if declared.is_none() {
+            *size.axis_mut(axis) = size.axis(axis).within_len(of.axis(axis));
+        }
+    }
+    size
+}
+
 /// What a widget declares a length of its box to be. `leftover` is not one: a
 /// share of what is left over is only a length to the widget dividing one,
 /// so it passes up in the size instead.
@@ -526,52 +669,101 @@ pub(crate) fn declared_lens(widgets: &Widgets, id: WidgetId) -> [Option<LayoutLe
     })
 }
 
-/// The box a drawing occupies: the size the widget reported, on the side of
-/// the box it was asked in that its alignment says. An axis reported as 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 is
-/// left alone too: `declared_box` already placed it, in the parent's box,
-/// and the rule's length is what the widget reports there.
+/// Whether what a widget reported along an axis is the whole of the box it
+/// is in rather than a part to be placed inside it. A share fills, because a
+/// share is a length only to whoever divides one, and whoever did is the one
+/// that handed down this box. A declared axis does too: the rule already gave
+/// the region its length, and the rule's length is what the widget reports
+/// there. And an axis the parent decided from the answer is
+/// the answer already.
+pub(crate) fn fills(reported: LayoutLen, declared: Option<LayoutLen>, decided: bool) -> bool {
+    reported.leftover != Weight::ZERO || declared.is_some() || decided
+}
+
+/// Where a widget's drawing goes inside the part its parent gave it: what
+/// it reported, on the side of the part its alignment says, and the whole
+/// part wherever the answer fills it.
 ///
-/// A reported fraction is a fraction of the box the widget drew in, where a
-/// declared one is a fraction of the box its parent handed down -- a span
-/// reporting `rel(1.0)` means all of what it was given, whatever that was a
-/// fraction of. So this scales by the box rather than composing into it.
-pub(crate) fn placed_box(
-    region: UiRegion,
+/// The length it reported is a length of its frame, and the part is one too,
+/// so this takes one from the other rather than composing it into the part.
+/// That is what makes a fraction the same fraction wherever the part it is
+/// placed in sits and however long it is -- the fraction is resolved once,
+/// here, against the frame it was reported of.
+pub(crate) fn placed_extent(
+    part: UiRegion,
     size: Size,
-    align: RegionAlign,
     declared: [Option<LayoutLen>; 2],
+    fill: [bool; 2],
+    align: RegionAlign,
 ) -> UiRegion {
-    let mut placed = region;
-    for (axis, declared) in AXES.into_iter().zip(declared) {
+    let mut placed = part;
+    for axis in AXES {
+        let n = axis as usize;
         let reported = size.axis(axis);
-        if reported.leftover != Weight::ZERO || declared.is_some() {
+        if fills(reported, declared[n], fill[n]) {
             continue;
         }
+        let len = Len::from_parts(reported.rel, reported.px);
         let span = placed.axis_mut(axis);
-        let len = span.len().scale(reported.rel) + Len::from_parts(Rel::ZERO, reported.px);
         span.start += (span.len() - len).scale(align.axis(axis).rel());
         span.end = span.start + len;
     }
     placed
 }
 
-/// Takes a widget's declared lengths in the box `region` is given in, since a
-/// fraction of a length means a fraction of that one, and puts what is left
-/// over on the side its alignment says. A caller that already reserved the
-/// space hands back the same length, so this is the identity for it.
-pub(crate) fn declared_box(
-    mut region: UiRegion,
-    declared: [Option<LayoutLen>; 2],
+/// The part of a widget's own box a `place` names, in the coordinates that
+/// box is in.
+pub(crate) fn part_of(extent: UiRegion, place: [Place; 2]) -> UiRegion {
+    let mut part = extent;
+    for axis in AXES {
+        *part.axis_mut(axis) = place[axis as usize].part().of(*extent.axis(axis));
+    }
+    part
+}
+
+/// The length a rule gives a child's frame, per axis: a fraction in it is a
+/// fraction of the frame the child was given, which is the one length the
+/// rule can mean.
+pub(crate) fn narrowed_by(declared: [Option<LayoutLen>; 2], frame: UiRegion) -> [Option<Len>; 2] {
+    AXES.map(|axis| {
+        declared[axis as usize]
+            .map(|len| Len::from_parts(len.rel, len.px).within_len(frame.axis(axis).len()))
+    })
+}
+
+/// The frame a child is asked in and the box its drawing goes in, both in
+/// the coordinates of the widget asking.
+///
+/// `frame` is what the caller said the child's fractions are of, and `part`
+/// what of the caller's own box the drawing takes. `narrow` is the length a
+/// declared rule gives the frame, which makes the frame the box the drawing
+/// goes in -- a rule is what decided where it goes, and there is nothing
+/// left to place inside it. A caller that narrowed the frame itself said the
+/// same thing.
+///
+/// The length is the caller's to supply so that a widget asked again gets
+/// the frame it already has rather than a second resolution of its rule.
+pub(crate) fn frame_and_extent(
+    mut frame: UiRegion,
+    part: UiRegion,
+    narrow: [Option<Len>; 2],
     align: RegionAlign,
-) -> UiRegion {
-    for (axis, len) in AXES.into_iter().zip(declared) {
-        let Some(len) = len else { continue };
-        let span = region.axis_mut(axis);
-        let len = Len::from_parts(len.rel, len.px);
-        span.start += (span.len() - len).scale(align.axis(axis).rel());
-        span.end = span.start + len;
+) -> (UiRegion, UiRegion) {
+    let mut extent = part;
+    for (axis, narrow) in AXES.into_iter().zip(narrow) {
+        let span = frame.axis_mut(axis);
+        let narrowed = match narrow {
+            Some(len) => {
+                let slot = part.axis(axis);
+                let start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
+                *span = UiSpan::new(start, start + len);
+                true
             }
-    region
+            None => *span != UiSpan::FULL,
+        };
+        if narrowed {
+            *extent.axis_mut(axis) = UiSpan::FULL;
+        }
+    }
+    (frame, extent)
 }

core/src/ui/place.rs

@@ -0,0 +1,61 @@
+use crate::{PrimitiveHandle, UiRegion, UiSpan};
+
+/// What of a widget's own box a child is given, along one axis.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Part {
+    /// The whole of it.
+    All,
+    /// Frame lengths from where the box starts, which is what a container
+    /// dividing room among its children speaks: a child's report is a length
+    /// of the frame, so the cursor that sums those reports is one too. A
+    /// moved box re-places every child by re-adding its start, exactly.
+    From(UiSpan),
+    /// A part of the box in its own coordinates, which is what a container
+    /// that insets one speaks: taking eleven pixels off the end needs no
+    /// length, where saying the same thing in frame lengths would make the
+    /// container read its own box -- and a box chosen from its own answer
+    /// then feeds back into the answer.
+    Of(UiSpan),
+}
+
+impl Part {
+    /// Where it lands in the coordinates `extent` is in.
+    pub(crate) fn of(self, extent: UiSpan) -> UiSpan {
+        match self {
+            Self::All => extent,
+            Self::From(span) => UiSpan::new(extent.start + span.start, extent.start + span.end),
+            Self::Of(span) => span.within(&extent),
+        }
+    }
+}
+
+/// Where a child goes along one axis, as a part of this widget's box.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Place {
+    /// The child's answer, aligned inside the part by the child's alignment.
+    Within(Part),
+    /// Exactly the part; the answer is not placed inside it again.
+    Fill(Part),
+}
+
+impl Place {
+    pub(crate) fn part(self) -> Part {
+        match self {
+            Self::Within(part) | Self::Fill(part) => part,
+        }
+    }
+
+    /// Whether the part is the drawing's box outright, rather than the box
+    /// the answer is placed inside.
+    pub(crate) fn fills(self) -> bool {
+        matches!(self, Self::Fill(_))
+    }
+}
+
+/// A primitive as it was written: its box in the widget's own box's
+/// coordinates, which is what a move of that box re-composes from.
+#[derive(Debug)]
+pub struct RetainedPrimitive {
+    pub handle: PrimitiveHandle,
+    pub region: UiRegion,
+}

core/src/util/slot.rs

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

examples/tabs/main.rs

@@ -20,10 +20,14 @@ impl DefaultAppState for Client {
         let pad_test = (
             rrect.color(Color::BLUE),
             (
+                // The square is one widget and the two shares of the row it
+                // sits centred in are another: a length is a property of a
+                // widget, so `.width` here would overwrite the `.sized`.
                 rrect
                     .color(Color::RED)
                     .sized((100, 100))
                     .center()
+                    .wrapper()
                     .width(leftover(2)),
                 (
                     rrect.color(Color::ORANGE),
@@ -143,7 +147,7 @@ impl DefaultAppState for Client {
             .span(Dir::DOWN)
             .add(rsc);
 
-        let main = WidgetPtr::new().add(rsc);
+        let main = Wrapper::new().add(rsc);
 
         let vals = Rc::new(RefCell::new((0, Vec::new())));
         let mut switch_button = |color, to: WeakWidget, label| {

examples/text.rs

@@ -28,10 +28,14 @@ impl DefaultAppState for State {
             .pad(16)
             .background(panel());
 
+        // Each one takes the whole width, because `text_align` puts the
+        // glyphs somewhere in the box the text is given and a text that
+        // reports the width of its own glyphs is given exactly that.
+        let label = |text: &str, align| wtext(text).size(24).text_align(align).width(rel(1.0));
         let aligned = (
-            wtext("left").size(24).text_align(Align::LEFT),
-            wtext("centred").size(24).text_align(Align::CENTER),
-            wtext("right").size(24).text_align(Align::RIGHT),
+            label("left", Align::LEFT),
+            label("centred", Align::H_CENTER),
+            label("right", Align::RIGHT),
         )
             .span(Dir::DOWN)
             .gap(8)

src/default/mod.rs

@@ -251,7 +251,7 @@ impl<State: DefaultAppState> AppState for DefaultApp<State> {
                 ui_state.renderer.draw();
             }
             WindowEvent::Resized(size) => {
-                render.resize((size.width, size.height));
+                render.resize((size.width, size.height), rsc.widgets_mut());
                 ui_state.renderer.resize(size)
             }
             WindowEvent::KeyboardInput { event, .. } => {

src/harness.rs

@@ -144,9 +144,9 @@ impl Harness {
         // bound that comes with `SyncSender` is far past anything a test
         // leaves unread.
         let (send, updates) = sync_channel(1024);
-        let rsc = DefaultRsc::init(Arc::new(Queue(send)));
+        let mut rsc = DefaultRsc::init(Arc::new(Queue(send)));
         let mut render = UiRenderState::new();
-        render.resize(size);
+        render.resize(size, rsc.widgets_mut());
         Self {
             rsc,
             render,
@@ -161,7 +161,7 @@ impl Harness {
     }
 
     pub fn resize(&mut self, size: impl Into<Vec2>) {
-        self.render.resize(size);
+        self.render.resize(size, self.rsc.widgets_mut());
     }
 
     /// Changes a length rule after the fact, the way `.width()` sets one.

src/random.rs

@@ -101,10 +101,6 @@ pub struct Tree {
     pub nodes: Vec<WidgetId>,
     pub spans: Vec<Spanned>,
     pub scrolls: Vec<WeakWidget<Scroll>>,
-    /// Children a `SpanEdit` took out, held so that dropping the last share
-    /// of one does not free its id for the next widget to be given -- which
-    /// would put the two trees' `ids` out of step.
-    pub detached: Vec<StrongWidget>,
 }
 
 /// Branches on a child's measured length. Comparing boxes catches a widget
@@ -121,16 +117,18 @@ pub struct Branch {
 
 impl Widget for Branch {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let mut top = UiRegion::FULL;
-        top.y.end = top.y.start.offset(Px::from_int(40));
-        let measured = painter.widget_within(&self.probe, top).len(Axis::X);
+        let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
+        let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
+        let measured = painter
+            .widget_at(&self.probe, UiRegion::FULL, [Place::Within(Part::All), top])
+            .len(Axis::X);
         let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
 
-        let mut below = UiRegion::FULL;
-        below.y.start = below.y.start.offset(Px::from_int(40));
+        let below = Place::Within(Part::Of(UiSpan::new(cut, Len::FULL)));
+        let place = [Place::Within(Part::All), below];
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_within(&self.wide, below),
-            false => painter.widget_within(&self.narrow, below),
+            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
+            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
         };
         Size::LEFTOVER
     }
@@ -138,15 +136,463 @@ impl Widget for Branch {
 
 pub struct Spanned {
     pub id: WeakWidget<Span>,
-    /// Leaves grown with the span whether or not they end up in it, so both
-    /// trees make the same widgets in the same order either way. Attaching
-    /// one moves it out of here: a widget belongs to one parent, and one that
-    /// belongs to nobody still has to be held or it reads as a leak.
+    /// Everything made for this span that it does not hold -- spares never
+    /// attached and children detached alike. A widget belongs to one parent,
+    /// and one that belongs to nobody still has to be held here: dropping
+    /// the last share of it frees its id for the next widget to be given,
+    /// which puts two trees out of step.
     pub spares: Vec<StrongWidget>,
     /// How many children it was grown with, before any edit.
     pub grown: usize,
 }
 
+/// A tree described rather than built: [`plan`] turns a seed into one of
+/// these and [`build`] turns it into widgets, where growing did both at once.
+///
+/// The split is what makes a counterexample readable. A failing seed used to
+/// be the entire record of one, because a grower that makes widgets as it
+/// draws leaves nothing to take apart -- a shrinker could only grow its own
+/// trees and hope to meet the same shape, which in practice it does not. A
+/// plan is reduced by [`Plan::smaller`] and built again, so any seed that
+/// fails can be cut down until what is left is small enough to read.
+#[derive(Clone, Debug, PartialEq)]
+pub struct Plan {
+    pub kind: Kind,
+    /// The declared size this widget carries. Whoever grows a widget offers
+    /// it one and the offer is taken or declined; a second offer to the same
+    /// widget is dropped, because two rules on one widget would settle in the
+    /// order they were applied rather than in grow order.
+    pub size: Option<Lens>,
+    /// The alignment it carries, under the same one-offer rule.
+    pub align: Option<Aligns>,
+    /// Whether it was offered a movable region of its own and what it
+    /// answered. `Some(false)` is an offer declined, which still uses up the
+    /// one offer, where `None` is an offer never made.
+    pub region_node: Option<bool>,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Kind {
+    /// Wrapped and unwrapped text, because only one of them reads the width
+    /// it is given and so only one has to be drawn again for a new one.
+    Wrapped,
+    OneLine,
+    Rect {
+        color: usize,
+        alpha: u8,
+    },
+    /// Scrolling reads the pixel length of its box, which nothing else here
+    /// does, and gives its child a box longer than its own.
+    Scroll {
+        axis: Axis,
+        inner: Box<Plan>,
+    },
+    /// All three sides are grown either way, so a tree that draws one has the
+    /// same ids as a tree that draws another.
+    Branch {
+        probe: Box<Plan>,
+        wide: Box<Plan>,
+        narrow: Box<Plan>,
+        threshold: f32,
+    },
+    /// Each side its own, since a padding that is the same all round hides
+    /// anything that treats one edge differently from another.
+    Pad {
+        padding: [i32; 4],
+        inner: Box<Plan>,
+    },
+    Stack {
+        children: Vec<Plan>,
+    },
+    Span {
+        dir: usize,
+        gap: i32,
+        /// Grown for this span, in the order they are made.
+        children: Vec<Plan>,
+        /// Grown beside it whether or not they end up in it, so the widget
+        /// after them has the same id in a tree that leaves them out as in
+        /// one that puts them in.
+        spares: Vec<Plan>,
+        /// Which of `children` then `spares` are actually in the span, and
+        /// in what order -- kept apart from the two lists above so that a
+        /// tree which detaches, attaches or reorders its children still
+        /// makes the same widgets in the same order, and two builds line up
+        /// index for index. Anything not named here is built and held
+        /// rather than dropped, since freeing an id hands it to the next
+        /// widget and puts two trees out of step.
+        order: Vec<usize>,
+    },
+}
+
+impl Plan {
+    /// A widget carrying nothing anybody has offered it yet.
+    fn bare(kind: Kind) -> Self {
+        Self {
+            kind,
+            size: None,
+            align: None,
+            region_node: None,
+        }
+    }
+
+    /// How many widgets building it makes, spares and detached children
+    /// included, since those are made either way.
+    pub fn size(&self) -> usize {
+        1 + match &self.kind {
+            Kind::Scroll { inner, .. } | Kind::Pad { inner, .. } => inner.size(),
+            Kind::Branch {
+                probe,
+                wide,
+                narrow,
+                ..
+            } => probe.size() + wide.size() + narrow.size(),
+            Kind::Stack { children } => children.iter().map(Plan::size).sum(),
+            Kind::Span {
+                children, spares, ..
+            } => children.iter().chain(spares).map(Plan::size).sum(),
+            _ => 0,
+        }
+    }
+
+    /// The trees to try instead of this one when reducing a counterexample,
+    /// biggest cut first: a shrinker takes the first that still fails, so
+    /// offering "this subtree alone" before "this subtree with one child
+    /// fewer" is what gets from six hundred widgets to six rather than to
+    /// five hundred and ninety.
+    ///
+    /// Every one of these is a tree the generator could have grown, so a
+    /// reduced plan is a counterexample in its own right rather than a
+    /// special case only the shrinker can make.
+    pub fn smaller(&self) -> Vec<Plan> {
+        let mut out = Vec::new();
+        // Standing in for the whole of it, which is the largest cut there is.
+        for kid in self.kids() {
+            out.push(kid.clone());
+        }
+        // Then what it carries, which costs nothing to put back if it was
+        // not the thing that mattered.
+        for dropped in [
+            self.region_node.map(|_| Plan {
+                region_node: None,
+                ..self.clone()
+            }),
+            self.align.map(|_| Plan {
+                align: None,
+                ..self.clone()
+            }),
+            self.size.map(|_| Plan {
+                size: None,
+                ..self.clone()
+            }),
+        ]
+        .into_iter()
+        .flatten()
+        {
+            out.push(dropped);
+        }
+        out.extend(self.kind.smaller().into_iter().map(|kind| Plan {
+            kind,
+            ..self.clone()
+        }));
+        out
+    }
+
+    /// Visits every widget in the order [`build`] makes them, so a count
+    /// kept by the visitor indexes the same widget as the matching [`Tree`]
+    /// vector does.
+    pub fn walk_mut(&mut self, at: &mut impl FnMut(&mut Plan)) {
+        match &mut self.kind {
+            Kind::Scroll { inner, .. } | Kind::Pad { inner, .. } => inner.walk_mut(at),
+            Kind::Branch {
+                probe,
+                wide,
+                narrow,
+                ..
+            } => {
+                probe.walk_mut(at);
+                wide.walk_mut(at);
+                narrow.walk_mut(at);
+            }
+            Kind::Stack { children } => {
+                for child in children {
+                    child.walk_mut(at);
+                }
+            }
+            Kind::Span {
+                children, spares, ..
+            } => {
+                for child in children.iter_mut().chain(spares) {
+                    child.walk_mut(at);
+                }
+            }
+            _ => {}
+        }
+        at(self);
+    }
+
+    /// The same tree with `edits` applied, by the indices the generator would
+    /// have used for them.
+    ///
+    /// [`plan`] resolves edits while drawing, which needs a seed. A scenario
+    /// needs them applied to a tree that already exists -- one it has built,
+    /// and one a shrinker may already have cut down, where no seed grows it
+    /// any more. Both routes take the same [`Edits`], so a case written
+    /// against one reads the same against the other.
+    pub fn edited(&self, edits: &Edits) -> Plan {
+        let mut out = self.clone();
+        let (mut sized, mut aligned, mut nodes, mut spans) = (0, 0, 0, 0);
+        out.walk_mut(&mut |plan| {
+            if let Kind::Span {
+                children,
+                spares,
+                order,
+                ..
+            } = &mut plan.kind
+            {
+                if let Some(edit) = edits.spans.get(&spans) {
+                    *order = span_edited(order, children.len(), spares.len(), edit);
+                }
+                spans += 1;
+            }
+            if let Kind::Branch { threshold, .. } = &mut plan.kind
+                && edits.fixed_branches
+            {
+                *threshold = f32::MIN;
+            }
+            if plan.size.is_some() {
+                if let Some(lens) = edits.sizes.get(&sized) {
+                    plan.size = Some(*lens);
+                }
+                sized += 1;
+            }
+            if plan.align.is_some() {
+                if let Some(align) = edits.aligns.get(&aligned) {
+                    plan.align = Some(*align);
+                }
+                aligned += 1;
+            }
+            if plan.region_node.is_some() {
+                if let Some(take) = edits.nodes.get(&nodes) {
+                    plan.region_node = Some(*take);
+                }
+                nodes += 1;
+            }
+        });
+        out
+    }
+
+    fn kids(&self) -> Vec<&Plan> {
+        match &self.kind {
+            Kind::Scroll { inner, .. } | Kind::Pad { inner, .. } => vec![inner],
+            Kind::Branch {
+                probe,
+                wide,
+                narrow,
+                ..
+            } => vec![probe, wide, narrow],
+            Kind::Stack { children } => children.iter().collect(),
+            Kind::Span { children, .. } => children.iter().collect(),
+            _ => Vec::new(),
+        }
+    }
+}
+
+impl Kind {
+    /// Simplifications of the shape alone, leaving what the widget carries to
+    /// [`Plan::smaller`]. Replacing a node with one of its children is there
+    /// rather than here, since it answers with a whole `Plan`.
+    fn smaller(&self) -> Vec<Kind> {
+        let mut out = Vec::new();
+        /// One child reduced at a time, rebuilt into the same shape. Every
+        /// answer has the same number of children as it was given, so it is
+        /// for the shapes whose child count is part of what they are.
+        fn reduced(kids: &[Plan], rebuild: &dyn Fn(Vec<Plan>) -> Kind) -> Vec<Kind> {
+            let mut out = Vec::new();
+            for (i, kid) in kids.iter().enumerate() {
+                for small in kid.smaller() {
+                    let mut next = kids.to_vec();
+                    next[i] = small;
+                    out.push(rebuild(next));
+                }
+            }
+            out
+        }
+
+        /// One child dropped, then [`reduced`]. For the shapes that hold any
+        /// number of children, where dropping one is the cut that matters.
+        fn each(kids: &[Plan], rebuild: &dyn Fn(Vec<Plan>) -> Kind) -> Vec<Kind> {
+            let mut out = Vec::new();
+            for i in 0..kids.len() {
+                if kids.len() > 1 {
+                    let mut less = kids.to_vec();
+                    less.remove(i);
+                    out.push(rebuild(less));
+                }
+            }
+            out.extend(reduced(kids, rebuild));
+            out
+        }
+        match self {
+            // The one leaf that reads the width it is given, then the one
+            // that does not, then the one that measures nothing at all.
+            Kind::Wrapped => out.push(Kind::OneLine),
+            Kind::OneLine => out.push(Kind::Rect {
+                color: 0,
+                alpha: 255,
+            }),
+            Kind::Rect { .. } => {}
+            Kind::Scroll { axis, inner } => {
+                let axis = *axis;
+                out.extend(each(std::slice::from_ref(inner), &|mut k| Kind::Scroll {
+                    axis,
+                    inner: Box::new(k.remove(0)),
+                }));
+            }
+            Kind::Branch {
+                probe,
+                wide,
+                narrow,
+                threshold,
+            } => {
+                let threshold = *threshold;
+                // All three sides stay: a branch is the widget that draws
+                // one of two on a measurement, and one with a side missing
+                // is a different widget rather than a smaller one. Dropping
+                // the branch for a side is offered by `Plan::smaller`.
+                let sides = [(**probe).clone(), (**wide).clone(), (**narrow).clone()];
+                out.extend(reduced(&sides, &|k| Kind::Branch {
+                    probe: Box::new(k[0].clone()),
+                    wide: Box::new(k[1].clone()),
+                    narrow: Box::new(k[2].clone()),
+                    threshold,
+                }));
+            }
+            Kind::Pad { padding, inner } => {
+                let padding = *padding;
+                if padding != [0; 4] {
+                    out.push(Kind::Pad {
+                        padding: [0; 4],
+                        inner: inner.clone(),
+                    });
+                }
+                out.extend(each(std::slice::from_ref(inner), &|mut k| Kind::Pad {
+                    padding,
+                    inner: Box::new(k.remove(0)),
+                }));
+            }
+            Kind::Stack { children } => {
+                out.extend(each(children, &|children| Kind::Stack { children }))
+            }
+            Kind::Span {
+                dir,
+                gap,
+                children,
+                spares,
+                order,
+            } => {
+                let (dir, gap, n) = (*dir, *gap, children.len());
+                let span = |children: Vec<Plan>, spares: Vec<Plan>, order: Vec<usize>| Kind::Span {
+                    dir,
+                    gap,
+                    children,
+                    spares,
+                    order,
+                };
+                let identity: Vec<usize> = (0..n).collect();
+                // An order the generator did not choose is part of the tree,
+                // so take that off before taking the tree apart.
+                if *order != identity {
+                    out.push(span(children.clone(), spares.clone(), identity));
+                }
+                // Spares exist to be attached; with none attached they are
+                // widgets the span never holds.
+                if !spares.is_empty() && order.iter().all(|&i| i < n) {
+                    out.push(span(children.clone(), Vec::new(), order.clone()));
+                }
+                if gap != 0 {
+                    out.push(Kind::Span {
+                        dir,
+                        gap: 0,
+                        children: children.clone(),
+                        spares: spares.clone(),
+                        order: order.clone(),
+                    });
+                }
+                for k in 0..n {
+                    if n > 1 {
+                        let mut less = children.clone();
+                        less.remove(k);
+                        // Everything after it shifts down, spares included,
+                        // since they are indexed past the children.
+                        let order = order
+                            .iter()
+                            .filter(|&&i| i != k)
+                            .map(|&i| if i > k { i - 1 } else { i })
+                            .collect();
+                        out.push(span(less, spares.clone(), order));
+                    }
+                }
+                for (i, kid) in children.iter().enumerate() {
+                    for small in kid.smaller() {
+                        let mut next = children.clone();
+                        next[i] = small;
+                        out.push(span(next, spares.clone(), order.clone()));
+                    }
+                }
+            }
+        }
+        out
+    }
+}
+
+/// A [`SpanEdit`] applied to the order a span already holds its children in.
+///
+/// `detach` names positions in that order and `attach` takes from the front
+/// of what the span is not holding, both of which is what a test changing a
+/// live span does -- so an edit means the same thing said to a tree and said
+/// to the plan it was built from. On a span nobody has edited the order is
+/// the children in the order they were grown, and this is then "leave these
+/// out and put that many spares on the end".
+fn span_edited(order: &[usize], children: usize, spares: usize, edit: &SpanEdit) -> Vec<usize> {
+    let mut detach = edit.detach.clone();
+    detach.sort_unstable();
+    detach.dedup();
+    let mut next: Vec<usize> = order
+        .iter()
+        .enumerate()
+        .filter(|(at, _)| !detach.contains(at))
+        .map(|(_, &which)| which)
+        .collect();
+    // What the span is not holding, in the order it hands them back: what it
+    // was already not holding first, in the order the widgets were made, and
+    // what this edit takes out after that, highest position first. A child
+    // just detached goes to the back rather than straight back in, which is
+    // what makes detaching one and attaching one a trade.
+    let mut free: Vec<usize> = (0..children + spares)
+        .filter(|i| !order.contains(i))
+        .collect();
+    free.extend(detach.iter().rev().filter_map(|&at| order.get(at).copied()));
+    next.extend(free.into_iter().take(edit.attach));
+    next
+}
+
+/// Plans the tree `seed` describes, `edits` replacing what it would otherwise
+/// have given the widgets that carry them.
+///
+/// The edits are resolved here rather than at build time, so that a plan is
+/// the whole of what a tree is and building one has nothing left to decide.
+pub fn plan(seed: u64, depth: usize, edits: &Edits) -> Plan {
+    let mut sow = Sow {
+        rng: Rng::new(seed),
+        edits,
+        sized: 0,
+        aligned: 0,
+        nodes: 0,
+        spans: 0,
+    };
+    sow.node(depth)
+}
+
 /// Grows the tree `seed` describes, `edits` replacing the declared sizes it
 /// would otherwise have given those wrappers.
 pub fn grow<Rsc: UiRsc + 'static>(
@@ -155,41 +601,32 @@ pub fn grow<Rsc: UiRsc + 'static>(
     depth: usize,
     edits: &Edits,
 ) -> (StrongWidget, Tree) {
-    let mut grow = Grow {
-        rsc,
-        rng: Rng::new(seed),
-        tree: Tree::default(),
-        edits,
-    };
-    let root = grow.node(depth);
-    (root, grow.tree)
+    build(rsc, &plan(seed, depth, edits))
 }
 
-struct Grow<'a, Rsc> {
-    rsc: &'a mut Rsc,
+/// Draws a plan out of the random stream. Every draw happens in the order it
+/// always has and before the decision it feeds, including the decisions that
+/// are then dropped, because a seed has to keep meaning the same tree.
+struct Sow<'a> {
     rng: Rng,
-    tree: Tree,
     edits: &'a Edits,
+    sized: usize,
+    aligned: usize,
+    nodes: usize,
+    spans: usize,
 }
 
-impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
-    fn leaf(&mut self) -> StrongWidget {
-        let id: StrongWidget = match self.rng.below(4) {
-            // Wrapped and unwrapped, because only one of them reads the width
-            // it is given and so only one has to be drawn again for a new one.
-            0 => wtext(WORDS).size(16).wrap(true).add_strong(self.rsc),
-            1 => wtext("one line, overflowing whatever it is given")
-                .size(16)
-                .wrap(false)
-                .add_strong(self.rsc),
+impl Sow<'_> {
+    fn leaf(&mut self) -> Plan {
+        Plan::bare(match self.rng.below(4) {
+            0 => Kind::Wrapped,
+            1 => Kind::OneLine,
             _ => {
-                let color = COLORS[self.rng.below(COLORS.len())];
+                let color = self.rng.below(COLORS.len());
                 let alpha = (self.rng.below(5) * 63) as u8;
-                rect(color.alpha(alpha)).add_strong(self.rsc)
+                Kind::Rect { color, alpha }
             }
-        };
-        self.tree.ids.push(id.id());
-        id
+        })
     }
 
     fn len(&mut self) -> Option<LayoutLen> {
@@ -200,100 +637,77 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
         }
     }
 
-    fn align(&mut self) -> Align {
-        let mut axis = || match self.rng.below(4) {
+    fn align(&mut self) -> Aligns {
+        let axis = |s: &mut Self| match s.rng.below(4) {
             0 => None,
             1 => Some(AxisAlign::NEG),
             2 => Some(AxisAlign::CENTER),
             _ => Some(AxisAlign::POS),
         };
-        let (mut x, y) = (axis(), axis());
+        let (x, y) = (axis(self), axis(self));
         // Aligning on neither axis leaves the branch unexercised.
-        if x.is_none() && y.is_none() {
-            x = Some(AxisAlign::CENTER);
+        match x.is_none() && y.is_none() {
+            true => [Some(AxisAlign::CENTER), y],
+            false => [x, y],
         }
-        Align { x, y }
     }
 
     /// A declared size over half the tree, kept where a test can change it.
-    fn sized(&mut self, inner: StrongWidget) -> StrongWidget {
-        // A rule is a property now, so a node already carrying one would take
-        // a second entry in `sized` -- and two edits naming one widget settle
-        // in the order they are applied, which is grow order cold and edit
-        // order warm. One entry per widget instead. Both draws are taken
-        // whatever is decided, and the decision is grow order alone, so the
-        // two trees consume the same random stream.
+    fn sized(&mut self, inner: &mut Plan) {
         let take = self.rng.chance();
         let lens = [self.len(), self.len()];
-        if !take || self.tree.sized.contains(&inner.id()) {
-            return inner;
+        if !take || inner.size.is_some() {
+            return;
         }
-        let idx = self.tree.sized.len();
-        let lens = self.edits.sizes.get(&idx).copied().unwrap_or(lens);
-        let id = inner.id();
-        self.rsc
-            .ui_mut()
-            .widgets
-            .set_size_rules(id, lens[0], lens[1]);
-        self.tree.sized.push(id);
-        inner
+        let idx = self.sized;
+        self.sized += 1;
+        inner.size = Some(self.edits.sizes.get(&idx).copied().unwrap_or(lens));
     }
 
     /// An alignment over some of the tree, kept where a test can change it.
-    /// One entry per widget for the reason `sized` gives.
-    fn aligned(&mut self, inner: StrongWidget) -> StrongWidget {
+    fn aligned(&mut self, inner: &mut Plan) {
         let align = self.align();
-        let align = [align.x, align.y];
-        if self.tree.aligned.contains(&inner.id()) {
-            return inner;
+        if inner.align.is_some() {
+            return;
         }
-        let idx = self.tree.aligned.len();
-        let align = self.edits.aligns.get(&idx).copied().unwrap_or(align);
-        let id = inner.id();
-        let widgets = &mut self.rsc.ui_mut().widgets;
-        for (axis, align) in [Axis::X, Axis::Y].into_iter().zip(align) {
-            widgets.set_alignment(id, axis, align.unwrap_or_default());
-        }
-        self.tree.aligned.push(id);
-        inner
+        let idx = self.aligned;
+        self.aligned += 1;
+        inner.align = Some(self.edits.aligns.get(&idx).copied().unwrap_or(align));
     }
 
     /// A movable region of its own over some of the tree. What it changes is
     /// how a move is written and how long a primitive's chain is, neither of
     /// which any other branch here varies.
-    fn noded(&mut self, inner: StrongWidget) -> StrongWidget {
+    fn noded(&mut self, inner: &mut Plan) {
         let take = self.rng.below(4) == 0;
-        if self.tree.nodes.contains(&inner.id()) {
-            return inner;
+        if inner.region_node.is_some() {
+            return;
         }
-        let idx = self.tree.nodes.len();
-        let take = self.edits.nodes.get(&idx).copied().unwrap_or(take);
-        let id = inner.id();
-        self.rsc.ui_mut().widgets.set_region_node(id, take);
-        self.tree.nodes.push(id);
-        inner
+        let idx = self.nodes;
+        self.nodes += 1;
+        inner.region_node = Some(self.edits.nodes.get(&idx).copied().unwrap_or(take));
     }
 
-    fn node(&mut self, depth: usize) -> StrongWidget {
+    fn offered(&mut self, inner: &mut Plan) {
+        self.sized(inner);
+        self.noded(inner);
+    }
+
+    fn node(&mut self, depth: usize) -> Plan {
         if depth == 0 {
             return self.leaf();
         }
         let positioned = self.rng.below(6);
         if positioned == 0 {
-            // Scrolling reads the pixel length of its box, which nothing
-            // else here does, and gives its child a box longer than its own.
-            let inner = self.node(depth - 1);
-            let inner = self.sized(inner);
-            let inner = self.noded(inner);
+            let mut inner = self.node(depth - 1);
+            self.offered(&mut inner);
             let axis = if self.rng.chance() { Axis::X } else { Axis::Y };
-            let id = Scroll::new(inner, axis).add(self.rsc);
-            self.tree.scrolls.push(id);
-            self.tree.ids.push(id.id());
-            return id.add_strong(self.rsc);
+            return Plan::bare(Kind::Scroll {
+                axis,
+                inner: Box::new(inner),
+            });
         }
         if positioned == 2 {
-            // Both sides are grown either way, so a tree that draws one has
-            // the same ids as a tree that draws the other.
             let probe = self.node(depth - 1);
             let wide = self.node(depth - 1);
             let narrow = self.node(depth - 1);
@@ -305,83 +719,188 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
                 true => f32::MIN,
                 false => measured,
             };
-            let id = Branch {
+            return Plan::bare(Kind::Branch {
+                probe: Box::new(probe),
+                wide: Box::new(wide),
+                narrow: Box::new(narrow),
+                threshold,
+            });
+        }
+        if positioned == 1 {
+            // Carries an alignment and makes no widget of its own, so the
+            // plan for it is the child it aligned.
+            let mut inner = self.node(depth - 1);
+            self.offered(&mut inner);
+            self.aligned(&mut inner);
+            return inner;
+        }
+        if self.rng.below(4) == 0 {
+            let mut inner = self.node(depth - 1);
+            self.offered(&mut inner);
+            let side = |s: &mut Self| s.rng.below(24) as i32;
+            let padding = [side(self), side(self), side(self), side(self)];
+            return Plan::bare(Kind::Pad {
+                padding,
+                inner: Box::new(inner),
+            });
+        }
+        let grown = 2 + self.rng.below(3);
+        let mut children = Vec::with_capacity(grown);
+        for _ in 0..grown {
+            let mut child = self.node(depth - 1);
+            self.offered(&mut child);
+            children.push(child);
+        }
+        if self.rng.chance() {
+            return Plan::bare(Kind::Stack { children });
+        }
+        let spares: Vec<Plan> = (0..SPARES).map(|_| self.leaf()).collect();
+        let idx = self.spans;
+        self.spans += 1;
+        let edit = self.edits.spans.get(&idx).cloned().unwrap_or_default();
+        let dir = self.rng.below(4);
+        // A row takes the height it is given rather than its tallest child,
+        // which is a rule beside it. Derived from an existing choice and
+        // consuming no randomness: a seed must keep growing the same tree
+        // when the generator gains another configuration.
+        let gap = self.rng.below(3) as i32 * 4;
+        let grown: Vec<usize> = (0..children.len()).collect();
+        let order = span_edited(&grown, children.len(), spares.len(), &edit);
+        Plan::bare(Kind::Span {
+            dir,
+            gap,
+            children,
+            spares,
+            order,
+        })
+    }
+}
+
+/// Builds a plan's widgets in the order it describes them, so two builds of
+/// one plan line up index for index and their boxes can be compared.
+pub fn build<Rsc: UiRsc + 'static>(rsc: &mut Rsc, plan: &Plan) -> (StrongWidget, Tree) {
+    let mut build = Build {
+        rsc,
+        tree: Tree::default(),
+    };
+    let root = build.node(plan);
+    (root, build.tree)
+}
+
+struct Build<'a, Rsc> {
+    rsc: &'a mut Rsc,
+    tree: Tree,
+}
+
+impl<Rsc: UiRsc + 'static> Build<'_, Rsc> {
+    fn node(&mut self, plan: &Plan) -> StrongWidget {
+        let built = self.kind(&plan.kind);
+        let id = built.id();
+        if let Some(lens) = plan.size {
+            self.rsc
+                .ui_mut()
+                .widgets
+                .set_size_rules(id, lens[0], lens[1]);
+            self.tree.sized.push(id);
+        }
+        if let Some(align) = plan.align {
+            let widgets = &mut self.rsc.ui_mut().widgets;
+            for (axis, align) in [Axis::X, Axis::Y].into_iter().zip(align) {
+                widgets.set_alignment(id, axis, align.unwrap_or_default());
+            }
+            self.tree.aligned.push(id);
+        }
+        if let Some(take) = plan.region_node {
+            self.rsc.ui_mut().widgets.set_region_node(id, take);
+            self.tree.nodes.push(id);
+        }
+        built
+    }
+
+    fn kind(&mut self, kind: &Kind) -> StrongWidget {
+        let id: StrongWidget = match kind {
+            Kind::Wrapped => wtext(WORDS).size(16).wrap(true).add_strong(self.rsc),
+            Kind::OneLine => wtext("one line, overflowing whatever it is given")
+                .size(16)
+                .wrap(false)
+                .add_strong(self.rsc),
+            Kind::Rect { color, alpha } => rect(COLORS[*color].alpha(*alpha)).add_strong(self.rsc),
+            Kind::Scroll { axis, inner } => {
+                let inner = self.node(inner);
+                let id = Scroll::new(inner, *axis).add(self.rsc);
+                self.tree.scrolls.push(id);
+                self.tree.ids.push(id.id());
+                return id.add_strong(self.rsc);
+            }
+            Kind::Branch {
                 probe,
                 wide,
                 narrow,
                 threshold,
+            } => {
+                let probe = self.node(probe);
+                let wide = self.node(wide);
+                let narrow = self.node(narrow);
+                let id = Branch {
+                    probe,
+                    wide,
+                    narrow,
+                    threshold: *threshold,
                 }
                 .add(self.rsc);
                 self.tree.ids.push(id.id());
                 return id.add_strong(self.rsc);
             }
-        if positioned == 1 {
-            let inner = self.node(depth - 1);
-            let inner = self.sized(inner);
-            let inner = self.noded(inner);
-            return self.aligned(inner);
-        }
-        if self.rng.below(4) == 0 {
-            let inner = self.node(depth - 1);
-            let inner = self.sized(inner);
-            let inner = self.noded(inner);
-            // Each side its own, since a padding that is the same all round
-            // hides anything that treats one edge differently from another.
-            let mut side = || Px::from_int(self.rng.below(24) as i32);
+            Kind::Pad { padding, inner } => {
+                let inner = self.node(inner);
+                let [left, right, top, bottom] = padding.map(Px::from_int);
                 let padding = Padding {
-                left: side(),
-                right: side(),
-                top: side(),
-                bottom: side(),
+                    left,
+                    right,
+                    top,
+                    bottom,
                 };
-            let id = Pad { padding, inner }.add_strong(self.rsc);
-            self.tree.ids.push(id.id());
-            return id;
+                Pad { padding, inner }.add_strong(self.rsc)
             }
-        let grown = 2 + self.rng.below(3);
-        let mut children = Vec::with_capacity(grown);
-        for _ in 0..grown {
-            let child = self.node(depth - 1);
-            let child = self.sized(child);
-            let child = self.noded(child);
-            children.push(child);
-        }
-        if self.rng.chance() {
-            let id = Stack {
+            Kind::Stack { children } => {
+                let children = children.iter().map(|c| self.node(c)).collect();
+                Stack {
                     children,
                     size: StackSize::Child(0),
                 }
-            .add_strong(self.rsc);
-            self.tree.ids.push(id.id());
-            return id;
+                .add_strong(self.rsc)
             }
-        // Grown either way, so the widget after them has the same id in a
-        // tree that leaves them out as in one that puts them in.
-        let mut spares: Vec<StrongWidget> = (0..SPARES).map(|_| self.leaf()).collect();
-        let idx = self.tree.spans.len();
-        let edit = self.edits.spans.get(&idx).cloned().unwrap_or_default();
-        // Highest first, so an index means the same child however many of its
-        // neighbours are going too.
-        let mut detach = edit.detach.clone();
-        detach.sort_unstable();
-        for j in detach.into_iter().rev() {
-            if j < children.len() {
-                self.tree.detached.push(children.remove(j));
-            }
-        }
-        let attach = edit.attach.min(spares.len());
-        children.extend(spares.drain(..attach));
-        let dir = [Dir::RIGHT, Dir::DOWN, Dir::LEFT, Dir::UP][self.rng.below(4)];
+            Kind::Span {
+                dir,
+                gap,
+                children,
+                spares,
+                order,
+            } => {
+                let grown = children.len();
+                // Every one of them is made, in this order, whether or not
+                // the span ends up holding it.
+                let made: Vec<StrongWidget> = children
+                    .iter()
+                    .chain(spares)
+                    .map(|c| self.node(c))
+                    .collect();
+                let mut left: Vec<Option<StrongWidget>> = made.into_iter().map(Some).collect();
+                let children: Vec<StrongWidget> = order
+                    .iter()
+                    .filter_map(|&i| left.get_mut(i).and_then(Option::take))
+                    .collect();
+                // What the span does not hold is still held here: dropping
+                // the last share of a widget frees its id for the next one
+                // to be given, which puts two trees out of step.
+                let spares: Vec<StrongWidget> = left.into_iter().flatten().collect();
+                let dir = [Dir::RIGHT, Dir::DOWN, Dir::LEFT, Dir::UP][*dir % 4];
                 let id = Span {
                     children,
                     dir,
-            gap: Px::from_int(self.rng.below(3) as i32 * 4),
+                    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 it. Derived from an existing choice and
-        // consuming no randomness: a seed must keep growing the same tree
-        // when the generator gains another configuration.
                 if dir.axis == Axis::X {
                     self.rsc
                         .widgets_mut()
@@ -389,6 +908,10 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
                 }
                 self.tree.ids.push(id.id());
                 self.tree.spans.push(Spanned { id, spares, grown });
-        id.add_strong(self.rsc)
+                return id.add_strong(self.rsc);
+            }
+        };
+        self.tree.ids.push(id.id());
+        id
     }
 }

src/widget/mask.rs

@@ -6,7 +6,7 @@ pub struct Masked {
 
 impl Widget for Masked {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        painter.set_mask(painter.region());
+        painter.set_mask(UiRegion::FULL);
         painter.widget(&self.inner);
         // What it occupies is its box, on both axes, for the reason `Scroll`
         // reports the same: it clips what is inside to that box, so it can

src/widget/mod.rs

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

src/widget/position/offset.rs

@@ -7,7 +7,14 @@ pub struct Offset {
 
 impl Widget for Offset {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let region = UiRegion::FULL.offset(self.amt);
-        painter.widget_within(&self.inner, region).size()
+        // The whole of this widget's box, moved: the frame passes through, so
+        // what the child declares or reports means the same as it would
+        // without the offset.
+        let moved = |len: Len, amt: Len| Place::Within(Part::From(UiSpan::new(amt, len + amt)));
+        let place = [
+            moved(painter.extent_len(Axis::X), self.amt.x),
+            moved(painter.extent_len(Axis::Y), self.amt.y),
+        ];
+        painter.widget_at(&self.inner, UiRegion::FULL, place).size()
     }
 }

src/widget/position/pad.rs

@@ -7,9 +7,30 @@ pub struct Pad {
 
 impl Widget for Pad {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let inner = painter
-            .widget_aligned(&self.inner, self.padding.region(), RegionAlign::NEAR)
-            .size();
+        // The inner's own alignment, not the near edge. This reports the
+        // inner's size plus the padding, so where the box is that answer the
+        // inset box is exactly the inner and alignment has no room to move
+        // it; where the box is bigger -- a share of a row, a rule over this
+        // widget -- the slack is the inner's to sit in, and forcing the near
+        // edge pinned it to a corner it had not asked for.
+        //
+        // The padding goes around what it pads: the frame passes through, so
+        // the inner's fractions mean what they would without it, and only
+        // the box it draws in is moved in by the pixels. Said as a part of
+        // this widget's own box in that box's own lengths, so nothing here
+        // reads how long the box is -- and a box chosen from this widget's
+        // own answer therefore does not feed back into that answer.
+        let inset = |lead: Px, trail: Px| {
+            Place::Within(Part::Of(UiSpan::new(
+                Len::from_parts(Rel::ZERO, lead),
+                Len::from_parts(Rel::ONE, -trail),
+            )))
+        };
+        let place = [
+            inset(self.padding.left, self.padding.right),
+            inset(self.padding.top, self.padding.bottom),
+        ];
+        let inner = painter.widget_at(&self.inner, UiRegion::FULL, place).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,
@@ -47,14 +68,18 @@ impl Padding {
             bottom: amt,
         }
     }
-    pub fn region(&self) -> UiRegion {
-        let mut region = UiRegion::FULL;
+    /// `region` less this padding on each side.
+    pub fn region_of(&self, mut region: UiRegion) -> UiRegion {
         region.x.start.px += self.left;
         region.y.start.px += self.top;
         region.x.end.px -= self.right;
         region.y.end.px -= self.bottom;
         region
     }
+
+    pub fn region(&self) -> UiRegion {
+        self.region_of(UiRegion::FULL)
+    }
     pub fn x(amt: impl UiNum) -> Self {
         let amt = Px::from_num(amt);
         Self {

src/widget/position/scroll.rs

@@ -12,15 +12,14 @@ pub struct Scroll {
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let container_len = painter.px_len(self.axis);
-        // Draw in the whole container only when its scrolling-axis length is
-        // not already known, then draw it at the scrolled offset.
-        let answer_len = match painter.known_len(&self.inner, self.axis, UiRegion::FULL) {
-            Some(len) => len,
-            None => painter.widget(&self.inner).size().axis(self.axis),
-        };
-        let content = answer_len.apply_leftover();
+        // Measured in the whole viewport, then drawn at the scrolled offset.
+        let whole = UiRegion::FULL;
+        let answer_len = painter
+            .widget_at(&self.inner, whole, [Place::Fill(Part::All); 2])
+            .len(self.axis);
+        let fixed = Len::from_parts(answer_len.rel, answer_len.px).to_px(container_len);
         self.container_len = container_len;
-        self.content_len = content.to_px(container_len);
+        self.content_len = fixed.max(container_len);
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
@@ -33,9 +32,9 @@ impl Widget for Scroll {
         // the drawing holds for that length alone. One scrolled part way sits
         // where it is until the box shrinks past what is left of it. Kept to
         // the end, it moves with every length.
-        let fixed_len = content.rel == Rel::ZERO;
+        let fixed_len = answer_len.rel == Rel::ZERO && answer_len.leftover == Weight::ZERO;
         if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
-            painter.holds(self.axis, self.content_len..=Px::MAX);
+            painter.holds(self.axis, fixed..=Px::MAX);
         } else if fixed_len && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, Px::MIN..=left);
@@ -46,7 +45,7 @@ impl Widget for Scroll {
         // have placed the whole scroll in a box longer than it.
         let slack = (self.container_len - self.content_len).max(Px::ZERO);
         let anchor = slack.mul(align.rel());
-        let mut region = UiRegion::FULL;
+        let mut content = UiSpan::FULL;
         // Content that fills the viewport and has not been scrolled is the
         // viewport, and is handed back as it came. Writing the same box as
         // its own length in pixels is the same box in another form, and the
@@ -55,15 +54,19 @@ impl Widget for Scroll {
         // each part of it.
         let moved = anchor != Px::ZERO || self.amt != Px::ZERO;
         if moved || self.content_len != self.container_len {
-            let offset = UiVec2::from_axis(
-                self.axis,
-                Len::from_parts(Rel::ZERO, anchor - self.amt),
-                Len::ZERO,
-            );
-            region = region.offset(offset);
-            region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
+            let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
+            content = UiSpan::new(start, start.offset(self.content_len));
         }
-        painter.widget_aligned(&self.inner, region, RegionAlign::NEAR);
+        // The viewport is the inner's frame, so a fraction it declares or
+        // reports is a fraction of what is on screen rather than of the
+        // content box its own answer decided. Where it is drawn is the
+        // content box, scrolled.
+        painter.widget_at(
+            &self.inner,
+            whole,
+            self.axis
+                .pair(Place::Fill(Part::From(content)), Place::Fill(Part::All)),
+        );
         // What it occupies is its box, on both axes: it clips its content to
         // that box, so it can neither take less of one nor honestly ask for
         // more. The content's length is what it scrolls through, not what it

src/widget/position/span.rs

@@ -10,20 +10,30 @@ pub struct Span {
 impl Widget for Span {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
-        // A length for every child before their final boxes are chosen: from
-        // a hint where one exists, and from drawing otherwise.
+        // The row: this span's own box, as a length of the frame its children
+        // are laid out against. Its start is nothing's business -- a slot is
+        // a length from it -- so what this reads is the length alone.
+        let far = painter.answer_extent_len(axis);
+        let measure_along = |from: Len, to: Len| match self.dir.sign {
+            Sign::Pos => UiSpan::new(from, to),
+            Sign::Neg => UiSpan::new(far - to, far - from),
+        };
+        // Across itself the child sits where its own alignment says, in the
+        // whole of the row: a span is what contains its children there, and
+        // nothing divides that axis.
+        let across = Place::Within(Part::All);
+        // A length for every child before their final slots are chosen. The
+        // frame passes through unchanged, so `rel(0.5)` is half the area this
+        // span was given whatever else is in it and wherever this child sits
+        // among them; what it is drawn in is the room left from the cursor,
+        // because a text has to wrap at the width actually there.
         let mut cursor = Len::rel_min();
         let mut lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let mut span = UiSpan::new(cursor, Len::rel_max());
-            if self.dir.sign == Sign::Neg {
-                span.flip();
-            }
-            let region = UiRegion::from_axis(axis, span, UiSpan::FULL);
-            let len = match painter.known_len(child, axis, region) {
-                Some(len) => len,
-                None => painter.widget_within(child, region).len(axis),
-            };
+            let room = Place::Fill(Part::From(measure_along(cursor, far)));
+            let len = painter
+                .widget_at(child, UiRegion::FULL, axis.pair(room, across))
+                .len(axis);
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
             lens.push(len);
@@ -40,45 +50,41 @@ impl Widget for Span {
             |sum, len| sum + *len,
         );
 
+        let fixed_total = Len::from_parts(total.rel, total.px);
+        // What is left for the shares to divide: the row less everything
+        // fixed, as a length of the frame rather than a number of pixels.
+        let room = far - fixed_total;
         // Whether anything is left over is a question in pixels: `rel(0.5)`
-        // beside 300 px is full at 600 and overfull at 400. The room to
-        // divide is `len * fixed - total.px`, and the length where it runs
-        // out is exactly the box a parent sizing itself from this answer
-        // hands back -- which is why this used to need a margin either side
-        // of the boundary, and why it does not now: that box and this sum are
-        // whole counts of the same step, and both routes to it land on the
-        // same count. What the generated oracle checks is the consequence,
-        // since which children exist at all turns on this.
-        let fixed = Rel::ONE - total.rel;
+        // beside 300 px is full at 600 and overfull at 400. Asked of `room`
+        // itself, and answered back through the same expression, so the
+        // boundary is the drawing's own and not a second way of finding it:
+        // the three cases a rounded division needed -- the fixed parts
+        // growing slower than the box, faster, or exactly with it -- are the
+        // sign of `room.rel`, which `through` already reads. What the
+        // generated oracle checks is the consequence, since which children
+        // exist at all turns on this.
         let mut shares = false;
         if total.leftover > Weight::ZERO {
-            let current = painter.px_len(axis);
-            let holds = if fixed > Rel::ZERO {
-                // The box length the fixed parts alone fill.
-                let full = total.px.div(fixed);
-                shares = current > full;
-                match shares {
-                    true => Holds::from(full.next_up()..=Px::MAX),
-                    false => Holds::from(Px::MIN..=full),
-                }
-            } else if fixed < Rel::ZERO {
-                // The relative parts grow faster than the box does, so here
-                // a shorter box is the one that leaves room.
-                let full = total.px.div(fixed);
-                shares = current < full;
-                match shares {
-                    true => Holds::from(Px::MIN..=full.next_down()),
-                    false => Holds::from(full..=Px::MAX),
-                }
-            } else {
-                // The relative parts take exactly the box, whatever it is, so
-                // the only room is what negative pixels leave.
-                shares = total.px < Px::ZERO;
-                Holds::ANY
+            shares = room.to_px(painter.frame_px_len(axis)) > Px::ZERO;
+            let holds = match shares {
+                true => Holds::from(Px::STEP..=Px::MAX),
+                false => Holds::from(Px::MIN..=Px::ZERO),
             };
-            painter.holds(axis, holds);
+            painter.frame_holds(axis, holds.through(room));
         }
 
+        if shares {
+            painter.drawing_uses_extent_len(axis, far);
+        }
+        let drawing_far = match shares {
+            true => far,
+            false => fixed_total,
+        };
+        let along = |from: Len, to: Len| match self.dir.sign {
+            Sign::Pos => UiSpan::new(from, to),
+            Sign::Neg => UiSpan::new(drawing_far - to, drawing_far - from),
+        };
+
         // Across itself a span is as long as its longest child -- unless a
         // rule beside it gives that length outright, and then reading them
         // answers nothing and makes its size depend on theirs for it. A rule
@@ -93,7 +99,6 @@ impl Widget for Span {
         // row.
         let mut fixed = Len::rel_min();
         let mut taken = Weight::ZERO;
-        let room = Len::rel_max() - Len::from_parts(total.rel, total.px);
         let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
         for (child, len) in self.children.iter().zip(&lens) {
@@ -106,20 +111,29 @@ impl Widget for Span {
                 fixed.px += self.gap;
                 continue;
             }
-            let mut span = UiSpan::FULL;
-            span.start = start;
+            let from = start;
             if len.leftover > Weight::ZERO && shares {
                 taken += len.leftover;
             }
             fixed.px += len.px;
             fixed.rel += len.rel;
             start = shared(fixed, taken, total.leftover, room);
-            span.end = start;
-            let mut region = UiRegion::from_axis(axis, span, UiSpan::FULL);
-            if self.dir.sign == Sign::Neg {
-                region.flip(axis);
-            }
-            let placed = painter.widget_within(child, region);
+            // Along the row the span says where the child goes, and that slot
+            // is the drawing's box outright rather than something to place an
+            // answer inside again.
+            let span = along(from, start);
+            let (frame, slot) = match len.leftover > Weight::ZERO && shares {
+                true => (
+                    UiRegion::from_axis(
+                        axis,
+                        painter.extent_part(axis, Part::From(span)),
+                        UiSpan::FULL,
+                    ),
+                    Place::Fill(Part::All),
+                ),
+                false => (UiRegion::FULL, Place::Fill(Part::From(span))),
+            };
+            let placed = painter.widget_at(child, frame, axis.pair(slot, across));
             if shrinks {
                 let used = placed.len(!axis);
                 // Choosing between a fixed and a relative length from the

src/widget/position/stack.rs

@@ -13,23 +13,35 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
-        // Whichever child sizes the stack decides the box every child gets.
-        // The stack reports that size, so a child given a longer box would
-        // draw outside what the stack says it occupies.
+        // Whichever child sizes the stack is given the stack's whole box --
+        // the stack is the length that child asked for, so placing that
+        // answer inside the box it decided would apply it twice.
         let size = match sizing.and_then(|i| self.children.get(i).map(|c| (i, c))) {
             // On the layer that child ends up on, so the ask below is a reuse
             // rather than a second drawing of it somewhere else: a retained
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter.widget(child).size()
+                painter
+                    .widget_at(child, UiRegion::FULL, [Place::Fill(Part::All); 2])
+                    .size()
             }
             None => Size::LEFTOVER,
         };
-        let region = painter.box_of(size);
         for (i, child) in self.children.iter().enumerate() {
+            if sizing == Some(i) {
+                continue;
+            }
             painter.child_layer_at(i);
-            painter.widget_aligned(child, region, RegionAlign::NEAR);
+            let place = [Axis::X, Axis::Y].map(|axis| {
+                let len = size.axis(axis);
+                let part = match len.leftover > Weight::ZERO {
+                    true => Part::All,
+                    false => Part::From(UiSpan::new(Len::ZERO, Len::from_parts(len.rel, len.px))),
+                };
+                Place::Within(part)
+            });
+            painter.widget_at(child, UiRegion::FULL, place);
         }
         size
     }

src/widget/text/mod.rs

@@ -50,15 +50,11 @@ impl TextView {
         let width = self.attrs.wrap.then(|| painter.px_len(Axis::X));
         // The shaper measures in floats, which is where a glyph advance comes
         // from; what it answers goes back on the grid.
-        let text = painter.render_text(&mut self.buf, &self.attrs, width.map(Px::to_f32));
-        // A greedy break is the same break at every width from its longest
-        // line up to the one it was made at: each line still fits, and none
-        // could take a word that did not fit in the wider box. A line too
-        // long to fit at all says nothing about narrower boxes.
-        if let Some(width) = width {
-            painter.holds(Axis::X, Px::from_f32(text.size.x).min(width)..=width);
+        painter.render_text(&mut self.buf, &self.attrs, width.map(Px::to_f32));
+        if width.is_some() {
+            painter.holds(Axis::X, self.buf.width_holds());
         }
-        text
+        self.buf.rendered().expect("render_text placed the glyphs")
     }
 
     pub fn tex(&self) -> Option<&RenderedText> {
@@ -78,9 +74,13 @@ impl TextView {
 
         let tex = self.render(painter);
         let region = tex.size.align(align);
-        let size = Size::px(tex.size);
-        let within = region.within(&painter.region());
-        painter.glyphs(tex, within);
+        // The step at or above what the shaper measured, so a parent that
+        // hands back the length this reports hands back a box the longest
+        // line fits in. Rounded to the nearest step it is half the time a
+        // hair under that line, and the break made in it is not the break a
+        // cold layout makes there.
+        let size = Size::from_px(PxVec2::ceil_from_f32(tex.size));
+        painter.glyphs(tex, region);
         (region, size)
     }
 

src/widget/trait_fns.rs

@@ -134,9 +134,13 @@ widget_trait! {
         |state| self.add(state)
     }
 
-    fn set_ptr(self, ptr: WeakWidget<WidgetPtr>, state: &mut Rsc) {
-        let id = self.add_strong(state);
-        state.ui_mut().widgets[ptr].inner = Some(id);
+    // Named for the type it makes rather than as `wrapped`, which would read
+    // as the text setting. `widget_trait!` takes no attributes, so what it is
+    // for is on `Wrapper` itself.
+    fn wrapper(self) -> impl WidgetFn<Rsc, Wrapper> {
+        |state| Wrapper {
+            inner: Some(self.add_strong(state)),
+        }
     }
 }
 

src/widget/wrapper.rs

@@ -1,11 +1,19 @@
 use crate::prelude::*;
 use std::marker::Unsize;
 
-pub struct WidgetPtr {
+/// One widget in a box of its own, doing as little as possible on the way:
+/// it draws its child in the whole of its box and reports back what the child
+/// said. It exists because a length and an alignment are properties of one
+/// widget, so a widget cannot both be 100 wide and take two shares of a row
+/// -- the two lengths need two widgets, and this is the smaller one.
+///
+/// Its child is optional so it can also be the swappable slot a tab bar
+/// needs, which is what it was written for.
+pub struct Wrapper {
     pub inner: Option<StrongWidget>,
 }
 
-impl Widget for WidgetPtr {
+impl Widget for Wrapper {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         match &self.inner {
             Some(id) => painter.widget(id).size(),
@@ -14,7 +22,7 @@ impl Widget for WidgetPtr {
     }
 }
 
-impl WidgetPtr {
+impl Wrapper {
     pub fn new() -> Self {
         Self::default()
     }
@@ -35,7 +43,7 @@ impl WidgetPtr {
     }
 }
 
-impl Default for WidgetPtr {
+impl Default for Wrapper {
     fn default() -> Self {
         Self::empty()
     }

tests/cases/determinism.rs

@@ -21,16 +21,18 @@ struct BranchesOnMeasurement {
 
 impl Widget for BranchesOnMeasurement {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let mut top = UiRegion::FULL;
-        top.y.end = top.y.start.offset(Px::from_int(40));
-        let measured = painter.widget_within(&self.probe, top).len(Axis::X);
+        let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
+        let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
+        let measured = painter
+            .widget_at(&self.probe, UiRegion::FULL, [Place::Within(Part::All), top])
+            .len(Axis::X);
         let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
 
-        let mut below = UiRegion::FULL;
-        below.y.start = below.y.start.offset(Px::from_int(40));
+        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
+        let place = [Place::Within(Part::All), below];
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_within(&self.wide, below),
-            false => painter.widget_within(&self.narrow, below),
+            true => painter.widget_at(&self.wide, UiRegion::FULL, place),
+            false => painter.widget_at(&self.narrow, UiRegion::FULL, place),
         };
         Size::LEFTOVER
     }

tests/cases/layout.rs

@@ -20,6 +20,88 @@ fn a_span_gives_each_child_the_width_it_asked_for() {
     assert_corners!(h, right, (100, 0), (400, 200));
 }
 
+/// A span places each child in the room left after the one before, because a
+/// text has to wrap at the width actually there, but the child's region is
+/// the whole row. So two children asking for half each take the whole row
+/// between them, however much of it was left when each was asked, and a third
+/// overflows -- and a span passes its own region on unchanged, so a child of
+/// a nested span asking for half asks for half of the same row.
+#[test]
+fn a_span_reads_a_child_report_as_a_fraction_of_the_row() {
+    let mut h = Harness::new((400, 100));
+    let half = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut h.rsc);
+    let nested = (inner,).span(Dir::RIGHT).add(&mut h.rsc);
+    let tail = rect(Color::BLUE).width(100).add(&mut h.rsc);
+    h.set_root((half, nested, tail).span(Dir::RIGHT).width(rel(1.0)));
+
+    // The nested span is placed at the length it reported, and its own child
+    // asks for half of the row rather than half of that placement.
+    assert_corners!(h, nested, (200, 0), (400, 100));
+    assert_corners!(h, inner, (200, 0), (400, 100));
+    assert_corners!(h, tail, (400, 0), (500, 100));
+}
+
+/// The same fraction either way round: after a 100 px child in a 400 px row,
+/// `rel(0.5)` is 100 to 300 whether the child's own rule says so or the child
+/// drew half of what it was offered and reported that. Half the row, not half
+/// of the 300 px left of it.
+#[test]
+fn a_reported_fraction_is_of_the_row_like_a_declared_one() {
+    let mut declaring = Harness::new((400, 100));
+    let head = rect(Color::RED).width(100).add(&mut declaring.rsc);
+    let declared = rect(Color::GREEN).width(rel(0.5)).add(&mut declaring.rsc);
+    declaring.set_root((head, declared).span(Dir::RIGHT).width(rel(1.0)));
+    assert_corners!(declaring, declared, (100, 0), (300, 100));
+
+    let mut reporting = Harness::new((400, 100));
+    let head = rect(Color::RED).width(100).add(&mut reporting.rsc);
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut reporting.rsc);
+    let reported = (inner,).span(Dir::RIGHT).add(&mut reporting.rsc);
+    reporting.set_root((head, reported).span(Dir::RIGHT).width(rel(1.0)));
+    assert_corners!(reporting, reported, (100, 0), (300, 100));
+}
+
+/// What the fraction a child reports is of and what box it is offered are
+/// two different lengths, and only the first is the whole row: a text still
+/// wraps at the room actually left after its neighbour, so the same
+/// paragraph is taller where less of the row is left for it.
+#[test]
+fn a_text_in_a_span_wraps_at_the_room_left_rather_than_the_whole_row() {
+    let paragraph = "Wrapping shapes one source into as many lines as the box \
+                     leaves room for, so a paragraph's height is an answer.";
+    let height_after = |head_width: i32| {
+        let mut h = Harness::new((400, 400));
+        let head = rect(Color::RED).width(head_width).add(&mut h.rsc);
+        let text = wtext(paragraph).size(16).wrap(true).add(&mut h.rsc);
+        h.set_root((head, text).span(Dir::RIGHT).width(rel(1.0)));
+        let region = h.region(&text).unwrap();
+        (region.bot_right.y - region.top_left.y).to_f32()
+    };
+
+    let (crowded, whole_row) = (height_after(300), height_after(0));
+    assert!(crowded > whole_row, "{crowded} against {whole_row}");
+}
+
+/// The same reading through a pad: padding goes around what it pads and
+/// does not narrow what a fraction under it is a fraction of, so half of the
+/// window plus the padding is what the pad takes and where the next child
+/// starts.
+#[test]
+fn a_pad_puts_its_padding_around_a_fraction_of_the_whole_box() {
+    let mut h = Harness::new((400, 100));
+    let inner = rect(Color::GREEN).width(rel(0.5)).add(&mut h.rsc);
+    let padded = (inner,).span(Dir::RIGHT).pad(10).add(&mut h.rsc);
+    let tail = rect(Color::BLUE).width(100).add(&mut h.rsc);
+    // Ruled to the window: a root reporting a fraction of it is otherwise
+    // placed inside it by its own alignment, which is not what is under test.
+    h.set_root((padded, tail).span(Dir::RIGHT).width(rel(1.0)));
+
+    assert_corners!(h, inner, (10, 10), (210, 90));
+    assert_corners!(h, padded, (0, 0), (220, 100));
+    assert_corners!(h, tail, (220, 0), (320, 100));
+}
+
 #[test]
 fn a_span_ruled_across_itself_does_not_measure_its_children_there() {
     let mut h = Harness::new((400, 200));
@@ -225,21 +307,21 @@ fn only_a_region_node_lengthens_the_chain_and_it_can_be_removed() {
     h.set_root((bar, buried).span(Dir::RIGHT));
 
     let move_idx = h.render.active[&leaf.id()].parent_move;
-    assert_eq!(h.render.moves.depth(move_idx), 1, "only the root region");
+    assert_eq!(h.render.moves.depth(move_idx), 0, "the window is no entry");
 
     h.rsc.widgets_mut().set_region_node(buried, true);
     h.frame();
     let move_idx = h.render.active[&leaf.id()].parent_move;
     assert_eq!(
         h.render.moves.depth(move_idx),
-        2,
-        "the opted-in widget's region and the root region"
+        1,
+        "the opted-in widget's region alone"
     );
 
     h.rsc.widgets_mut().set_region_node(buried, false);
     h.frame();
     let move_idx = h.render.active[&leaf.id()].parent_move;
-    assert_eq!(h.render.moves.depth(move_idx), 1);
+    assert_eq!(h.render.moves.depth(move_idx), 0);
 }
 
 /// A span that sizes from its children passes their `leftover` weight up
@@ -266,6 +348,11 @@ fn nested_spans_divide_the_space_once_however_deep_the_nesting_is() {
 
 /// The same space, unevenly nested: weights carried up mean a share is a
 /// share of the whole, not of whatever branch a widget happens to sit in.
+///
+/// Each edge lands on the even division or one step below it, since a share
+/// is a fraction of the room and a truncating multiply gives up what that
+/// fraction does not divide. What stays exact is that each share starts
+/// where the last one ended and the row ends at its own edge.
 #[test]
 fn an_uneven_nesting_still_gives_every_share_the_same_length() {
     let mut h = Harness::new((400, 200));
@@ -279,10 +366,24 @@ fn an_uneven_nesting_still_gives_every_share_the_same_length() {
     let three = (b, c, d).span(Dir::RIGHT).add(&mut h.rsc);
     h.set_root((one, three).span(Dir::RIGHT));
 
+    let mut start = Px::ZERO;
     for (i, id) in [a, b, c, d].into_iter().enumerate() {
-        let x = i as f32 * 100.0;
-        assert_corners!(h, id, (x, 0), (x + 100.0, 200));
+        let got = h.region(&id).expect("widget drew nothing");
+        let even = Px::from_int((i as i32 + 1) * 100);
+        assert_eq!(got.top_left, PxVec2::new(start, Px::ZERO), "share {i}");
+        assert_eq!(got.bot_right.y, Px::from_int(200), "share {i}");
+        assert!(
+            got.bot_right.x == even || got.bot_right.x == even.next_down(),
+            "share {i} ends at {:?}, not {even:?}",
+            got.bot_right.x
+        );
+        start = got.bot_right.x;
     }
+    assert_eq!(
+        start,
+        Px::from_int(400),
+        "the row stopped short of its edge"
+    );
 }
 
 /// However many ways a row is divided, the shares add up to the row: each
@@ -322,16 +423,16 @@ fn a_row_of_equal_shares_fills_it_exactly() {
     }
 }
 
-/// Where the shader puts an edge: the two parts of a scalar are floored
-/// apart, so a fraction and a pixel offset snap independently, and each is
-/// taken to the boundary it composes to within half a step of. Kept in step
-/// with `snap_floor` in `prelude.wgsl`.
+/// Where the shader puts an edge: the fraction resolved against the window
+/// plus the pixel offset, taken to the boundary it composes to within half
+/// a step of. Kept in step with `snap_floor` in `prelude.wgsl`.
 fn drawn_edges(h: &Harness, id: WidgetId, axis: Axis) -> (f32, f32) {
     let active = &h.render.active[&id];
-    let region = h.render.moves.resolve(active.parent_move, active.region);
+    let drawn = active.extent.within(&active.frame_abs);
+    let region = h.render.moves.resolve(active.parent_move, drawn);
     let dim = h.size().axis(axis);
     let snap = |v: f32| (v + Px::STEP.to_f32() * 0.5).floor();
-    let edge = |s: Len| snap(s.rel.to_f32() * dim) + snap(s.px.to_f32());
+    let edge = |s: Len| snap(s.rel.to_f32() * dim + s.px.to_f32());
     let span = region.axis(axis);
     (edge(span.start), edge(span.end))
 }
@@ -480,3 +581,162 @@ fn leftover_children_disappear_at_the_exact_fixed_content_boundary() {
     assert!(h.region(&a).is_none());
     assert!(h.region(&b).is_none());
 }
+
+/// **A stack child smaller than the stack sits where its own alignment
+/// says.** `Stack` gives every child the box its sizing child defines and
+/// used to force the near edge on all of them; that override is owed only to
+/// the sizing child, which has already placed its own content in the box the
+/// stack derived from its answer. Every other child is handed a box that owes
+/// nothing to it, so where it sits in one bigger than itself is its own
+/// business -- and with the override it could not be aligned at all, which is
+/// what moved the `tabs` example's counters to the wrong corner.
+#[test]
+fn a_stack_child_smaller_than_the_stack_keeps_its_own_alignment() {
+    let mut h = Harness::new((400, 200));
+    let big = rect(Color::BLUE).add(&mut h.rsc);
+    let small = rect(Color::RED).sized((50, 50)).add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_alignment(small.id(), Axis::X, AxisAlign::POS);
+    let (a, b) = (big.add_strong(&mut h.rsc), small.add_strong(&mut h.rsc));
+    let children: Vec<StrongWidget> = vec![a, b];
+    h.set_root(Stack {
+        children,
+        size: StackSize::Default,
+    });
+
+    assert_corners!(h, big, (0, 0), (400, 200));
+    // The far edge on X because it asked for it, the middle on Y because
+    // that is the default.
+    assert_corners!(h, small, (350, 75), (400, 125));
+}
+/// Five children of one span, buried under three containers that are each a
+/// fraction of their parent so no length reaches the window without being
+/// composed and rounded on the way. Returns each child's drawn width and
+/// each gap between them, in pixels.
+fn row_under_fractions(kid: Option<LayoutLen>, gap: f32, box_w: f32) -> (Vec<Px>, Vec<Px>) {
+    let mut h = Harness::new((box_w, 400.0));
+    let mut ids = Vec::new();
+    let mut kids: Vec<StrongWidget> = Vec::new();
+    for _ in 0..5 {
+        let r = rect(Color::RED).add(&mut h.rsc);
+        if let Some(len) = kid {
+            h.rsc
+                .widgets_mut()
+                .set_size_rule(r.id(), Axis::X, SizeRule::Exact(len));
+        }
+        ids.push(r.id());
+        kids.push(r.add_strong(&mut h.rsc));
+    }
+    let span = Span {
+        children: kids,
+        dir: Dir::RIGHT,
+        gap: Px::from_f32(gap),
+    }
+    .add(&mut h.rsc);
+    let a = (span.width(rel(0.9)),).span(Dir::RIGHT).add(&mut h.rsc);
+    let b = (a.width(rel(0.8)),).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((b.width(rel(0.7)),).span(Dir::RIGHT));
+    let boxes: Vec<_> = ids
+        .iter()
+        .map(|id| h.region(id).expect("a child drew nothing"))
+        .collect();
+    (
+        boxes.iter().map(|b| b.bot_right.x - b.top_left.x).collect(),
+        boxes
+            .windows(2)
+            .map(|p| p[1].top_left.x - p[0].bot_right.x)
+            .collect(),
+    )
+}
+
+/// **A length given in pixels is that many pixels, wherever it ends up.** A
+/// gap and a declared width compose additively -- `Len::within` adds a part's
+/// own pixels rather than scaling them, and both ends of a gap carry the same
+/// fraction, so the multiply that rounds is the same on each -- which is why
+/// nesting the row inside fractions of fractions cannot move them. Swept over
+/// 2,100 box widths when this was written and exact at every one; five here,
+/// including widths that divide badly by five.
+#[test]
+fn a_length_in_pixels_is_that_many_pixels_however_it_is_nested() {
+    for box_w in [300.0, 1000.0, 1001.0, 1003.0, 1920.0] {
+        let want = Px::from_int(7);
+        let (_, gaps) = row_under_fractions(None, 7.0, box_w);
+        assert!(
+            gaps.iter().all(|g| *g == want),
+            "box {box_w}: gaps between leftover children are {gaps:?}"
+        );
+        let (widths, gaps) = row_under_fractions(Some(LayoutLen::px(100.0)), 7.0, box_w);
+        assert!(
+            gaps.iter().all(|g| *g == want),
+            "box {box_w}: gaps between fixed children are {gaps:?}"
+        );
+        assert!(
+            widths.iter().all(|w| *w == Px::from_int(100)),
+            "box {box_w}: declared widths came out {widths:?}"
+        );
+    }
+}
+
+/// **Children asking for the same share of a row are not the same length**,
+/// and this pins by how much rather than claiming they are equal. A position
+/// is the quantity that gets rounded, so the row fills exactly and no two
+/// children leave a seam; what that costs is a step or two between lengths
+/// that were asked for identically. Exact composition would shrink the
+/// spread, not remove it: five equal lengths cannot fill a row whose step
+/// count is not a multiple of five.
+#[test]
+fn equal_shares_differ_by_at_most_two_steps_and_fill_the_row() {
+    for kid in [None, Some(LayoutLen::rel(0.2))] {
+        for box_w in [300.0, 1000.0, 1001.0, 1003.0, 1920.0] {
+            let (widths, gaps) = row_under_fractions(kid, 0.0, box_w);
+            let spread = *widths.iter().max().unwrap() - *widths.iter().min().unwrap();
+            assert!(
+                spread <= Px::from_raw(2),
+                "box {box_w}, {kid:?}: widths {widths:?} spread {spread:?}"
+            );
+            assert!(
+                gaps.iter().all(|g| *g == Px::ZERO),
+                "box {box_w}, {kid:?}: children left seams {gaps:?}"
+            );
+        }
+    }
+}
+
+#[test]
+fn a_stack_sized_by_a_child_does_not_take_that_childs_fraction_twice() {
+    let mut h = Harness::new((400, 200));
+    let half = rect(Color::RED).width(rel(0.5)).add(&mut h.rsc);
+    let behind = rect(Color::BLUE).add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![behind.add_strong(&mut h.rsc), half.add_strong(&mut h.rsc)],
+        size: StackSize::Child(1),
+    }
+    .add(&mut h.rsc);
+    h.set_root((stack,).span(Dir::RIGHT).width(rel(1.0)));
+
+    assert_corners!(h, stack, (0, 0), (200, 200));
+    assert_corners!(h, half, (0, 0), (200, 200));
+    assert_corners!(h, behind, (0, 0), (200, 200));
+}
+
+#[test]
+fn a_fixed_child_is_centered_in_its_wrappers_share() {
+    let mut h = Harness::new((600, 300));
+    let leaf = rect(Color::RED).sized((100, 100)).center().add(&mut h.rsc);
+    let wrapper = leaf
+        .wrapper()
+        .width(leftover(2))
+        .height(rel(1.0))
+        .add(&mut h.rsc);
+    let other = rect(Color::BLUE).width(200).add(&mut h.rsc);
+    h.set_root((other, wrapper).span(Dir::RIGHT));
+
+    assert_corners!(h, wrapper, (200, 0), (600, 300));
+    assert_corners!(h, leaf, (350, 100), (450, 200));
+
+    h.resize((900, 400));
+    h.frame();
+    assert_corners!(h, wrapper, (200, 0), (900, 400));
+    assert_corners!(h, leaf, (500, 150), (600, 250));
+}

tests/cases/plan.rs

@@ -0,0 +1,121 @@
+//! The tree a seed describes, as a value rather than as widgets.
+//!
+//! Two things have to hold for a plan to be worth having. Editing a plan has
+//! to mean what growing with those edits means, or a scenario reads one thing
+//! and the oracle another. And reducing a plan has to end, or a shrinker
+//! searching for the smallest counterexample never returns.
+
+use iris::random::{Edits, Kind, Plan, Rng, SpanEdit, plan};
+use std::collections::HashMap;
+
+fn some_edits(seed: u64, of: &Plan) -> Edits {
+    let mut rng = Rng::new(seed);
+    let (mut sized, mut aligned, mut nodes, mut spans) = (0, 0, 0, 0);
+    let mut of = of.clone();
+    of.walk_mut(&mut |p| {
+        if matches!(p.kind, Kind::Span { .. }) {
+            spans += 1;
+        }
+        sized += p.size.is_some() as usize;
+        aligned += p.align.is_some() as usize;
+        nodes += p.region_node.is_some() as usize;
+    });
+    let pick =
+        |n: usize, rng: &mut Rng| -> Vec<usize> { (0..n).filter(|_| rng.chance()).collect() };
+    Edits {
+        sizes: pick(sized, &mut rng)
+            .into_iter()
+            .map(|i| (i, [Some(LayoutLen::LEFTOVER), None]))
+            .collect(),
+        aligns: pick(aligned, &mut rng)
+            .into_iter()
+            .map(|i| (i, [Some(AxisAlign::POS), None]))
+            .collect(),
+        nodes: pick(nodes, &mut rng)
+            .into_iter()
+            .map(|i| (i, true))
+            .collect(),
+        spans: pick(spans, &mut rng)
+            .into_iter()
+            .map(|i| {
+                (
+                    i,
+                    SpanEdit {
+                        detach: vec![0],
+                        attach: 2,
+                    },
+                )
+            })
+            .collect::<HashMap<_, _>>(),
+        fixed_branches: false,
+    }
+}
+
+use iris::prelude::*;
+
+/// The two routes to an edited tree are one tree. `plan` resolves edits out
+/// of the random stream as it draws; `edited` puts them on a tree that
+/// already exists, which is the only route a shrunk plan has, since no seed
+/// grows one. A scenario written against either has to read the same.
+#[test]
+fn editing_a_plan_is_growing_one_with_those_edits() {
+    for seed in 1..=60 {
+        let bare = plan(seed, 5, &Edits::default());
+        let edits = some_edits(seed, &bare);
+        assert_eq!(
+            bare.edited(&edits),
+            plan(seed, 5, &edits),
+            "seed {seed}: edited and grown-with-edits disagree"
+        );
+    }
+}
+
+/// Every simplification is strictly smaller, so taking them in turn reaches a
+/// fixed point instead of circling. A shrinker that can return to a tree it
+/// has already tried does not stop.
+#[test]
+fn every_simplification_of_a_plan_is_smaller_than_it() {
+    for seed in 1..=60 {
+        let tree = plan(seed, 4, &Edits::default());
+        let mut queue = vec![tree];
+        let mut seen = 0;
+        while let Some(node) = queue.pop() {
+            seen += 1;
+            if seen > 400 {
+                break;
+            }
+            for small in node.smaller() {
+                assert!(
+                    small.size() <= node.size(),
+                    "seed {seed}: a simplification grew from {} to {}",
+                    node.size(),
+                    small.size()
+                );
+                if small.size() < node.size() {
+                    queue.push(small);
+                }
+            }
+        }
+    }
+}
+
+/// Reducing until nothing reduces ends, and ends at something small enough to
+/// read rather than at the tree it started from.
+#[test]
+fn reducing_a_plan_all_the_way_ends() {
+    for seed in 1..=30 {
+        let mut node = plan(seed, 5, &Edits::default());
+        let grown = node.size();
+        let mut steps = 0;
+        while let Some(next) = node.smaller().into_iter().next() {
+            node = next;
+            steps += 1;
+            assert!(steps < 10_000, "seed {seed}: reducing did not end");
+        }
+        assert!(
+            node.size() < grown.max(2),
+            "seed {seed}: reduced {grown} widgets to {}",
+            node.size()
+        );
+    }
+}

tests/cases/retained.rs

@@ -12,6 +12,7 @@ struct Counted {
     draws: Rc<Cell<usize>>,
     size: Size,
     reads_box: bool,
+    reads_answer_box: bool,
 }
 
 impl Widget for Counted {
@@ -19,6 +20,8 @@ impl Widget for Counted {
         self.draws.set(self.draws.get() + 1);
         if self.reads_box {
             painter.px_size();
+        } else if self.reads_answer_box {
+            painter.answer_px_size();
         }
         self.size
     }
@@ -38,11 +41,18 @@ fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>
         draws: draws.clone(),
         size,
         reads_box,
+        reads_answer_box: false,
     }
     .add(&mut h.rsc);
     (id, Counts(draws))
 }
 
+fn answer_counted(h: &mut Harness, size: Size) -> (WeakWidget<Counted>, Counts) {
+    let (id, draws) = counted(h, size, false);
+    h.rsc[id].reads_answer_box = true;
+    (id, draws)
+}
+
 struct Layered {
     children: [StrongWidget<Rect>; 2],
     _revision: usize,
@@ -156,16 +166,9 @@ fn a_span_child_that_declares_its_length_is_drawn_once() {
     h.set_root((hinted, asked).span(Dir::RIGHT));
 
     assert_eq!(told_draws.get(), 1);
-    // Reading its box makes its drawing hold for the measuring box alone,
-    // and it reports less than that box: so it is drawn again in the box its
-    // answer places it in, and once more in the final box the span chooses.
-    // A widget that says what it holds for, as text does, skips the middle
-    // one.
-    assert_eq!(
-        asked_draws.get(),
-        3,
-        "drawn to be measured, in its placed box, then in its final box"
-    );
+    // Its final slot is a parent decision, so it is evaluated there after
+    // the provisional ask established its length.
+    assert_eq!(asked_draws.get(), 2);
 }
 
 #[test]
@@ -220,9 +223,12 @@ struct FromHint {
 impl Widget for FromHint {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
-        let mut region = UiRegion::FULL;
-        region.y.end = region.y.start.offset(len.px);
-        painter.widget_within(&self.inner, region);
+        let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
+        painter.widget_at(
+            &self.inner,
+            UiRegion::FULL,
+            [Place::Within(Part::All), Place::Within(Part::From(top))],
+        );
         Size::LEFTOVER
     }
 }
@@ -244,7 +250,7 @@ fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
     assert_corners!(h, inner, (0, 0), (400, 120));
 }
 
-/// Reads its box's size, which nothing but its own draw can put right.
+/// Reads its box's size to compute its answer.
 struct ReadsBox {
     draws: Rc<Cell<usize>>,
 }
@@ -252,26 +258,36 @@ struct ReadsBox {
 impl Widget for ReadsBox {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
-        Size::from_px(painter.px_size().div_int(4))
+        Size::from_px(painter.answer_px_size().div_int(4))
     }
 }
 
 /// Reads its box across one axis only, so its drawing holds for a taller
 /// box on its own and only a wider one is worth a draw.
 ///
-/// Both of these report a quarter of what they read, without saying that the
-/// drawing holds there too, so each length they are asked at costs two draws:
-/// one to answer, and one in the quarter-sized box that answer places them
-/// in. The counts below are in those pairs.
+/// Both report a quarter of what they read. Their empty drawings are
+/// independent of that read, so a changed question costs one draw.
 struct ReadsWidth {
     draws: Rc<Cell<usize>>,
 }
 
+struct ReadsDrawingWidth {
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for ReadsDrawingWidth {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.px_len(Axis::X);
+        Size::LEFTOVER
+    }
+}
+
 impl Widget for ReadsWidth {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
         Size::from_px(PxVec2::new(
-            painter.px_len(Axis::X).div_int(4),
+            painter.answer_px_len(Axis::X).div_int(4),
             Px::from_int(20),
         ))
     }
@@ -312,6 +328,52 @@ fn a_span_ruled_across_itself_moves_its_child_without_redrawing_it() {
     assert_eq!(h.render.active[&span.id()].size.y, LayoutLen::rel(1.0));
 }
 
+/// A row places its children as lengths from where its own box starts, so a
+/// child that grew moves the ones after it and nothing else: each of them is
+/// the same box in a new place, which the retained drawing follows without
+/// being made again. Both kinds of length: one the row resolves from a rule,
+/// and one it takes from what the child reported.
+#[test]
+fn a_row_moves_what_follows_a_child_that_grew_rather_than_drawing_it() {
+    for declared in [false, true] {
+        let mut h = Harness::new((400, 200));
+        let first = rect(Color::RED).width(50).add(&mut h.rsc);
+        let ruled = Rc::new(Cell::new(0));
+        let second = Counted {
+            draws: ruled.clone(),
+            size: Size::LEFTOVER,
+            reads_box: false,
+            reads_answer_box: false,
+        };
+        let second = match declared {
+            true => second.width(rel(0.25)).add(&mut h.rsc),
+            false => second.width(60).add(&mut h.rsc),
+        };
+        let (third, reported) = counted(&mut h, Size::from((70, 20)), false);
+        h.set_root((first, second, third).span(Dir::RIGHT).width(rel(1.0)));
+        let (was_ruled, was_reported) = (ruled.get(), reported.get());
+        // A quarter of the row is a quarter of the row, wherever it sits in
+        // it and whatever the first child takes.
+        let width = match declared {
+            true => 100,
+            false => 60,
+        };
+        assert_corners!(h, second, (50, 0), (50 + width, 200));
+
+        h.set_len(first, Axis::X, 80);
+        h.frame();
+
+        assert_eq!(ruled.get(), was_ruled, "the ruled child was drawn again");
+        assert_eq!(
+            reported.get(),
+            was_reported,
+            "the reported child was drawn again"
+        );
+        assert_corners!(h, second, (80, 0), (80 + width, 200));
+        assert_corners!(h, third, (80 + width, 90), (150 + width, 110));
+    }
+}
+
 /// The output is the root of the box chain, so a resize is a box that changed
 /// length like any other -- there is not a second rule for the window. A
 /// drawing that holds for one length is drawn again whichever box moved.
@@ -343,7 +405,7 @@ fn a_resize_redraws_what_read_its_box() {
     h.resize((800, 100));
     h.frame();
 
-    assert_eq!(draws.get(), settled + 2);
+    assert_eq!(draws.get(), settled + 1);
 }
 
 #[test]
@@ -363,7 +425,7 @@ fn a_resize_only_redraws_read_axes() {
 
     h.resize((800, 300));
     h.frame();
-    assert_eq!(draws.get(), settled + 2, "width changes its answer");
+    assert_eq!(draws.get(), settled + 1, "width changes its answer");
 }
 
 /// A window is measured onto the grid like everything else, so a resize too
@@ -390,7 +452,7 @@ fn a_resize_within_one_step_is_not_a_resize() {
 
     h.resize((400.0 + step, 200.0));
     h.frame();
-    assert_eq!(draws.get(), settled + 2);
+    assert_eq!(draws.get(), settled + 1);
 }
 
 /// The same for a box that changes because a sibling did: what is compared
@@ -459,7 +521,7 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     // Every wrapper up to the outer pad read the size below it, so the outer
     // pad is what draws again -- and the span it hands the box to is the same
     // size as before, which is what lets a draw reuse its way past the leaf.
-    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
+    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), false);
     let padded = leaf.pad(10).add(&mut h.rsc);
     let below = rect(Color::RED).add(&mut h.rsc);
     h.set_root((padded, below).span(Dir::DOWN).pad(12));
@@ -613,3 +675,695 @@ fn a_stacks_sizing_child_is_drawn_once_where_it_belongs() {
     assert_ne!(layer(front.id()), layer(background.id()));
     assert_eq!(draws.get(), 1);
 }
+
+/// A widget's own mask is not the one it inherited, and a redraw of it
+/// inherits the second: handing back the first is handing it its own mask to
+/// set a second time, which `set_mask` asserts against.
+#[test]
+fn a_masked_widget_redrawn_on_its_own_sets_its_mask_again() {
+    let mut h = Harness::new((400, 200));
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    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.frame();
+    assert_corners!(h, inner, (100, 0), (400, 200));
+}
+
+/// The two spans a subtree changes hands between, and the branch that is not
+/// in the tree yet -- kept alive by the test until it is.
+struct Handover {
+    leaf: WidgetId,
+    first: WeakWidget<Span>,
+    second: WeakWidget<Span>,
+    root: WeakWidget<Span>,
+    spare: StrongWidget,
+}
+
+/// A subtree that changes hands while its box does not move, so nothing about
+/// reusing its drawing says it changed parents. `deeper` puts a span between
+/// the root and `second`, so it changes depth by changing hands as well.
+fn plant_handover(h: &mut Harness, moved: bool, deeper: bool, width: f32) -> Handover {
+    let leaf = rect(Color::RED).add(&mut h.rsc);
+    let sized = leaf.width(width).add(&mut h.rsc);
+    let holder = (sized,).span(Dir::RIGHT).add(&mut h.rsc);
+    let first = Span {
+        children: match moved {
+            true => Vec::new(),
+            false => vec![holder.add_strong(&mut h.rsc)],
+        },
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let second = Span {
+        children: match moved {
+            true => vec![holder.add_strong(&mut h.rsc)],
+            false => Vec::new(),
+        },
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let branch = match deeper {
+        true => (second,).span(Dir::RIGHT).add_strong(&mut h.rsc),
+        false => second.add_strong(&mut h.rsc),
+    };
+    let (in_tree, spare) = match moved {
+        true => (branch, first.add_strong(&mut h.rsc)),
+        false => (first.add_strong(&mut h.rsc), branch),
+    };
+    let root = Span {
+        children: vec![in_tree],
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    h.state.root = Some(root.add_strong(&mut h.rsc));
+    Handover {
+        leaf: sized.id(),
+        first,
+        second,
+        root,
+        spare,
+    }
+}
+
+/// Moves the subtree and swaps the branch it sits in for the one it left.
+fn hand_over(h: &mut Harness, tree: Handover) -> WidgetId {
+    let holder = h.rsc[tree.first].children.remove(0);
+    h.rsc[tree.second].children.push(holder);
+    h.rsc[tree.root].children.clear();
+    h.rsc[tree.root].children.push(tree.spare);
+    h.frame();
+    tree.leaf
+}
+
+#[test]
+fn a_subtree_that_changed_parents_is_not_undrawn_by_the_one_it_left() {
+    let mut warm = Harness::new((400, 200));
+    let tree = plant_handover(&mut warm, false, false, 40.0);
+    warm.frame();
+    let leaf = hand_over(&mut warm, tree);
+
+    let mut cold = Harness::new((400, 200));
+    let grown = plant_handover(&mut cold, true, false, 40.0);
+    cold.frame();
+
+    assert_eq!(
+        warm.region(&leaf),
+        cold.region(&grown.leaf),
+        "the span it left still listed it and undrew it"
+    );
+}
+
+#[test]
+fn a_subtree_that_changed_parents_settles_at_the_depth_it_moved_to() {
+    let mut warm = Harness::new((400, 200));
+    let tree = plant_handover(&mut warm, false, true, 40.0);
+    warm.frame();
+    let leaf = hand_over(&mut warm, tree);
+    // After it has changed hands, so what has to reach the new parent is a
+    // change made under the subtree it now holds.
+    warm.set_len(leaf, Axis::X, LayoutLen::px(90.0));
+    warm.frame();
+
+    let mut cold = Harness::new((400, 200));
+    let grown = plant_handover(&mut cold, true, true, 90.0);
+    cold.frame();
+
+    assert_eq!(
+        warm.region(&leaf),
+        cold.region(&grown.leaf),
+        "the span it moved to is the one the change has to reach"
+    );
+}
+
+fn primitive_bounds(h: &Harness, id: WidgetId) -> Vec<PixelRegion> {
+    h.render.active[&id]
+        .primitives
+        .iter()
+        .map(|primitive| {
+            let handle = &primitive.handle;
+            let instance = &h.render.layers[handle.layer].primitives()[handle.kind as usize]
+                .as_ref()
+                .unwrap()
+                .instances()[handle.inst_idx];
+            h.render
+                .moves
+                .resolve(instance.move_idx, instance.region)
+                .to_px(h.render.output_size())
+        })
+        .collect()
+}
+
+#[test]
+fn changing_an_inherited_extent_keeps_the_original_measurement_offer() {
+    fn build(h: &mut Harness, width: i32, text: &str) -> (WeakWidget<Text>, WeakWidget<Rect>) {
+        let first = rect(Color::RED).width(width).add(&mut h.rsc);
+        let words = wtext(text).size(20).wrap(true).add(&mut h.rsc);
+        let through = Stretchy {
+            inner: words.add_strong(&mut h.rsc),
+            draws: Rc::new(Cell::new(0)),
+        }
+        .add(&mut h.rsc);
+        h.set_root((first, through).span(Dir::RIGHT));
+        (words, first)
+    }
+    let short = "one two";
+    let long = "one two three four five six seven eight nine ten eleven twelve";
+    let mut warm = Harness::new((400, 200));
+    let (words, first) = build(&mut warm, 50, short);
+    warm.set_len(first, Axis::X, 200);
+    warm.frame();
+    *warm.rsc[words].content = long.to_string();
+    warm.frame();
+    let mut cold = Harness::new((400, 200));
+    let (other, _) = build(&mut cold, 200, long);
+    assert_eq!(warm.region(&words), cold.region(&other));
+    assert_eq!(
+        primitive_bounds(&warm, words.id()),
+        primitive_bounds(&cold, other.id())
+    );
+}
+
+#[test]
+fn widening_text_without_soft_breaks_reuses_its_drawing() {
+    struct CountedText {
+        text: Text,
+        draws: Rc<Cell<usize>>,
+    }
+    impl Widget for CountedText {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.draws.set(self.draws.get() + 1);
+            self.text.draw(painter)
+        }
+    }
+    for content in ["Short text", "Two hard\nline breaks\nhere", ""] {
+        let plant = |h: &mut Harness| {
+            let mut text = Text::new(content);
+            text.wrap = true;
+            let draws = Rc::new(Cell::new(0));
+            let root = CountedText {
+                text,
+                draws: draws.clone(),
+            }
+            .add(&mut h.rsc);
+            h.set_root(root);
+            (root, draws)
+        };
+        let mut warm = Harness::new((300, 200));
+        let (root, draws) = plant(&mut warm);
+        let before = draws.get();
+        warm.resize((500, 200));
+        warm.frame();
+        assert_eq!(draws.get(), before, "{content:?}");
+        let mut cold = Harness::new((500, 200));
+        let (other, _) = plant(&mut cold);
+        assert_eq!(warm.region(&root), cold.region(&other));
+        assert_eq!(
+            primitive_bounds(&warm, root.id()),
+            primitive_bounds(&cold, other.id())
+        );
+    }
+}
+
+#[test]
+fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
+    struct Frame {
+        child: StrongWidget,
+        region: UiRegion,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget_at(
+                &self.child,
+                UiRegion::FULL,
+                [
+                    Place::Within(Part::From(self.region.x)),
+                    Place::Within(Part::From(self.region.y)),
+                ],
+            );
+            Size::LEFTOVER
+        }
+    }
+    struct Painted(Rc<Cell<usize>>);
+    impl Widget for Painted {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.0.set(self.0.get() + 1);
+            painter.set_mask(UiRegion::FULL);
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::LEFTOVER
+        }
+    }
+    let fixed = |start, end| UiRegion::new(UiSpan::new(Len::px(start), Len::px(end)), UiSpan::FULL);
+    for node in [false, true] {
+        let plant = |h: &mut Harness, region| {
+            let draws = Rc::new(Cell::new(0));
+            let leaf = Painted(draws.clone()).add(&mut h.rsc);
+            h.rsc.widgets_mut().set_region_node(leaf, node);
+            let inner = Frame {
+                child: leaf.add_strong(&mut h.rsc),
+                region: UiRegion::new(UiSpan::new(Len::rel(0.23), Len::rel(0.83)), UiSpan::FULL),
+            }
+            .add_strong(&mut h.rsc);
+            let root = Frame {
+                child: inner,
+                region,
+            }
+            .add(&mut h.rsc);
+            h.set_root(root);
+            (root, leaf, draws)
+        };
+        let mut warm = Harness::new((400, 200));
+        let (root, leaf, draws) = plant(&mut warm, fixed(7.0, 104.0));
+        let before = draws.get();
+        warm.rsc[root].region = fixed(19.0, 180.0);
+        warm.frame();
+        assert_eq!(draws.get(), before);
+        let mut cold = Harness::new((400, 200));
+        let (_, other, _) = plant(&mut cold, fixed(19.0, 180.0));
+        assert_eq!(warm.region(&leaf), cold.region(&other));
+        assert_eq!(
+            primitive_bounds(&warm, leaf.id()),
+            primitive_bounds(&cold, other.id())
+        );
+        let mask = |h: &Harness, id: WidgetId| {
+            let active = &h.render.active[&id];
+            let mask = &h.rsc.ui().masks[active.mask.idx()];
+            h.render
+                .moves
+                .resolve(mask.move_idx, mask.region)
+                .to_px(h.render.output_size())
+        };
+        assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
+    }
+}
+
+#[test]
+fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
+    struct Glyphs {
+        buffer: TextBuffer,
+        draws: Rc<Cell<usize>>,
+    }
+    impl Widget for Glyphs {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.draws.set(self.draws.get() + 1);
+            let text = painter.render_text(&mut self.buffer, &TextAttrs::default(), None);
+            let origin = UiRegion::new(
+                UiSpan::new(Len::rel(0.23) + Len::px(-7.125), Len::FULL),
+                UiSpan::new(Len::rel(0.37) + Len::px(3.25), Len::FULL),
+            );
+            painter.glyphs(text, origin);
+            Size::LEFTOVER
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        region: UiRegion,
+        extent: UiRegion,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget_at(
+                &self.child,
+                self.region,
+                [
+                    Place::Fill(Part::From(self.extent.x)),
+                    Place::Fill(Part::From(self.extent.y)),
+                ],
+            );
+            Size::LEFTOVER
+        }
+    }
+    for node in [false, true] {
+        let mut h = Harness::new((403, 211));
+        let draws = Rc::new(Cell::new(0));
+        let text = Glyphs {
+            buffer: TextBuffer::new("Glyphs: gj AV\nsecond line"),
+            draws: draws.clone(),
+        }
+        .add(&mut h.rsc);
+        h.rsc.widgets_mut().set_region_node(text, node);
+        let root = Frame {
+            child: text.add_strong(&mut h.rsc),
+            region: UiRegion::FULL,
+            extent: UiRegion::FULL,
+        }
+        .add(&mut h.rsc);
+        h.set_root(root);
+        for (start, end) in [(0.13, 0.83), (-0.17, 1.23), (0.31, 0.67)] {
+            let before = draws.get();
+            h.rsc[root].region.x = UiSpan::new(Len::px(13.125), Len::px(287.375));
+            h.rsc[root].extent = UiRegion::new(
+                UiSpan::new(Len::rel(start), Len::rel(end)),
+                UiSpan::new(Len::px(7.25), Len::rel(end)),
+            );
+            h.frame();
+            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.frame();
+            assert!(draws.get() > before);
+            assert_eq!(retained, primitive_bounds(&h, text.id()));
+        }
+    }
+}
+
+#[test]
+fn resizing_does_not_remeasure_a_fixed_stack_for_its_unmeasured_overlay() {
+    let mut h = Harness::new((400, 200));
+    let (sizing, _) = counted(&mut h, Size::from((100, 80)), false);
+    let (overlay, draws) = counted(&mut h, Size::LEFTOVER, true);
+    h.set_root((sizing, overlay).stack().size(StackSize::Child(0)));
+    let settled = draws.get();
+
+    h.resize((800, 300));
+    h.frame();
+
+    assert_eq!(draws.get(), settled);
+    assert_corners!(h, overlay, (350, 110), (450, 190));
+}
+
+struct Unmeasured {
+    child: StrongWidget,
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for Unmeasured {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.widget(&self.child);
+        Size::LEFTOVER
+    }
+}
+
+#[test]
+fn a_declared_size_change_stops_at_an_independent_parent() {
+    let mut h = Harness::new((400, 200));
+    let leaf = rect(Color::RED).width(100).add(&mut h.rsc);
+    let parent = Unmeasured {
+        child: leaf.add_strong(&mut h.rsc),
+        draws: Rc::new(Cell::new(0)),
+    }
+    .add_strong(&mut h.rsc);
+    let draws = Rc::new(Cell::new(0));
+    h.set_root(Unmeasured {
+        child: parent,
+        draws: draws.clone(),
+    });
+    let settled = draws.get();
+
+    h.set_len(leaf, Axis::X, 150);
+    h.frame();
+
+    assert_corners!(h, leaf, (125, 0), (275, 200));
+    assert_eq!(draws.get(), settled);
+}
+
+#[test]
+fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsDrawingWidth {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root((leaf,).stack());
+    let settled = draws.get();
+
+    h.resize((800, 200));
+    h.frame();
+
+    assert!(draws.get() > settled);
+    assert_corners!(h, leaf, (0, 0), (800, 200));
+}
+
+#[test]
+fn changed_drawing_dependencies_reach_ancestors_without_a_size_change() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, false);
+    h.set_root(((leaf,).stack(),).stack());
+
+    h.rsc[leaf].reads_box = true;
+    h.frame();
+    let settled = draws.get();
+    h.resize((800, 200));
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 1);
+    assert_corners!(h, leaf, (0, 0), (800, 200));
+}
+
+#[test]
+fn widening_and_restoring_a_contract_does_not_invalidate_its_reader() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, leaf_draws) = counted(&mut h, Size::LEFTOVER, true);
+    let draws = Rc::new(Cell::new(0));
+    let child = leaf.add_strong(&mut h.rsc);
+    h.set_root(Unmeasured {
+        child,
+        draws: draws.clone(),
+    });
+    let settled = draws.get();
+    for reads_box in [false, true, false, true] {
+        h.rsc[leaf].reads_box = reads_box;
+        h.frame();
+        assert_eq!(draws.get(), settled);
+    }
+    let settled = leaf_draws.get();
+    h.resize((800, 200));
+    h.frame();
+    assert_eq!(leaf_draws.get(), settled + 1);
+}
+#[test]
+fn padding_and_stack_boxes_follow_the_extent_without_drawing_again() {
+    struct Observed<W> {
+        widget: W,
+        draws: Rc<Cell<usize>>,
+    }
+    impl<W: Widget> Widget for Observed<W> {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.draws.set(self.draws.get() + 1);
+            self.widget.draw(painter)
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        extent: UiRegion,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget_at(
+                &self.child,
+                UiRegion::FULL,
+                [
+                    Place::Fill(Part::From(self.extent.x)),
+                    Place::Fill(Part::From(self.extent.y)),
+                ],
+            );
+            Size::LEFTOVER
+        }
+    }
+    for node in [false, true] {
+        let plant = |h: &mut Harness, extent| {
+            let draws = Rc::new(Cell::new(0));
+            let leaf = rect(Color::BLUE).masked().add(&mut h.rsc);
+            h.rsc.widgets_mut().set_region_node(leaf, node);
+            let fixed = rect(Color::RED).width(31).height(19).add(&mut h.rsc);
+            let stack = Observed {
+                widget: Stack {
+                    children: vec![leaf.add_strong(&mut h.rsc), fixed.add_strong(&mut h.rsc)],
+                    size: StackSize::Default,
+                },
+                draws: draws.clone(),
+            }
+            .add_strong(&mut h.rsc);
+            let pad = Observed {
+                widget: Pad {
+                    inner: stack,
+                    padding: Padding::uniform(7).with_left(13),
+                },
+                draws: draws.clone(),
+            }
+            .add_strong(&mut h.rsc);
+            let root = Frame { child: pad, extent }.add(&mut h.rsc);
+            h.set_root(root);
+            (root, leaf, fixed, draws)
+        };
+        // The same box in three places. A pad places its child as lengths of
+        // its own box measured from where that box starts, so moving it is
+        // nothing to the pad -- where changing its length is a different
+        // question, and does draw it again.
+        let at = |start: f32| {
+            let span = |start: Len| UiSpan::new(start, start + Len::rel(0.4));
+            UiRegion::new(span(Len::rel(start) + Len::px(3.125)), span(Len::px(11.25)))
+        };
+        let mut warm = Harness::new((403, 211));
+        let (root, leaf, fixed, draws) = plant(&mut warm, at(0.13));
+        for start in [0.13, -0.17, 0.31] {
+            let extent = at(start);
+            let before = draws.get();
+            warm.rsc[root].extent = extent;
+            warm.frame();
+            assert_eq!(draws.get(), before);
+            let mut cold = Harness::new((403, 211));
+            let (_, other, other_fixed, _) = plant(&mut cold, extent);
+            for (a, b) in [(leaf.id(), other.id()), (fixed.id(), other_fixed.id())] {
+                assert_eq!(warm.region(&a), cold.region(&b));
+                assert_eq!(primitive_bounds(&warm, a), primitive_bounds(&cold, b));
+            }
+            let mask = |h: &Harness, id: WidgetId| {
+                let active = &h.render.active[&id];
+                let mask = &h.rsc.ui().masks[active.mask.idx()];
+                h.render
+                    .moves
+                    .resolve(mask.move_idx, mask.region)
+                    .to_px(h.render.output_size())
+            };
+            assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
+        }
+    }
+}
+
+#[test]
+fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
+    struct Measured;
+    impl Widget for Measured {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            let width = painter.answer_px_len(Axis::X);
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        start: f32,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget_at(
+                &self.child,
+                UiRegion::FULL,
+                [
+                    Place::Fill(Part::From(UiSpan::new(
+                        Len::px(self.start),
+                        Len::px(self.start + 200.0),
+                    ))),
+                    Place::Fill(Part::From(UiSpan::FULL)),
+                ],
+            );
+            Size::LEFTOVER
+        }
+    }
+    let mut h = Harness::new((400, 200));
+    let leaf = Measured.add(&mut h.rsc);
+    let stack = (leaf,).stack().add_strong(&mut h.rsc);
+    let root = Frame {
+        child: stack,
+        start: 0.0,
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+    assert_corners!(h, leaf, (60, 80), (140, 120));
+    h.rsc[root].start = 30.0;
+    h.frame();
+    assert_corners!(h, leaf, (90, 80), (170, 120));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf).unwrap()]
+    );
+}
+
+#[test]
+fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
+    struct Container {
+        child: StrongWidget,
+        region: UiRegion,
+    }
+    impl Widget for Container {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter
+                .widget_at(
+                    &self.child,
+                    UiRegion::FULL,
+                    [
+                        Place::Within(Part::From(self.region.x)),
+                        Place::Within(Part::From(self.region.y)),
+                    ],
+                )
+                .size()
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        extent: UiRegion,
+        answer: Rc<Cell<Size>>,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.answer.set(
+                painter
+                    .widget_at(
+                        &self.child,
+                        UiRegion::FULL,
+                        [
+                            Place::Fill(Part::From(self.extent.x)),
+                            Place::Fill(Part::From(self.extent.y)),
+                        ],
+                    )
+                    .size(),
+            );
+            Size::LEFTOVER
+        }
+    }
+    for fractional in [false, true] {
+        for region in [
+            UiRegion::FULL,
+            UiRegion::new(UiSpan::new(Len::rel(0.13), Len::rel(0.79)), UiSpan::FULL),
+        ] {
+            let plant = |h: &mut Harness, extent| {
+                let size = if fractional {
+                    Size {
+                        x: rel(0.5),
+                        y: LayoutLen::px(27),
+                    }
+                } else {
+                    Size::from((80, 27))
+                };
+                let (leaf, _) = match fractional {
+                    true => counted(h, size, false),
+                    false => answer_counted(h, size),
+                };
+                let child = Container {
+                    child: leaf.add_strong(&mut h.rsc),
+                    region,
+                }
+                .add_strong(&mut h.rsc);
+                let answer = Rc::new(Cell::new(Size::ZERO));
+                let root = Frame {
+                    child,
+                    extent,
+                    answer: answer.clone(),
+                }
+                .add(&mut h.rsc);
+                h.set_root(root);
+                (root, leaf, answer)
+            };
+            let mut warm = Harness::new((403, 211));
+            let (root, leaf, answer) = plant(&mut warm, UiRegion::FULL);
+            for width in [191.125, 297.25, 83.75] {
+                let extent =
+                    UiRegion::new(UiSpan::new(Len::px(13.125), Len::px(width)), UiSpan::FULL);
+                warm.rsc[root].extent = extent;
+                warm.frame();
+                let mut cold = Harness::new((403, 211));
+                let (_, other, other_answer) = plant(&mut cold, extent);
+                assert_eq!(answer.get(), other_answer.get());
+                assert_eq!(warm.region(&leaf), cold.region(&other));
+            }
+        }
+    }
+}

tests/cases/scroll.rs

@@ -60,6 +60,69 @@ fn a_wheel_scrolls_the_content_and_stops_at_its_end() {
     assert_corners!(h, top, (0, 0), (400, 200));
 }
 
+#[test]
+fn fixed_content_and_a_share_fill_one_viewport() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED)
+        .width(LayoutLen {
+            px: Px::from_int(600),
+            rel: Rel::ZERO,
+            leftover: Weight::ONE,
+        })
+        .add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (0, 0), (900, 100));
+}
+
+#[test]
+fn fixed_content_wider_than_the_viewport_still_scrolls() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED).width(1200).add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (-300, 0), (900, 100));
+}
+
+#[test]
+fn a_lone_share_fills_without_scrolling() {
+    let mut h = Harness::new((900, 100));
+    let content = rect(Color::RED).width(LayoutLen::LEFTOVER).add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+    h.set_root(scroll);
+
+    assert_corners!(h, content, (0, 0), (900, 100));
+}
+
+#[test]
+fn wrapping_content_beside_a_fixed_length_is_stable_warm_and_cold() {
+    fn plant(h: &mut Harness) -> (WidgetId, WidgetId) {
+        let fixed = rect(Color::RED).width(600).add(&mut h.rsc);
+        let text = wtext("Wrapping shapes one source into as many lines as the box leaves room for, so a paragraph's height is an answer and not a setting.")
+            .size(16)
+            .wrap(true)
+            .width(LayoutLen::LEFTOVER)
+            .add(&mut h.rsc);
+        let content = (fixed, text).span(Dir::RIGHT).add(&mut h.rsc);
+        let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X);
+        h.set_root(scroll);
+        (text.id(), content.id())
+    }
+
+    let mut warm = Harness::new((900, 300));
+    let (text, content) = plant(&mut warm);
+    warm.rsc.widgets_mut().get_dyn_mut(text);
+    warm.frame();
+
+    let mut cold = Harness::new((900, 300));
+    let (cold_text, cold_content) = plant(&mut cold);
+
+    assert_eq!(warm.region(&text), cold.region(&cold_text));
+    assert_eq!(warm.region(&content), cold.region(&cold_content));
+}
+
 /// A widget that clips to its box may not report more than the box: its
 /// parent would place the part it cut off, and the framework would put a
 /// drawing longer than its box somewhere. `Masked` is the second of these
@@ -71,7 +134,7 @@ fn a_clipping_widget_reporting_more_than_its_box_is_caught() {
 
     impl Widget for Clipper {
         fn draw(&mut self, painter: &mut Painter) -> Size {
-            painter.set_mask(painter.region());
+            painter.set_mask(UiRegion::FULL);
             painter.widget(&self.0).size()
         }
     }

tests/cases/unsettled.rs

@@ -3,12 +3,190 @@
 //! frame that had not settled: a wrapping text shaped at a width it was
 //! measured in rather than the one it was given. The rest are a widget
 //! measured again in a box its own answer had decided, where the old answer
-//! is a fixed point whatever the content now says. The last is neither: one
-//! box length, composed two ways, landing either side of the boundary that
-//! decided whether a child was drawn at all.
+//! is a fixed point whatever the content now says. The last three are
+//! neither: one box length, composed two ways, landing either side of the
+//! boundary that decided whether a child was drawn at all, and two boxes
+//! 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.
 
 use iris::harness::Harness;
 use iris::prelude::*;
+use iris::random::Branch;
+
+fn assert_same_regions(
+    warm: &Harness,
+    warm_ids: &[WidgetId],
+    cold: &Harness,
+    cold_ids: &[WidgetId],
+) {
+    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));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Ten widgets, shrunk from seed 2 at depth 5. The stack is as tall as its
+/// first child, so its other children belong in that one-line box. A cold
+/// layout used to keep the span's answer from the larger measuring box while
+/// a repaint asked it in the stack's final box.
+fn plant_stack_in_its_sizing_childs_box(h: &mut Harness) -> Vec<WidgetId> {
+    let sizing = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let filler = rect(Color::CYAN.alpha(252)).add(&mut h.rsc);
+    let plain = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let span = (filler, plain).span(Dir::DOWN).add(&mut h.rsc);
+    let pad = Pad {
+        padding: Padding::ZERO,
+        inner: span.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    let probe = rect(Color::RED).add(&mut h.rsc);
+    let wide = rect(Color::YELLOW.alpha(252)).add(&mut h.rsc);
+    let narrow = rect(Color::RED).add(&mut h.rsc);
+    let branch = Branch {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 55.0,
+    }
+    .add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![
+            sizing.add_strong(&mut h.rsc),
+            pad.add_strong(&mut h.rsc),
+            branch.add_strong(&mut h.rsc),
+        ],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_size_rules(stack.id(), Some(LayoutLen::LEFTOVER), None);
+    h.set_root(stack);
+    vec![
+        sizing.id(),
+        filler.id(),
+        plain.id(),
+        span.id(),
+        pad.id(),
+        probe.id(),
+        wide.id(),
+        narrow.id(),
+        branch.id(),
+        stack.id(),
+    ]
+}
+
+#[test]
+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.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let cold_ids = plant_stack_in_its_sizing_childs_box(&mut cold);
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
+/// Ten widgets, shrunk from seed 108 at depth 5. The nested reverse spans
+/// evaluate the branch in successively narrower boxes. The answer from the
+/// final, decided box must be the one retained after every span is reordered.
+fn plant_branch_in_nested_reverse_spans(
+    h: &mut Harness,
+    reordered: bool,
+) -> (Vec<WidgetId>, [WeakWidget<Span>; 3]) {
+    let pair = |first: StrongWidget, second: StrongWidget| match reordered {
+        true => vec![second, first],
+        false => vec![first, second],
+    };
+    let probe = rect(Color::RED.alpha(63)).add(&mut h.rsc);
+    let wide = rect(Color::RED).add(&mut h.rsc);
+    let narrow = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let branch = Branch {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 483.0,
+    }
+    .add(&mut h.rsc);
+    let wrapped = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let down = Span {
+        children: pair(
+            branch.add_strong(&mut h.rsc),
+            wrapped.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::DOWN,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let inner_filler = rect(Color::CYAN.alpha(63)).add(&mut h.rsc);
+    let inner = Span {
+        children: pair(
+            down.add_strong(&mut h.rsc),
+            inner_filler.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::LEFT,
+        gap: Px::ZERO,
+    }
+    .height(LayoutLen::rel(1.0))
+    .add(&mut h.rsc);
+    let outer_filler = rect(Color::GREEN.alpha(63)).add(&mut h.rsc);
+    let outer = Span {
+        children: pair(
+            inner.add_strong(&mut h.rsc),
+            outer_filler.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::LEFT,
+        gap: Px::ZERO,
+    }
+    .height(LayoutLen::rel(1.0))
+    .add(&mut h.rsc);
+    h.set_root(outer);
+    (
+        vec![
+            probe.id(),
+            wide.id(),
+            narrow.id(),
+            branch.id(),
+            wrapped.id(),
+            down.id(),
+            inner_filler.id(),
+            inner.id(),
+            outer_filler.id(),
+            outer.id(),
+        ],
+        [down, inner, outer],
+    )
+}
+
+#[test]
+fn reordering_nested_spans_keeps_the_answer_from_the_decided_box() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, spans) = plant_branch_in_nested_reverse_spans(&mut warm, false);
+    for span in spans {
+        warm.rsc[span].children.rotate_left(1);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _) = plant_branch_in_nested_reverse_spans(&mut cold, true);
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
 
 /// Six 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
@@ -398,3 +576,273 @@ fn a_box_that_only_rounds_past_its_fixed_children_leaves_nothing_over() {
     }
     assert!(wrong.is_empty(), "{}", wrong.join("\n"));
 }
+
+/// Five widgets, shrunk by `tests/shrink.rs` from the 277 the oracle's seed
+/// 18 grows at depth 6. A scroll inside a scroll, the inner one owning a
+/// movable region of its own, and only its text marked for redraw. Nothing
+/// about the tree changes, so no box may.
+fn plant_nested_scrolls(h: &mut Harness) -> Vec<WidgetId> {
+    let text = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let inner = Scroll::new(text.add_strong(&mut h.rsc), Axis::X).add(&mut h.rsc);
+    h.rsc.widgets_mut().set_region_node(inner.id(), true);
+    let filler = rect(Color::RED).add(&mut h.rsc);
+    h.rsc.widgets_mut().set_size_rules(
+        filler.id(),
+        Some(LayoutLen::px(87.0)),
+        Some(LayoutLen::px(24.0)),
+    );
+    let span = Span {
+        children: vec![inner.add_strong(&mut h.rsc), filler.add_strong(&mut h.rsc)],
+        dir: Dir::DOWN,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let root = Scroll::new(span.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    h.set_root(root);
+    vec![text.id(), inner.id(), filler.id(), span.id(), root.id()]
+}
+
+/// A local redraw asks a dirty widget in the box its parent gave it, and only
+/// where that box is as long as the one it was offered; anything else is a
+/// question its parent has to ask. This inner scroll's offer is the outer
+/// scroll's whole viewport and the box it was given is 24px shorter -- the
+/// height of the sized child the outer scroll snaps to the end of -- so what
+/// it must not do is settle itself. It was drawn at its offer once, and the
+/// inner scroll and its text stayed 24px too low.
+#[test]
+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.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let cold_ids = plant_nested_scrolls(&mut cold);
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Ten widgets, of the shape `tests/shrink.rs` reduces the oracle's seed 220
+/// to. The pad owns a movable region and is the scroll's content, so the box
+/// the scroll places it in is as long as that content while the box it was
+/// offered is the viewport -- and with no padding to tell those two apart,
+/// the span inside it looked like it was still at its offer. So everything
+/// under the pad was asked again in the *placed* box, the offer resolving
+/// against the node's own entry, which holds that box: the texts kept the
+/// widths they had, the content stayed the length those widths make, and the
+/// old answer confirmed itself. What the branch adds is a tree that differs
+/// rather than a box that moved, since a probe measured at the wrong width
+/// takes the other side.
+fn plant_under_a_node(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget<Span>; 2]) {
+    let probe = rect(Color::RED).add(&mut h.rsc);
+    let wide = rect(Color::GREEN).add(&mut h.rsc);
+    let narrow = rect(Color::BLUE).add(&mut h.rsc);
+    let branch = Branch {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 213.0,
+    }
+    .add(&mut h.rsc);
+    let wrapped = wtext(
+        "Wrapping shapes one source into as many lines as the box \
+         leaves room for, so a paragraph's height is an answer and not a setting.",
+    )
+    .size(16)
+    .wrap(true)
+    .add(&mut h.rsc);
+    let plain = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let row = |h: &mut Harness, mut children: Vec<StrongWidget>| {
+        if swapped {
+            children.rotate_left(1);
+        }
+        Span {
+            children,
+            dir: Dir::RIGHT,
+            gap: Px::ZERO,
+        }
+        .add(&mut h.rsc)
+    };
+    let texts: Vec<StrongWidget> =
+        vec![wrapped.add_strong(&mut h.rsc), plain.add_strong(&mut h.rsc)];
+    let inner = row(h, texts);
+    let pair: Vec<StrongWidget> = vec![branch.add_strong(&mut h.rsc), inner.add_strong(&mut h.rsc)];
+    let outer = row(h, pair);
+    let pad = Pad {
+        padding: Padding::ZERO,
+        inner: outer.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    h.rsc.widgets_mut().set_region_node(pad.id(), true);
+    let root = Scroll::new(pad.add_strong(&mut h.rsc), Axis::X).add(&mut h.rsc);
+    h.set_root(root);
+    (
+        vec![
+            probe.id(),
+            wide.id(),
+            narrow.id(),
+            branch.id(),
+            wrapped.id(),
+            plain.id(),
+            inner.id(),
+            outer.id(),
+            pad.id(),
+            root.id(),
+        ],
+        [outer, inner],
+    )
+}
+
+#[test]
+fn a_widget_under_a_region_node_is_asked_in_the_box_that_node_was_offered() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, spans) = plant_under_a_node(&mut warm, false);
+    warm.frame();
+    for span in spans {
+        warm.rsc[span].children.rotate_left(1);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _) = plant_under_a_node(&mut cold, true);
+    cold.frame();
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+const PARAGRAPH: &str = "Wrapping shapes one source into as many lines as the \
+    box leaves room for, so a paragraph's height is an answer and not a setting.";
+
+/// Eight widgets, shrunk from a 118-widget tree (seed 1121, depth 4,
+/// `shuffle-swap-for-three`). The stack takes its size from the span above,
+/// the span takes its width from the longest line of the texts in it, and
+/// the text below the span is then wrapped at that width -- so a width the
+/// shaper measured comes back to it as the box to break in.
+fn plant_a_measured_width(h: &mut Harness, swapped: bool) -> (WeakWidget<Span>, WidgetId) {
+    let first: StrongWidget = rect(Color::YELLOW).add_strong(&mut h.rsc);
+    let mut inner = Span::empty(Dir::UP);
+    inner.children = match swapped {
+        true => swapped_in(h),
+        false => vec![first],
+    };
+    let inner = inner.height(142).add(&mut h.rsc);
+    let text = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![inner.add_strong(&mut h.rsc), text.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    h.set_root((stack,).span(Dir::DOWN).width(195));
+    (inner, text.id())
+}
+
+/// What the span holds once its children have been swapped, which is what
+/// the warm tree is changed to and what the cold one is grown with.
+fn swapped_in(h: &mut Harness) -> Vec<StrongWidget> {
+    let paragraph = |h: &mut Harness| -> StrongWidget {
+        wtext(PARAGRAPH).size(16).wrap(true).add_strong(&mut h.rsc)
+    };
+    vec![
+        paragraph(h),
+        rect(Color::YELLOW).add_strong(&mut h.rsc),
+        paragraph(h),
+    ]
+}
+
+/// A text handed back the width it measured breaks there the way it broke
+/// when it measured it. The width the shaper answers is not on the grid, and
+/// a report rounded to the nearest step is under the longest line half the
+/// time: a warm tree then keeps a break made in a wider box while a cold one
+/// makes a narrower break in the same box, and the paragraph gains a line.
+#[test]
+fn a_text_is_given_back_a_box_the_line_it_measured_fits_in() {
+    let mut warm = Harness::new((900, 1200));
+    let (inner, text) = plant_a_measured_width(&mut warm, false);
+    warm.frame();
+    warm.rsc[inner].children = swapped_in(&mut warm);
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (_, cold_text) = plant_a_measured_width(&mut cold, true);
+    cold.frame();
+
+    assert_eq!(warm.region(&text), cold.region(&cold_text));
+}
+
+#[test]
+fn adding_text_to_a_reverse_row_keeps_its_shared_height() {
+    fn build(
+        h: &mut Harness,
+        changed: bool,
+    ) -> (WeakWidget<Span>, WeakWidget<Text>, Vec<StrongWidget>) {
+        let wrap = wtext("Wrapping shapes one source into as many lines as the box leaves room for, so a paragraph's height is an answer and not a setting.").size(16).wrap(true).add_strong(&mut h.rsc);
+        let one = || {
+            wtext("one line, overflowing whatever it is given")
+                .size(16)
+                .wrap(false)
+        };
+        let plain = one().add_strong(&mut h.rsc);
+        let shared = one()
+            .width(LayoutLen::LEFTOVER)
+            .height(LayoutLen::LEFTOVER)
+            .add(&mut h.rsc);
+        let mut extra: Vec<StrongWidget> = vec![
+            rect(Color::RED).add_strong(&mut h.rsc),
+            one().add_strong(&mut h.rsc),
+            one().add_strong(&mut h.rsc),
+        ];
+        let children: Vec<StrongWidget> = if changed {
+            let mut children: Vec<StrongWidget> = vec![plain, shared.add_strong(&mut h.rsc)];
+            children.append(&mut extra);
+            children
+        } else {
+            vec![wrap, plain, shared.add_strong(&mut h.rsc)]
+        };
+        let row = Span {
+            children,
+            dir: Dir::LEFT,
+            gap: Px::ZERO,
+        }
+        .height(LayoutLen::rel(1.0))
+        .add(&mut h.rsc);
+        let fill: StrongWidget = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let children: Vec<StrongWidget> = vec![fill, row.add_strong(&mut h.rsc)];
+        let root = Span {
+            children,
+            dir: Dir::RIGHT,
+            gap: Px::from_int(4),
+        }
+        .height(LayoutLen::rel(1.0))
+        .add(&mut h.rsc);
+        h.set_root(root);
+        (row, shared, extra)
+    }
+    let mut warm = Harness::new((900, 1200));
+    let (row, shared, extra) = build(&mut warm, false);
+    warm.rsc[row].children.remove(0);
+    warm.rsc[row].children.extend(extra);
+    warm.frame();
+    let mut cold = Harness::new((900, 1200));
+    let (_, other, _) = build(&mut cold, true);
+    assert_eq!(warm.region(&shared), cold.region(&other));
+}

tests/generated.rs

@@ -1,19 +1,21 @@
-//! Random trees, checked against building the same tree cold.
+//! Laying a tree out again has to land where growing it that way would.
 //!
-//! A frame reaches its layout by keeping most of the last one: movable regions
-//! or primitive boxes rewritten, some widgets drawn again, the rest untouched.
-//! The result must be the tree a cold start would have produced, so anything
-//! wrongly retained shows up as a difference in somebody's box.
+//! Every case is one of `scenario`'s, over the trees `iris::random` grows
+//! from a seed. The fast test takes a handful of seeds and the ignored one
+//! takes as many as it is asked for; both run the same cases the shrinker
+//! does over the same trees, so a seed that fails here is reduced by
 //!
-//! `iris::random` grows the tree and `examples/random.rs` draws one. A seed is
-//! the whole reproduction; `a_long_run_of_seeds_agrees` is the ignored sweep
-//! for when it is worth spending the time.
+//!     SHRINK_SEED=<seed> SHRINK_DEPTH=<depth> SHRINK_CASE=<case> \
+//!       cargo test --release --test shrink -- --ignored --nocapture
+//!
+//! `IRIS_GENERATED_SEED`, `IRIS_GENERATED_SEEDS` and `IRIS_GENERATED_DEPTH`
+//! select what the long run covers.
 
-use std::collections::HashMap;
+#[path = "scenario/mod.rs"]
+mod scenario;
 
-use iris::harness::Harness;
-use iris::prelude::*;
-use iris::random::{Aligns, Edits, Lens, Rng, SpanEdit, Tree, grow};
+use iris::random::{Edits, plan};
+use scenario::{ALL, Case, diverges, env, over_seeds};
 
 /// How deep the generator branches. The generator widens two to four ways per
 /// level, so depth is exponential in width and a deep narrow tree is not
@@ -23,562 +25,102 @@ fn depth() -> usize {
     env("IRIS_GENERATED_DEPTH", 4)
 }
 
-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)
-}
-const SEEDS: [u64; 9] = [1, 2, 3, 5, 8, 10, 13, 86, 98];
+/// The seeds the ordinary tests take. Seven that have never failed; 86,
+/// which a `Scroll` fixed point once settled differently on; and 20, which
+/// caught a locally redrawn widget being placed twice in the box its parent
+/// had already placed it in.
+const SEEDS: [u64; 10] = [1, 2, 3, 5, 8, 10, 13, 20, 86, 98];
 
-/// The same box, to a step of the grid per level of nesting between the two
-/// ways of reaching it. A move, a repaint and a row of shares land on the
-/// same number now; what is left is a box centred in a fraction of its parent
-/// against the same box centred in its own pixels. A step is a thousandth of
-/// a pixel, where this was a twentieth of one before any of it was on a grid.
-const AGREE_STEPS: i32 = 2;
-
-fn same_region(got: Option<PixelRegion>, want: Option<PixelRegion>) -> bool {
-    match (got, want) {
-        (Some(got), Some(want)) => {
-            let same = |a: Px, b: Px| (a - b).abs() <= Px::STEP.mul_int(AGREE_STEPS);
-            same(got.top_left.x, want.top_left.x)
-                && same(got.top_left.y, want.top_left.y)
-                && same(got.bot_right.x, want.bot_right.x)
-                && same(got.bot_right.y, want.bot_right.y)
-        }
-        (None, None) => true,
-        _ => false,
+fn check(seed: u64, depth: usize, case: Case) {
+    let grown = plan(seed, depth, &Edits::default());
+    if let Some(how) = diverges(&grown, case, seed) {
+        panic!(
+            "seed {seed} at depth {depth} differs after {}: {how}\n\
+             reduce it with SHRINK_SEED={seed} SHRINK_DEPTH={depth} \
+             SHRINK_CASE={} cargo test --release --test shrink -- --ignored --nocapture",
+            case.name(),
+            case.name(),
+        );
     }
 }
 
-fn plant(h: &mut Harness, seed: u64, edits: &Edits) -> Tree {
-    let (root, tree) = grow(&mut h.rsc, seed, depth(), edits);
-    h.state.root = Some(root);
-    h.frame();
-    tree
+macro_rules! case {
+    ($name:ident, $case:expr) => {
+        #[test]
+        fn $name() {
+            for seed in SEEDS {
+                check(seed, depth(), $case);
             }
-
-fn resize_one(h: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Lens {
-    let lens = [
-        Some(LayoutLen::px(20.0 + rng.below(180) as f32)),
-        Some(LayoutLen::px(20.0 + rng.below(180) as f32)),
-    ];
-    h.rsc
-        .widgets_mut()
-        .set_size_rules(tree.sized[idx], lens[0], lens[1]);
-    lens
-}
-
-/// Changes a few of the declared sizes, and says which, so the cold tree can
-/// be grown with the same ones.
-fn edit(h: &mut Harness, tree: &Tree, rng: &mut Rng) -> HashMap<usize, Lens> {
-    let mut edits = HashMap::new();
-    for _ in 0..4 {
-        let idx = rng.below(tree.sized.len());
-        edits.insert(idx, resize_one(h, tree, idx, rng));
-    }
-    edits
-}
-
-/// Every declared size at once, so every reader of a size in the tree has a
-/// changed descendant in the same frame and the whole dirty set has to settle
-/// together.
-fn edit_every(h: &mut Harness, tree: &Tree, rng: &mut Rng) -> HashMap<usize, Lens> {
-    (0..tree.sized.len())
-        .map(|idx| (idx, resize_one(h, tree, idx, rng)))
-        .collect()
-}
-
-/// A way of changing what a span holds. Each is a shape worth its own case:
-/// taking a child out of the middle is not the same as emptying a span, and
-/// adding one is not the same as adding three.
-#[derive(Clone, Copy, Debug)]
-enum Shuffle {
-    /// Every other child, so what is left is interleaved with what went.
-    EveryOther,
-    /// Everything but the first, which is the last step before empty.
-    AllButFirst,
-    /// Three more on the end at once.
-    AddThree,
-    /// The first out and three more on, so the count moves both ways.
-    SwapForThree,
-    /// One out of the middle and one on the end.
-    TradeOne,
-}
-
-const SHUFFLES: [Shuffle; 5] = [
-    Shuffle::EveryOther,
-    Shuffle::AllButFirst,
-    Shuffle::AddThree,
-    Shuffle::SwapForThree,
-    Shuffle::TradeOne,
-];
-
-impl Shuffle {
-    fn of(self, grown: usize) -> SpanEdit {
-        let all = |step: usize, from: usize| (from..grown).step_by(step).collect();
-        match self {
-            Self::EveryOther => SpanEdit {
-                detach: all(2, 0),
-                attach: 0,
-            },
-            Self::AllButFirst => SpanEdit {
-                detach: all(1, 1),
-                attach: 0,
-            },
-            Self::AddThree => SpanEdit {
-                detach: Vec::new(),
-                attach: 3,
-            },
-            Self::SwapForThree => SpanEdit {
-                detach: vec![0],
-                attach: 3,
-            },
-            Self::TradeOne => SpanEdit {
-                detach: vec![grown / 2],
-                attach: 1,
-            },
-        }
-    }
-}
-
-/// Applies `shuffle` to every third span, and says what it did so the cold
-/// tree can be grown that way. The widgets it takes out are given back: the
-/// last share of one must outlive the comparison, or its id is handed to
-/// something else and the two trees stop lining up.
-fn reshuffle(
-    h: &mut Harness,
-    tree: &mut Tree,
-    shuffle: Shuffle,
-) -> (HashMap<usize, SpanEdit>, Vec<StrongWidget>) {
-    let mut edits = HashMap::new();
-    let mut detached = Vec::new();
-    for (idx, span) in tree.spans.iter_mut().enumerate().step_by(3) {
-        let span_edit = shuffle.of(span.grown);
-        let mut take = span_edit.detach.clone();
-        take.sort_unstable();
-        let children = &mut h.rsc[span.id].children;
-        // Highest first, so an index means the same child however many of
-        // its neighbours are going too.
-        for j in take.into_iter().rev() {
-            if j < children.len() {
-                detached.push(children.remove(j));
-            }
-        }
-        let attach = span_edit.attach.min(span.spares.len());
-        children.extend(span.spares.drain(..attach));
-        edits.insert(idx, span_edit);
-    }
-    (edits, detached)
-}
-
-/// What a widget was configured with, so a tree the generator found can be
-/// written out by hand. A fuzz failure is a lead; the fast test that replaces
-/// it has to be buildable from what the failure printed.
-fn describe(id: WidgetId, h: &Harness) -> String {
-    let rules = h.rsc.widgets().size_rules(id);
-    let rule = |r: SizeRule| match r.exact() {
-        Some(len) => format!("{len}"),
-        None => "-".into(),
-    };
-    let align = h.rsc.widgets().alignment(id);
-    let side = |a: AxisAlign| {
-        if a == AxisAlign::NEG {
-            "neg".into()
-        } else if a == AxisAlign::CENTER {
-            "mid".into()
-        } else if a == AxisAlign::POS {
-            "pos".into()
-        } else {
-            format!("{:.2}", a.rel())
         }
     };
-    // A rule and an alignment are properties of whatever carries them, so
-    // they print with that widget rather than as widgets of their own.
-    let mut out = describe_widget(id, h);
-    if (rules.x, rules.y) != (SizeRule::Free, SizeRule::Free) {
-        out += &format!("[x:{},y:{}]", rule(rules.x), rule(rules.y));
-    }
-    if align != RegionAlign::default() {
-        out += &format!("@{},{}", side(align.x), side(align.y));
-    }
-    out
 }
 
-fn describe_widget(id: WidgetId, h: &Harness) -> String {
-    let label = h.rsc.widgets().label(id).to_string();
-    let Some(widget) = h.rsc.widgets().get_dyn(id) else {
-        return label;
-    };
-    let any: &dyn std::any::Any = widget;
-    if let Some(w) = any.downcast_ref::<Span>() {
-        let sign = if w.dir.sign == Sign::Neg { "-" } else { "+" };
-        return format!(
-            "Span{{dir:{:?}{sign},gap:{},n:{}}}",
-            w.dir.axis,
-            w.gap,
-            w.children.len()
+case!(
+    many_widgets_redrawing_at_once_leaves_every_box_where_it_was,
+    Case::RepaintSome
 );
-    }
-    if let Some(w) = any.downcast_ref::<Pad>() {
-        let p = &w.padding;
-        return format!(
-            "Pad{{l:{},r:{},t:{},b:{}}}",
-            p.left, p.right, p.top, p.bottom
+case!(
+    everything_redrawing_at_once_leaves_every_box_where_it_was,
+    Case::Repaint
 );
-    }
-    if let Some(w) = any.downcast_ref::<Stack>() {
-        return format!("Stack{{n:{}}}", w.children.len());
-    }
-    label
-}
-
-/// Every widget in one tree against the matching widget in the other. A
-/// mismatch prints the widget's ancestry, marking region nodes, since where
-/// two trees disagree is rarely where the cause is.
-fn assert_same(seed: u64, what: &str, warm: (&Harness, &Tree), cold: (&Harness, &Tree)) {
-    let ((wh, wt), (ch, ct)) = (warm, cold);
-    assert_eq!(wt.ids.len(), ct.ids.len(), "seed {seed}: different trees");
-    let mut drawn = 0;
-    let mut wrong = 0;
-    for (i, (&w, &c)) in wt.ids.iter().zip(&ct.ids).enumerate() {
-        let (got, want) = (wh.region(&w), ch.region(&c));
-        drawn += usize::from(got.is_some());
-        // This oracle cares where rasterization lands, not whether equivalent
-        // arithmetic produced the same f32. Keep the tolerance to one
-        // twentieth of a physical pixel, while whether a widget drew remains
-        // exact.
-        if same_region(got, want) {
-            continue;
-        }
-        wrong += 1;
-        if wrong <= 3 {
-            let mut chain = Vec::new();
-            let mut at = Some(w);
-            while let Some(id) = at {
-                let active = &wh.render.active[&id];
-                let node = match active.move_idx == active.parent_move {
-                    true => "",
-                    false => "*",
-                };
-                chain.push(format!("{}{node}", describe(id, wh)));
-                at = active.parent;
-            }
-            println!(
-                "seed {seed} after {what}: widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}",
-                chain.join(" < ")
+case!(
+    a_resize_lands_where_starting_at_that_size_would,
+    Case::Resize
 );
-        }
-    }
-    assert!(drawn > 0, "seed {seed}: nothing was drawn");
-    assert_eq!(wrong, 0, "seed {seed}: {wrong} widgets differ after {what}");
-}
-
-fn changed_size(seed: u64) {
-    let mut warm = Harness::new((900, 1200));
-    let grown = plant(&mut warm, seed, &Edits::default());
-    // Not every tree grows a declared size to change.
-    if grown.sized.is_empty() {
-        return;
-    }
-
-    let mut rng = Rng::new(seed ^ 0x5eed);
-    let sizes = edit(&mut warm, &grown, &mut rng);
-    warm.frame();
-
-    let mut cold = Harness::new((900, 1200));
-    let same = plant(
-        &mut cold,
-        seed,
-        &Edits {
-            sizes,
-            ..Default::default()
-        },
+case!(
+    a_resize_and_a_repaint_land_where_starting_that_way_would,
+    Case::ResizeRepaint
 );
-    assert_same(seed, "a size change", (&warm, &grown), (&cold, &same));
-}
-
-/// Moves one widget to a different corner of the box it is given.
-fn realign_one(h: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Aligns {
-    let mut side = || match rng.below(4) {
-        0 => None,
-        1 => Some(AxisAlign::NEG),
-        2 => Some(AxisAlign::CENTER),
-        _ => Some(AxisAlign::POS),
-    };
-    let aligns = [side(), side()];
-    for (axis, align) in [Axis::X, Axis::Y].into_iter().zip(aligns) {
-        h.rsc
-            .widgets_mut()
-            .set_alignment(tree.aligned[idx], axis, align.unwrap_or_default());
-    }
-    aligns
-}
-
-fn changed_alignment(seed: u64) {
-    let mut warm = Harness::new((900, 1200));
-    let grown = plant(&mut warm, seed, &Edits::default());
-    if grown.aligned.is_empty() {
-        return;
-    }
-
-    let mut rng = Rng::new(seed ^ 0xa11);
-    let aligns = (0..grown.aligned.len())
-        .step_by(3)
-        .map(|idx| (idx, realign_one(&mut warm, &grown, idx, &mut rng)))
-        .collect();
-    warm.frame();
-
-    let mut cold = Harness::new((900, 1200));
-    let same = plant(
-        &mut cold,
-        seed,
-        &Edits {
-            aligns,
-            ..Default::default()
-        },
+case!(
+    a_size_change_after_a_resize_lands_the_same_way,
+    Case::ResizeSize
 );
-    assert_same(seed, "an alignment change", (&warm, &grown), (&cold, &same));
-}
-
-/// Giving a widget a movable region of its own, or taking it away, is a
-/// structural change: every primitive under it changes which chain resolves
-/// it. A cold tree built that way is what says the rebuild was complete.
-fn changed_region_node(seed: u64) {
-    let mut warm = Harness::new((900, 1200));
-    let grown = plant(&mut warm, seed, &Edits::default());
-    if grown.nodes.is_empty() {
-        return;
-    }
-
-    let nodes: HashMap<usize, bool> = (0..grown.nodes.len())
-        .step_by(2)
-        .map(|idx| {
-            let id = grown.nodes[idx];
-            let was = warm.rsc.widgets().is_region_node(id);
-            warm.rsc.widgets_mut().set_region_node(id, !was);
-            (idx, !was)
-        })
-        .collect();
-    warm.frame();
-
-    let mut cold = Harness::new((900, 1200));
-    let same = plant(
-        &mut cold,
-        seed,
-        &Edits {
-            nodes,
-            ..Default::default()
-        },
+case!(
+    a_size_change_lands_where_growing_it_that_way_would,
+    Case::Size
 );
-    assert_same(
-        seed,
-        "a region-node change",
-        (&warm, &grown),
-        (&cold, &same),
+case!(
+    every_size_changing_at_once_lands_where_growing_it_that_way_would,
+    Case::EverySize
 );
-}
-
-fn reshuffled(seed: u64, shuffle: Shuffle) {
-    let mut warm = Harness::new((900, 1200));
-    let mut grown = plant(&mut warm, seed, &Edits::default());
-    // Some seeds grow nothing but wrappers, and a shuffle with no span to
-    // shuffle is not the same thing as one that had no effect. A span behind
-    // a branch nobody took is the same kind of nothing: it is not drawn, so
-    // shuffling it cannot move anything.
-    let shuffles = grown
-        .spans
-        .iter()
-        .step_by(3)
-        .any(|span| warm.region(&span.id.id()).is_some());
-    if !shuffles {
-        return;
-    }
-    let (spans, _held) = reshuffle(&mut warm, &mut grown, shuffle);
-    warm.frame();
-
-    let mut cold = Harness::new((900, 1200));
-    let same = plant(
-        &mut cold,
-        seed,
-        &Edits {
-            spans,
-            ..Default::default()
-        },
+case!(
+    an_alignment_change_lands_where_growing_it_that_way_would,
+    Case::Align
 );
-
-    let what = format!("{shuffle:?}");
-    assert_same(seed, &what, (&warm, &grown), (&cold, &same));
-}
-
-fn changed_every_size(seed: u64) {
-    let mut warm = Harness::new((900, 1200));
-    let grown = plant(&mut warm, seed, &Edits::default());
-    if grown.sized.is_empty() {
-        return;
-    }
-
-    let mut rng = Rng::new(seed ^ 0xa11);
-    let sizes = edit_every(&mut warm, &grown, &mut rng);
-    warm.frame();
-
-    let mut cold = Harness::new((900, 1200));
-    let same = plant(
-        &mut cold,
-        seed,
-        &Edits {
-            sizes,
-            ..Default::default()
-        },
+case!(
+    giving_and_taking_a_movable_region_rebuilds_what_resolves_it,
+    Case::RegionNode
 );
-    assert_same(seed, "every size at once", (&warm, &grown), (&cold, &same));
-}
-
-/// Marks a spread of widgets for redraw at once. Nothing changes, so no box
-/// may either; what this exercises is the order a frame settles a dirty set
-/// in, which the other cases reach one dependency path at a time.
-fn repainted_together(seed: u64) {
-    let mut warm = Harness::new((900, 1200));
-    let grown = plant(&mut warm, seed, &Edits::default());
-    for &id in grown.ids.iter().step_by(5) {
-        warm.rsc.widgets_mut().get_dyn_mut(id);
-    }
-    assert!(
-        !warm.rsc.widgets().needs_redraw.is_empty(),
-        "seed {seed}: nothing was marked"
+case!(
+    reordering_a_span_lands_where_growing_it_that_way_would,
+    Case::Reorder
 );
-    warm.frame();
-
-    let mut cold = Harness::new((900, 1200));
-    let same = plant(&mut cold, seed, &Edits::default());
-
-    let what = "many repaints at once";
-    assert_same(seed, what, (&warm, &grown), (&cold, &same));
-}
-
-fn resized(seed: u64) {
-    let mut warm = Harness::new((1920, 1200));
-    let grown = plant(&mut warm, seed, &Edits::default());
-    warm.resize((640, 900));
-    warm.frame();
-
-    let mut cold = Harness::new((640, 900));
-    let same = plant(&mut cold, seed, &Edits::default());
-
-    assert_same(seed, "a resize", (&warm, &grown), (&cold, &same));
-}
-
-fn resized_then_changed(seed: u64) {
-    let mut warm = Harness::new((1920, 1200));
-    let grown = plant(&mut warm, seed, &Edits::default());
-    if grown.sized.is_empty() {
-        return;
-    }
-    warm.resize((640, 900));
-    warm.frame();
-
-    let mut rng = Rng::new(seed ^ 0xb0a7);
-    let sizes = edit(&mut warm, &grown, &mut rng);
-    warm.frame();
-
-    let mut cold = Harness::new((640, 900));
-    let same = plant(
-        &mut cold,
-        seed,
-        &Edits {
-            sizes,
-            ..Default::default()
-        },
-    );
-
-    let what = "a resize then a size change";
-    assert_same(seed, what, (&warm, &grown), (&cold, &same));
-}
-
-#[test]
-fn a_changed_size_lands_where_growing_it_that_way_would() {
-    SEEDS.into_iter().for_each(changed_size);
-}
-
-#[test]
-fn a_changed_alignment_lands_where_growing_it_that_way_would() {
-    SEEDS.into_iter().for_each(changed_alignment);
-}
-
-#[test]
-fn a_toggled_region_node_lands_where_growing_it_that_way_would() {
-    SEEDS.into_iter().for_each(changed_region_node);
-}
-
-#[test]
-fn every_size_changing_at_once_lands_where_growing_it_that_way_would() {
-    SEEDS.into_iter().for_each(changed_every_size);
-}
-
-#[test]
-fn many_widgets_redrawing_at_once_leaves_every_box_where_it_was() {
-    SEEDS.into_iter().for_each(repainted_together);
-}
-
-#[test]
-fn a_resize_lands_where_starting_at_that_size_would() {
-    SEEDS.into_iter().for_each(resized);
-}
-
-#[test]
-fn a_size_change_after_a_resize_lands_the_same_way() {
-    SEEDS.into_iter().for_each(resized_then_changed);
-}
 
 #[test]
 fn adding_and_removing_span_children_lands_where_growing_it_that_way_would() {
-    for shuffle in SHUFFLES {
+    for case in ALL {
+        if matches!(case, Case::Shuffle(_)) {
             for seed in SEEDS {
-            reshuffled(seed, shuffle);
+                check(seed, depth(), case);
+            }
         }
     }
 }
 
-/// The same property over a hundred seeds and every scenario. What it has
-/// found so far was never where the trees disagreed: a text measured in a box
-/// it was not going to get, and a widget re-measured in a box its own answer
-/// had decided. `tests/shrink.rs` is how a seed from here becomes a tree
-/// small enough to read.
 #[test]
-#[ignore = "a hundred seeds, rather than the nine the others check"]
+#[ignore = "as many seeds as it is asked for, rather than the nine the others check"]
 fn a_long_run_of_seeds_agrees() {
-    let seeds = std::env::var("IRIS_GENERATED_SEED")
+    let depth = depth();
+    let seeds: Vec<u64> = match std::env::var("IRIS_GENERATED_SEED")
         .ok()
-        .and_then(|seed| seed.parse().ok())
-        .map(|seed| seed..=seed)
-        .unwrap_or_else(|| 1..=env("IRIS_GENERATED_SEEDS", 100));
-    over_seeds(seeds.collect(), |seed| {
-        changed_size(seed);
-        changed_every_size(seed);
-        repainted_together(seed);
-        resized(seed);
-        resized_then_changed(seed);
-        for shuffle in SHUFFLES {
-            reshuffled(seed, shuffle);
-        }
-    });
-}
-
-/// Every seed on its own thread's share of them. A tree is grown, laid out
-/// and dropped inside one call, so seeds share nothing, and this is most of
-/// the time a run takes. A thread that fails takes the scope down with it,
-/// which is the same panic libtest would have seen.
-///
-/// One core short of all of them, so the machine this runs on stays usable.
-pub fn over_seeds(seeds: Vec<u64>, run: impl Fn(u64) + Sync) {
-    let threads =
-        std::thread::available_parallelism().map_or(1, |n| n.get().saturating_sub(1).max(1));
-    let chunk = seeds.len().div_ceil(threads).max(1);
-    std::thread::scope(|scope| {
-        for part in seeds.chunks(chunk) {
-            let run = &run;
-            scope.spawn(move || part.iter().for_each(|&seed| run(seed)));
+        .and_then(|v| v.parse().ok())
+    {
+        Some(seed) => vec![seed],
+        None => (1..=env("IRIS_GENERATED_SEEDS", 100_u64)).collect(),
+    };
+    over_seeds(seeds, |seed| {
+        for case in ALL {
+            check(seed, depth, case);
         }
     });
 }

tests/layout_diagnostics.rs

@@ -5,11 +5,11 @@
 //!     cargo test --release --features layout-diagnostics \
 //!       --test layout_diagnostics -- --ignored --nocapture
 //!
-//! Uninstrumented hardware totals for one phase:
+//! Build the uninstrumented test with `cargo test --release --test
+//! layout_diagnostics --no-run`, then run the emitted executable directly:
 //!
-//!     IRIS_PHASE=resize IRIS_FRAMES=1000 perf stat \
-//!       -e cycles:u,instructions:u cargo test --release \
-//!       --test layout_diagnostics -- --ignored --nocapture
+//!     IRIS_PHASE=resize IRIS_FRAMES=10000 perf stat -r 7 \
+//!       -e cycles:u,instructions:u /path/to/layout_diagnostics --ignored --nocapture
 //!
 //! `IRIS_PHASE` is `cold`, `repaint`, `many`, `size`, `scroll`, `resize`, or
 //! `all`. `IRIS_SEED`, `IRIS_DEPTH`, and `IRIS_FRAMES` select the load, and
@@ -134,6 +134,9 @@ fn report(label: &str, mut elapsed: Vec<f64>, _harness: &Harness) {
     {
         let diagnostics = iris::core::layout_diagnostics::take();
         print!("{}", diagnostics.per_frame(frames));
+        for event in diagnostics.traces() {
+            println!("    {event:?}");
+        }
         for callsite in diagnostics.hot_text().iter().take(3) {
             let mut ancestry = Vec::new();
             let mut id = Some(callsite.id);

tests/scenario/mod.rs

@@ -0,0 +1,438 @@
+//! The scenarios both fuzzers run, over the tree a [`Plan`] describes.
+//!
+//! One implementation rather than two. The oracle grew its trees from a seed
+//! and the shrinker grew its own, with every scenario written out on each
+//! side, so a failure the oracle found could not be handed to the shrinker:
+//! there was no tree to pass it, only a seed, and a seed cannot be made
+//! smaller. Both take a plan now, so whatever finds a counterexample can also
+//! reduce it.
+//!
+//! Each target compiles this for itself, so what only one of them calls is
+//! dead code in the other.
+#![allow(dead_code)]
+
+use iris::harness::Harness;
+use iris::prelude::*;
+use iris::random::{Aligns, Edits, Kind, Lens, Plan, Rng, SpanEdit, Tree, build};
+use std::collections::HashMap;
+
+/// A seed per thread but one, since a seed grows, lays out and drops its tree
+/// alone. A failing seed still shrinks and panics on its own thread.
+pub fn over_seeds(seeds: Vec<u64>, run: impl Fn(u64) + Sync) {
+    let threads =
+        std::thread::available_parallelism().map_or(1, |n| n.get().saturating_sub(1).max(1));
+    let chunk = seeds.len().div_ceil(threads).max(1);
+    std::thread::scope(|scope| {
+        for part in seeds.chunks(chunk) {
+            let run = &run;
+            scope.spawn(move || part.iter().for_each(|&seed| run(seed)));
+        }
+    });
+}
+
+pub fn env<T: std::str::FromStr>(name: &str, fallback: T) -> T {
+    std::env::var(name)
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(fallback)
+}
+
+/// The window a tree is grown in, and the one a resize takes it to.
+const OUTER: (f32, f32) = (1920.0, 1200.0);
+const INNER: (f32, f32) = (640.0, 900.0);
+const STILL: (f32, f32) = (900.0, 1200.0);
+
+/// A way of changing what a span holds. Each is a shape worth its own case:
+/// taking a child out of the middle is not the same as emptying a span, and
+/// adding one is not the same as adding three.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Shuffle {
+    /// Every other child, so what is left is interleaved with what went.
+    EveryOther,
+    /// Everything but the first, which is the last step before empty.
+    AllButFirst,
+    /// Three more on the end at once.
+    AddThree,
+    /// The first out and three more on, so the count moves both ways.
+    SwapForThree,
+    /// One out of the middle and one on the end.
+    TradeOne,
+}
+
+impl Shuffle {
+    fn of(self, grown: usize) -> SpanEdit {
+        let all = |step: usize, from: usize| (from..grown).step_by(step).collect();
+        match self {
+            Self::EveryOther => SpanEdit {
+                detach: all(2, 0),
+                attach: 0,
+            },
+            Self::AllButFirst => SpanEdit {
+                detach: all(1, 1),
+                attach: 0,
+            },
+            Self::AddThree => SpanEdit {
+                detach: Vec::new(),
+                attach: 3,
+            },
+            Self::SwapForThree => SpanEdit {
+                detach: vec![0],
+                attach: 3,
+            },
+            Self::TradeOne => SpanEdit {
+                detach: vec![grown / 2],
+                attach: 1,
+            },
+        }
+    }
+}
+
+/// What a warm tree is put through before it is compared with a cold one
+/// grown the way it was left.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Case {
+    /// Nothing changes, so no box may either. What this exercises is the
+    /// order a frame settles a dirty set in.
+    Repaint,
+    /// Every fifth widget rather than all of them: marking all of them
+    /// redraws the whole tree, which is a cold start reached the long way,
+    /// where the mixed case leaves a redrawn subtree beside a retained one.
+    RepaintSome,
+    Resize,
+    ResizeRepaint,
+    /// A resize and then a size change, so a retained answer is asked to
+    /// survive two different kinds of invalidation in a row.
+    ResizeSize,
+    /// A few declared sizes.
+    Size,
+    /// Every declared size at once, so every reader of a size has a changed
+    /// descendant in the same frame and the whole dirty set settles together.
+    EverySize,
+    Align,
+    /// Giving a widget a movable region of its own, or taking it away, is a
+    /// structural change: every primitive under it changes which chain
+    /// resolves it.
+    RegionNode,
+    /// The same children in a different order, which moves every one of them
+    /// without changing what any of them is.
+    Reorder,
+    Shuffle(Shuffle),
+}
+
+pub const ALL: [Case; 15] = [
+    Case::Repaint,
+    Case::RepaintSome,
+    Case::Resize,
+    Case::ResizeRepaint,
+    Case::ResizeSize,
+    Case::Size,
+    Case::EverySize,
+    Case::Align,
+    Case::RegionNode,
+    Case::Reorder,
+    Case::Shuffle(Shuffle::EveryOther),
+    Case::Shuffle(Shuffle::AllButFirst),
+    Case::Shuffle(Shuffle::AddThree),
+    Case::Shuffle(Shuffle::SwapForThree),
+    Case::Shuffle(Shuffle::TradeOne),
+];
+
+impl Case {
+    /// The name `CASE` selects it by, and the one a failure prints.
+    pub fn name(self) -> &'static str {
+        match self {
+            Self::Repaint => "repaint",
+            Self::RepaintSome => "repaint-some",
+            Self::Resize => "resize",
+            Self::ResizeRepaint => "resize-repaint",
+            Self::ResizeSize => "resize-size",
+            Self::Size => "size",
+            Self::EverySize => "every-size",
+            Self::Align => "align",
+            Self::RegionNode => "region-node",
+            Self::Reorder => "reorder",
+            Self::Shuffle(Shuffle::EveryOther) => "shuffle-every-other",
+            Self::Shuffle(Shuffle::AllButFirst) => "shuffle-all-but-first",
+            Self::Shuffle(Shuffle::AddThree) => "shuffle-add-three",
+            Self::Shuffle(Shuffle::SwapForThree) => "shuffle-swap-for-three",
+            Self::Shuffle(Shuffle::TradeOne) => "shuffle-trade-one",
+        }
+    }
+
+    pub fn named(name: &str) -> Option<Self> {
+        ALL.into_iter().find(|case| case.name() == name)
+    }
+
+    /// Grown in the first, compared in the second.
+    fn window(self) -> ((f32, f32), (f32, f32)) {
+        match self {
+            Self::Resize | Self::ResizeRepaint | Self::ResizeSize => (OUTER, INNER),
+            _ => (STILL, STILL),
+        }
+    }
+}
+
+fn mark(warm: &mut Harness, tree: &Tree, step: usize) {
+    for &id in tree.ids.iter().step_by(step) {
+        warm.rsc.widgets_mut().get_dyn_mut(id);
+    }
+}
+
+fn a_len(rng: &mut Rng) -> Option<LayoutLen> {
+    Some(LayoutLen::px(20.0 + rng.below(180) as f32))
+}
+
+fn resize_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Lens {
+    let lens = [a_len(rng), a_len(rng)];
+    warm.rsc
+        .widgets_mut()
+        .set_size_rules(tree.sized[idx], lens[0], lens[1]);
+    lens
+}
+
+fn realign_one(warm: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Aligns {
+    let side = |rng: &mut Rng| match rng.below(4) {
+        0 => None,
+        1 => Some(AxisAlign::NEG),
+        2 => Some(AxisAlign::CENTER),
+        _ => Some(AxisAlign::POS),
+    };
+    let align = [side(rng), side(rng)];
+    let id = tree.aligned[idx];
+    for (axis, align) in [Axis::X, Axis::Y].into_iter().zip(align) {
+        warm.rsc
+            .widgets_mut()
+            .set_alignment(id, axis, align.unwrap_or_default());
+    }
+    align
+}
+
+/// Every span's children in a different order, said both to the warm tree and
+/// to the plan the cold one is grown from.
+fn reorder(warm: &mut Harness, tree: &Tree, plan: &Plan) -> Plan {
+    for span in &tree.spans {
+        let children = &mut warm.rsc[span.id].children;
+        if !children.is_empty() {
+            children.rotate_left(1);
+        }
+    }
+    let mut out = plan.clone();
+    out.walk_mut(&mut |node| {
+        if let Kind::Span { order, .. } = &mut node.kind
+            && !order.is_empty()
+        {
+            order.rotate_left(1);
+        }
+    });
+    out
+}
+
+/// Applies `shuffle` to every third span. What it takes out is given back to
+/// the span's spares: the last share of a widget must outlive the comparison,
+/// or its id is handed to something else and the two trees stop lining up.
+fn reshuffle(warm: &mut Harness, tree: &mut Tree, shuffle: Shuffle) -> HashMap<usize, SpanEdit> {
+    let mut edits = HashMap::new();
+    for (idx, span) in tree.spans.iter_mut().enumerate().step_by(3) {
+        let edit = shuffle.of(span.grown);
+        let mut take = edit.detach.clone();
+        take.sort_unstable();
+        let children = &mut warm.rsc[span.id].children;
+        // Highest first, so an index means the same child however many of its
+        // neighbours are going too.
+        for j in take.into_iter().rev() {
+            if j < children.len() {
+                span.spares.push(children.remove(j));
+            }
+        }
+        let attach = edit.attach.min(span.spares.len());
+        let moved: Vec<_> = span.spares.drain(..attach).collect();
+        warm.rsc[span.id].children.extend(moved);
+        edits.insert(idx, edit);
+    }
+    edits
+}
+
+/// Changes the warm tree and answers with the plan a cold tree grown that way
+/// comes from. Each arm settles its own frame, so a case that changes nothing
+/// does not get a second one that could settle what the first left.
+fn change(case: Case, warm: &mut Harness, tree: &mut Tree, plan: &Plan, rng: &mut Rng) -> Plan {
+    let some_sizes = |warm: &mut Harness, tree: &Tree, rng: &mut Rng| {
+        let mut sizes = HashMap::new();
+        for _ in 0..4 {
+            if tree.sized.is_empty() {
+                break;
+            }
+            let idx = rng.below(tree.sized.len());
+            sizes.insert(idx, resize_one(warm, tree, idx, rng));
+        }
+        sizes
+    };
+    let edits = match case {
+        Case::Resize => return plan.clone(),
+        Case::Repaint | Case::ResizeRepaint => {
+            mark(warm, tree, 1);
+            warm.frame();
+            return plan.clone();
+        }
+        Case::RepaintSome => {
+            mark(warm, tree, 5);
+            warm.frame();
+            return plan.clone();
+        }
+        Case::Reorder => {
+            let out = reorder(warm, tree, plan);
+            warm.frame();
+            return out;
+        }
+        Case::Size | Case::ResizeSize => Edits {
+            sizes: some_sizes(warm, tree, rng),
+            ..Default::default()
+        },
+        Case::EverySize => Edits {
+            sizes: (0..tree.sized.len())
+                .map(|idx| (idx, resize_one(warm, tree, idx, rng)))
+                .collect(),
+            ..Default::default()
+        },
+        Case::Align => Edits {
+            aligns: (0..tree.aligned.len())
+                .step_by(3)
+                .map(|idx| (idx, realign_one(warm, tree, idx, rng)))
+                .collect(),
+            ..Default::default()
+        },
+        Case::RegionNode => {
+            let mut nodes = HashMap::new();
+            for idx in (0..tree.nodes.len()).step_by(2) {
+                let id = tree.nodes[idx];
+                let take = !warm.rsc.widgets().is_region_node(id);
+                warm.rsc.widgets_mut().set_region_node(id, take);
+                nodes.insert(idx, take);
+            }
+            Edits {
+                nodes,
+                ..Default::default()
+            }
+        }
+        Case::Shuffle(shuffle) => Edits {
+            spans: reshuffle(warm, tree, shuffle),
+            ..Default::default()
+        },
+    };
+    warm.frame();
+    plan.edited(&edits)
+}
+
+/// What a widget was configured with, so a tree a fuzzer found can be written
+/// out by hand. A failure is a lead; the fast test that replaces it has to be
+/// buildable from what the failure printed.
+fn describe(id: WidgetId, h: &Harness) -> String {
+    let rules = h.rsc.widgets().size_rules(id);
+    let rule = |r: SizeRule| match r.exact() {
+        Some(len) => format!("{len}"),
+        None => "-".into(),
+    };
+    let align = h.rsc.widgets().alignment(id);
+    let side = |a: AxisAlign| {
+        if a == AxisAlign::NEG {
+            "neg".into()
+        } else if a == AxisAlign::CENTER {
+            "mid".into()
+        } else if a == AxisAlign::POS {
+            "pos".into()
+        } else {
+            format!("{:.2}", a.rel())
+        }
+    };
+    // A rule and an alignment are properties of whatever carries them, so
+    // they print with that widget rather than as widgets of their own.
+    let mut out = describe_widget(id, h);
+    if (rules.x, rules.y) != (SizeRule::Free, SizeRule::Free) {
+        out += &format!("[x:{},y:{}]", rule(rules.x), rule(rules.y));
+    }
+    if align != RegionAlign::default() {
+        out += &format!("@{},{}", side(align.x), side(align.y));
+    }
+    out
+}
+
+fn describe_widget(id: WidgetId, h: &Harness) -> String {
+    let label = h.rsc.widgets().label(id).to_string();
+    let Some(widget) = h.rsc.widgets().get_dyn(id) else {
+        return label;
+    };
+    let any: &dyn std::any::Any = widget;
+    if let Some(w) = any.downcast_ref::<Span>() {
+        let sign = if w.dir.sign == Sign::Neg { "-" } else { "+" };
+        return format!(
+            "Span{{dir:{:?}{sign},gap:{},n:{}}}",
+            w.dir.axis,
+            w.gap,
+            w.children.len()
+        );
+    }
+    if let Some(w) = any.downcast_ref::<Pad>() {
+        let p = &w.padding;
+        return format!(
+            "Pad{{l:{},r:{},t:{},b:{}}}",
+            p.left, p.right, p.top, p.bottom
+        );
+    }
+    if let Some(w) = any.downcast_ref::<Stack>() {
+        return format!("Stack{{n:{}}}", w.children.len());
+    }
+    label
+}
+
+/// Runs `case` on the tree `plan` describes, warm and cold, and says where
+/// the two disagree. `seed` chooses only the values a case picks at random,
+/// so one plan under one case is one comparison however it was reached.
+pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
+    let (start, end) = case.window();
+    let mut warm = Harness::new(start);
+    let (root, mut tree) = build(&mut warm.rsc, plan);
+    warm.state.root = Some(root);
+    // The frame that makes it warm: without it nothing is retained and the
+    // comparison is two cold starts agreeing with each other.
+    warm.frame();
+    if start != end {
+        warm.resize(end);
+        warm.frame();
+    }
+    let cold_plan = change(case, &mut warm, &mut tree, plan, &mut Rng::new(seed));
+
+    let mut cold = Harness::new(end);
+    let (root, cold_tree) = build(&mut cold.rsc, &cold_plan);
+    cold.state.root = Some(root);
+    cold.frame();
+
+    let mut drawn = 0;
+    for (i, (&w, &c)) in tree.ids.iter().zip(&cold_tree.ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        drawn += got.is_some() as usize;
+        if got == want {
+            continue;
+        }
+        // Where two trees disagree is rarely where the cause is, so the
+        // ancestry comes with it, marking the widgets that own a region.
+        let mut chain = Vec::new();
+        let mut at = Some(w);
+        while let Some(id) = at {
+            let active = &warm.render.active[&id];
+            let node = match active.move_idx == active.parent_move {
+                true => "",
+                false => "*",
+            };
+            chain.push(format!("{}{node}", describe(id, &warm)));
+            at = active.parent;
+        }
+        return Some(format!(
+            "widget {i}\n  warm {got:?}\n  cold {want:?}\n  {}",
+            chain.join(" < ")
+        ));
+    }
+    match drawn {
+        0 => Some("nothing was drawn".into()),
+        _ => None,
+    }
+}

tests/shrink.rs

@@ -1,556 +1,38 @@
-//! A property test that shrinks its own counterexample.
+//! A fuzzer that reduces its own counterexample.
 //!
-//! `generated.rs` reproduces a failure from a seed, but a seed is not a lead
-//! anybody can read: the tree is hundreds of widgets, and reconstructing the
-//! part that matters by hand has failed every time it has been tried. This
-//! grows trees it can take apart, so a failure is reduced to the smallest
-//! tree that still shows it and printed as something to write a fast test
-//! from.
+//! A seed is not a lead anybody can read: the tree is hundreds of widgets,
+//! and reconstructing the part that matters by hand has failed every time it
+//! has been tried. This grows the trees `iris::random` describes, takes them
+//! apart, and prints the smallest one that still fails as something to write
+//! a fast test from.
 //!
 //!     cargo test --release --test shrink -- --ignored --nocapture
 //!
 //! `SHRINK_SEEDS` how many trees to try, `SHRINK_DEPTH` how deep to grow
-//! them, `SHRINK_CASE` which scenario. 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.
+//! them, `SHRINK_CASE` which scenario or `all` for every one. `SHRINK_SEED`
+//! takes a single seed, which is how a failure `generated` printed is handed
+//! straight here: the two run the same cases over the same trees, so a seed
+//! that fails there fails here and is reduced.
+//!
+//! It is a fuzzer: run it once the ordinary tests pass, and turn what it
+//! finds into a test of its own rather than leaving a seed as the record.
 
-use iris::harness::Harness;
-use iris::prelude::*;
-use iris::random::{Branch, Rng};
+#[path = "scenario/mod.rs"]
+mod scenario;
 
-/// The same two leaves `iris::random` grows, since only one of them reads the
-/// width it is given and that is the difference that matters.
-const WORDS: &[&str] = &[
-    "Wrapping",
-    "shapes",
-    "one",
-    "source",
-    "into",
-    "as",
-    "many",
-    "lines",
-    "as",
-    "the",
-    "box",
-    "leaves",
-    "room",
-    "for,",
-    "so",
-    "a",
-    "paragraph's",
-    "height",
-    "is",
-    "an",
-    "answer",
-    "and",
-    "not",
-    "a",
-    "setting.",
-];
+use iris::random::{Edits, Plan, plan};
+use scenario::{ALL, Case, diverges, env, over_seeds};
 
-const ONE_LINE: &str = "one line, overflowing whatever it is given";
-
-const OUTER: (f32, f32) = (1920.0, 1200.0);
-/// Steps of the grid two ways of reaching a box may differ by: one per level
-/// of nesting between them, and these trees are five deep. See
-/// `docs/HANDOFF.md`'s "Fixed point" in `ai-app-2` for what is left.
-const AGREE_STEPS: i32 = 2;
-const INNER: (f32, f32) = (640.0, 900.0);
-
-#[derive(Clone, Debug, PartialEq)]
-enum Node {
-    /// Words taken from [`WORDS`], and whether it wraps.
-    Text(usize, bool),
-    /// The leaf that overflows whatever box it is given rather than wrapping.
-    OneLine,
-    Rect,
-    /// Direction, gap, children in creation order, and the order they are
-    /// attached in -- separate so a tree that reorders its children
-    /// still makes the same widgets in the same order, and two
-    /// builds line up index for index.
-    Span(bool, f32, Vec<Node>, Vec<usize>),
-    Stack(Vec<Node>),
-    Pad(f32, Box<Node>),
-    Aligned(u8, u8, Box<Node>),
-    Sized(Option<LayoutLen>, Option<LayoutLen>, Box<Node>),
-    Scroll(bool, Box<Node>),
-    Branch(Box<Node>, Box<Node>, Box<Node>, f32),
-}
-
-fn axis_align(v: u8) -> Option<AxisAlign> {
-    match v % 4 {
-        0 => None,
-        1 => Some(AxisAlign::NEG),
-        2 => Some(AxisAlign::CENTER),
-        _ => Some(AxisAlign::POS),
-    }
-}
-
-fn dir(down: bool) -> Dir {
-    if down { Dir::DOWN } else { Dir::RIGHT }
-}
-
-impl Node {
-    /// Builds into `h`, pushing every id in tree order, so two builds of one
-    /// node line up index for index and their boxes can be compared.
-    fn build(
-        &self,
-        h: &mut Harness,
-        out: &mut Vec<WidgetId>,
-        spans: &mut Vec<WeakWidget<Span>>,
-        sized: &mut Vec<WidgetId>,
-    ) -> StrongWidget {
-        let id: StrongWidget = match self {
-            Node::Text(words, wrap) => {
-                let n = (*words).clamp(1, WORDS.len());
-                wtext(WORDS[..n].join(" "))
-                    .size(16)
-                    .wrap(*wrap)
-                    .add_strong(&mut h.rsc)
-            }
-            Node::OneLine => wtext(ONE_LINE).size(16).wrap(false).add_strong(&mut h.rsc),
-            Node::Rect => rect(Color::RED).add_strong(&mut h.rsc),
-            Node::Span(down, gap, kids, order) => {
-                let mut built: Vec<_> = kids
-                    .iter()
-                    .map(|k| Some(k.build(h, out, spans, sized)))
-                    .collect();
-                // `order` is a permutation, so each is taken exactly once.
-                let children = order
-                    .iter()
-                    .map(|&i| built[i].take().expect("order repeats an index"))
-                    .collect();
-                let handle = Span {
-                    children,
-                    dir: dir(*down),
-                    gap: Px::from_f32(*gap),
-                }
-                .add(&mut h.rsc);
-                // A row takes the height it is given; a column is as wide
-                // as its widest child, which needs no rule.
-                if !*down {
-                    h.rsc
-                        .widgets_mut()
-                        .set_size_rules(handle, None, Some(LayoutLen::rel(1.0)));
-                }
-                spans.push(handle);
-                handle.add_strong(&mut h.rsc)
-            }
-            Node::Stack(kids) => {
-                let children = kids.iter().map(|k| k.build(h, out, spans, sized)).collect();
-                Stack {
-                    children,
-                    size: StackSize::Child(0),
-                }
-                .add_strong(&mut h.rsc)
-            }
-            Node::Pad(p, kid) => {
-                let inner = kid.build(h, out, spans, sized);
-                Pad {
-                    padding: Padding::uniform(*p),
-                    inner,
-                }
-                .add_strong(&mut h.rsc)
-            }
-            Node::Aligned(x, y, kid) => {
-                let inner = kid.build(h, out, spans, sized);
-                for (axis, align) in [(Axis::X, axis_align(*x)), (Axis::Y, axis_align(*y))] {
-                    if let Some(align) = align {
-                        h.rsc.widgets_mut().set_alignment(&inner, axis, align);
-                    }
-                }
-                inner
-            }
-            Node::Sized(x, y, kid) => {
-                let inner = kid.build(h, out, spans, sized);
-                h.rsc.widgets_mut().set_size_rules(&inner, *x, *y);
-                sized.push(inner.id());
-                inner
-            }
-            Node::Scroll(down, kid) => {
-                let inner = kid.build(h, out, spans, sized);
-                let axis = if *down { Axis::Y } else { Axis::X };
-                Scroll::new(inner, axis).add_strong(&mut h.rsc)
-            }
-            Node::Branch(probe, a, b, at) => {
-                let probe = probe.build(h, out, spans, sized);
-                let wide = a.build(h, out, spans, sized);
-                let narrow = b.build(h, out, spans, sized);
-                Branch {
-                    probe,
-                    wide,
-                    narrow,
-                    threshold: *at,
-                }
-                .add_strong(&mut h.rsc)
-            }
-        };
-        out.push(id.id());
-        id
-    }
-
-    /// The lengths every `Sized` node would carry after `resized`, in the
-    /// order `build` pushes them.
-    fn sized_lens(&self, out: &mut Vec<(Option<LayoutLen>, Option<LayoutLen>)>) {
-        match self {
-            Node::Text(..) | Node::OneLine | Node::Rect => {}
-            Node::Span(_, _, kids, _) | Node::Stack(kids) => {
-                kids.iter().for_each(|k| k.sized_lens(out));
-            }
-            Node::Pad(_, k) | Node::Aligned(_, _, k) | Node::Scroll(_, k) => k.sized_lens(out),
-            Node::Sized(x, y, k) => {
-                k.sized_lens(out);
-                out.push((resized_len(*x), resized_len(*y)));
-            }
-            Node::Branch(p, a, b, _) => {
-                p.sized_lens(out);
-                a.sized_lens(out);
-                b.sized_lens(out);
-            }
-        }
-    }
-
-    fn size(&self) -> usize {
-        1 + match self {
-            Node::Text(..) | Node::OneLine | Node::Rect => 0,
-            Node::Span(_, _, kids, _) | Node::Stack(kids) => kids.iter().map(Node::size).sum(),
-            Node::Pad(_, k)
-            | Node::Aligned(_, _, k)
-            | Node::Sized(_, _, k)
-            | Node::Scroll(_, k) => k.size(),
-            Node::Branch(p, a, b, _) => p.size() + a.size() + b.size(),
-        }
-    }
-
-    /// Every one-step simplification: a wrapper replaced by what it wrapped, a
-    /// child dropped, a length or a word count reduced. Ordered cheapest-first
-    /// so the greedy walk takes the biggest bites early.
-    fn smaller(&self) -> Vec<Node> {
-        let mut out = Vec::new();
-        let leaf = Node::Rect;
-        match self {
-            Node::Text(words, wrap) => {
-                if *words > 1 {
-                    out.push(Node::Text(words / 2, *wrap));
-                    out.push(Node::Text(words - 1, *wrap));
-                }
-                if *wrap {
-                    out.push(Node::Text(*words, false));
-                }
-                out.push(leaf);
-            }
-            Node::OneLine => out.push(Node::Rect),
-            Node::Rect => {}
-            Node::Span(down, gap, kids, order) => {
-                out.extend(order.iter().map(|&i| kids[i].clone()));
-                for i in 0..kids.len() {
-                    if kids.len() > 1 {
-                        let mut less = kids.clone();
-                        less.remove(i);
-                        let order = (0..less.len()).collect();
-                        out.push(Node::Span(*down, *gap, less, order));
-                    }
-                }
-                if *gap != 0.0 {
-                    out.push(Node::Span(*down, 0.0, kids.clone(), order.clone()));
-                }
-                for (i, kid) in kids.iter().enumerate() {
-                    for small in kid.smaller() {
-                        let mut next = kids.clone();
-                        next[i] = small;
-                        out.push(Node::Span(*down, *gap, next, order.clone()));
-                    }
-                }
-            }
-            Node::Stack(kids) => {
-                out.extend(kids.iter().cloned());
-                for i in 0..kids.len() {
-                    if kids.len() > 1 {
-                        let mut less = kids.clone();
-                        less.remove(i);
-                        out.push(Node::Stack(less));
-                    }
-                }
-                for (i, kid) in kids.iter().enumerate() {
-                    for small in kid.smaller() {
-                        let mut next = kids.clone();
-                        next[i] = small;
-                        out.push(Node::Stack(next));
-                    }
-                }
-            }
-            Node::Pad(p, kid) => {
-                out.push((**kid).clone());
-                if *p != 0.0 {
-                    out.push(Node::Pad(0.0, kid.clone()));
-                }
-                out.extend(
-                    kid.smaller()
-                        .into_iter()
-                        .map(|k| Node::Pad(*p, Box::new(k))),
-                );
-            }
-            Node::Aligned(x, y, kid) => {
-                out.push((**kid).clone());
-                for (nx, ny) in [(0, *y), (*x, 0)] {
-                    if (nx, ny) != (*x, *y) {
-                        out.push(Node::Aligned(nx, ny, kid.clone()));
-                    }
-                }
-                out.extend(
-                    kid.smaller()
-                        .into_iter()
-                        .map(|k| Node::Aligned(*x, *y, Box::new(k))),
-                );
-            }
-            Node::Sized(x, y, kid) => {
-                out.push((**kid).clone());
-                if x.is_some() {
-                    out.push(Node::Sized(None, *y, kid.clone()));
-                }
-                if y.is_some() {
-                    out.push(Node::Sized(*x, None, kid.clone()));
-                }
-                out.extend(
-                    kid.smaller()
-                        .into_iter()
-                        .map(|k| Node::Sized(*x, *y, Box::new(k))),
-                );
-            }
-            Node::Scroll(down, kid) => {
-                out.push((**kid).clone());
-                out.extend(
-                    kid.smaller()
-                        .into_iter()
-                        .map(|k| Node::Scroll(*down, Box::new(k))),
-                );
-            }
-            Node::Branch(p, a, b, at) => {
-                out.push((**p).clone());
-                out.push((**a).clone());
-                out.push((**b).clone());
-                for small in p.smaller() {
-                    out.push(Node::Branch(Box::new(small), a.clone(), b.clone(), *at));
-                }
-                for small in a.smaller() {
-                    out.push(Node::Branch(p.clone(), Box::new(small), b.clone(), *at));
-                }
-                for small in b.smaller() {
-                    out.push(Node::Branch(p.clone(), a.clone(), Box::new(small), *at));
-                }
-            }
-        }
-        out
-    }
-}
-
-/// A declared size over about half the tree, the way `iris::random` puts them
-/// in: on the way into every child rather than as a node kind of its own, so
-/// readers of a size are dense rather than occasional.
-fn sized(rng: &mut Rng, inner: Node) -> Node {
-    if !rng.chance() {
-        return inner;
-    }
-    let len = |rng: &mut Rng| match rng.below(4) {
-        0 => Some(LayoutLen::px(20.0 + rng.below(180) as f32)),
-        1 => Some(LayoutLen::LEFTOVER),
-        _ => None,
-    };
-    Node::Sized(len(rng), len(rng), Box::new(inner))
-}
-
-fn grow(rng: &mut Rng, depth: usize) -> Node {
-    if depth == 0 {
-        return match rng.below(4) {
-            0 => Node::Text(1 + rng.below(WORDS.len()), true),
-            1 => Node::OneLine,
-            _ => Node::Rect,
-        };
-    }
-    let len = |rng: &mut Rng| match rng.below(4) {
-        0 => Some(LayoutLen::px(20.0 + rng.below(180) as f32)),
-        1 => Some(LayoutLen::LEFTOVER),
-        2 => Some(LayoutLen::rel(0.25 + rng.below(3) as f32 * 0.25)),
-        _ => None,
-    };
-    let kid = |rng: &mut Rng| {
-        let inner = grow(rng, depth - 1);
-        sized(rng, inner)
-    };
-    match rng.below(8) {
-        0 => Node::Scroll(rng.chance(), Box::new(kid(rng))),
-        1 => Node::Aligned(rng.below(4) as u8, rng.below(4) as u8, Box::new(kid(rng))),
-        2 => Node::Pad(rng.below(24) as f32, Box::new(kid(rng))),
-        3 => Node::Sized(len(rng), len(rng), Box::new(kid(rng))),
-        4 => Node::Branch(
-            Box::new(kid(rng)),
-            Box::new(kid(rng)),
-            Box::new(kid(rng)),
-            rng.below(500) as f32,
-        ),
-        5 => Node::Stack((0..2 + rng.below(2)).map(|_| kid(rng)).collect()),
-        _ => {
-            let kids: Vec<_> = (0..2 + rng.below(3)).map(|_| kid(rng)).collect();
-            let order = (0..kids.len()).collect();
-            Node::Span(rng.chance(), rng.below(3) as f32 * 4.0, kids, order)
-        }
-    }
-}
-
-#[derive(Clone, Copy, PartialEq)]
-enum Case {
-    Resize,
-    Repaint,
-    ResizeRepaint,
-    Reorder,
-    SizeChange,
-}
-
-/// A different declared length, kept the same kind so the change is to the
-/// value alone.
-fn resized_len(len: Option<LayoutLen>) -> Option<LayoutLen> {
-    let half = Rel::from_f32(0.5);
-    len.map(|len| LayoutLen {
-        px: len.px.mul(half) + Px::from_int(13),
-        rel: len.rel.mul(half),
-        leftover: len.leftover,
-    })
-}
-
-/// Every declared size changed, as a tree rather than as a change.
-fn resized(node: &Node) -> Node {
-    match node {
-        Node::Span(down, gap, kids, order) => Node::Span(
-            *down,
-            *gap,
-            kids.iter().map(resized).collect(),
-            order.clone(),
-        ),
-        Node::Stack(kids) => Node::Stack(kids.iter().map(resized).collect()),
-        Node::Pad(p, k) => Node::Pad(*p, Box::new(resized(k))),
-        Node::Aligned(x, y, k) => Node::Aligned(*x, *y, Box::new(resized(k))),
-        Node::Sized(x, y, k) => Node::Sized(resized_len(*x), resized_len(*y), Box::new(resized(k))),
-        Node::Scroll(d, k) => Node::Scroll(*d, Box::new(resized(k))),
-        Node::Branch(p, a, b, at) => Node::Branch(
-            Box::new(resized(p)),
-            Box::new(resized(a)),
-            Box::new(resized(b)),
-            *at,
-        ),
-        leaf => leaf.clone(),
-    }
-}
-
-/// Every span's children rotated by one, as a tree rather than as a change:
-/// what a warm frame reaches by moving them has to be where growing them that
-/// way lands.
-fn reordered(node: &Node) -> Node {
-    match node {
-        Node::Span(down, gap, kids, order) => {
-            let kids = kids.iter().map(reordered).collect::<Vec<_>>();
-            let mut order = order.clone();
-            order.rotate_left(1);
-            Node::Span(*down, *gap, kids, order)
-        }
-        Node::Stack(kids) => Node::Stack(kids.iter().map(reordered).collect()),
-        Node::Pad(p, k) => Node::Pad(*p, Box::new(reordered(k))),
-        Node::Aligned(x, y, k) => Node::Aligned(*x, *y, Box::new(reordered(k))),
-        Node::Sized(x, y, k) => Node::Sized(*x, *y, Box::new(reordered(k))),
-        Node::Scroll(d, k) => Node::Scroll(*d, Box::new(reordered(k))),
-        Node::Branch(p, a, b, at) => Node::Branch(
-            Box::new(reordered(p)),
-            Box::new(reordered(a)),
-            Box::new(reordered(b)),
-            *at,
-        ),
-        leaf => leaf.clone(),
-    }
-}
-
-/// Runs one scenario warm and cold and says where they disagree.
-fn diverges(node: &Node, case: Case) -> Option<String> {
-    let resizes = matches!(case, Case::Resize | Case::ResizeRepaint);
-    let repaints = matches!(case, Case::Repaint | Case::ResizeRepaint);
-    let start = if resizes { OUTER } else { INNER };
-    let mut warm = Harness::new(start);
-    let mut warm_ids = Vec::new();
-    let mut warm_spans = Vec::new();
-    let mut warm_sized = Vec::new();
-    let root = node.build(&mut warm, &mut warm_ids, &mut warm_spans, &mut warm_sized);
-    warm.state.root = Some(root);
-    // The frame that makes it warm: without it there is nothing retained and
-    // the comparison is two cold starts agreeing with each other.
-    warm.frame();
-    if resizes {
-        warm.resize(INNER);
-        warm.frame();
-    }
-    if repaints {
-        for &id in &warm_ids {
-            warm.rsc.widgets_mut().get_dyn_mut(id);
-        }
-        warm.frame();
-    }
-    if case == Case::Reorder {
-        for span in &warm_spans {
-            warm.rsc[*span].children.rotate_left(1);
-        }
-        warm.frame();
-    }
-    if case == Case::SizeChange {
-        let mut lens = Vec::new();
-        node.sized_lens(&mut lens);
-        for (id, (x, y)) in warm_sized.iter().zip(lens) {
-            warm.rsc.widgets_mut().set_size_rules(*id, x, y);
-        }
-        warm.frame();
-    }
-
-    // What the warm tree was moved into, grown that way from the start.
-    let want = match case {
-        Case::Reorder => reordered(node),
-        Case::SizeChange => resized(node),
-        _ => node.clone(),
-    };
-    let mut cold = Harness::new(INNER);
-    let mut cold_ids = Vec::new();
-    let mut cold_spans = Vec::new();
-    let mut cold_sized = Vec::new();
-    let root = want.build(&mut cold, &mut cold_ids, &mut cold_spans, &mut cold_sized);
-    cold.state.root = Some(root);
-    cold.frame();
-
-    for (i, (&w, &c)) in warm_ids.iter().zip(&cold_ids).enumerate() {
-        let (got, want) = (warm.region(&w), cold.region(&c));
-        // To a couple of steps of the grid, each a thousandth of a pixel: a
-        // move or a resize lands on the same number now, and a length
-        // measured one way against the same length composed another can
-        // still be a step out per composition between them.
-        let same = match (got, want) {
-            (Some(g), Some(c)) => {
-                let d = |a: Px, b: Px| (a - b).abs() <= Px::STEP.mul_int(AGREE_STEPS);
-                d(g.top_left.x, c.top_left.x)
-                    && d(g.top_left.y, c.top_left.y)
-                    && d(g.bot_right.x, c.bot_right.x)
-                    && d(g.bot_right.y, c.bot_right.y)
-            }
-            (None, None) => true,
-            _ => false,
-        };
-        if !same {
-            return Some(format!("widget {i}: warm {got:?} cold {want:?}"));
-        }
-    }
-    None
-}
-
-/// Takes the first simplification that still fails, until none does.
-fn shrink(mut node: Node, case: Case) -> Node {
+/// Takes the first simplification that still fails, until none does. The
+/// simplifications come biggest first, so this walks down rather than
+/// nibbling: a six-hundred-widget tree reaches single figures in a few
+/// hundred builds.
+fn shrink(mut node: Plan, case: Case, seed: u64) -> Plan {
     loop {
         let Some(next) = node
             .smaller()
             .into_iter()
-            .find(|small| diverges(small, case).is_some())
+            .find(|small| diverges(small, case, seed).is_some())
         else {
             return node;
         };
@@ -558,62 +40,60 @@ fn shrink(mut node: Node, case: Case) -> Node {
     }
 }
 
-/// One thread per core but one, each taking a share of the seeds: a tree is
-/// grown, laid out and dropped within a seed, so nothing is shared. A seed
-/// that fails shrinks on its own thread and panics there, which brings the
-/// scope down with it.
-fn over_seeds(seeds: Vec<u64>, run: impl Fn(u64) + Sync) {
-    let threads =
-        std::thread::available_parallelism().map_or(1, |n| n.get().saturating_sub(1).max(1));
-    let chunk = seeds.len().div_ceil(threads).max(1);
-    std::thread::scope(|scope| {
-        for part in seeds.chunks(chunk) {
-            let run = &run;
-            scope.spawn(move || part.iter().for_each(|&seed| run(seed)));
+fn cases() -> Vec<Case> {
+    match env("SHRINK_CASE", String::from("all")).as_str() {
+        "all" => ALL.to_vec(),
+        name => match Case::named(name) {
+            Some(case) => vec![case],
+            None => panic!(
+                "unknown SHRINK_CASE {name:?}; one of all, {}",
+                ALL.map(Case::name).join(", ")
+            ),
+        },
     }
-    });
-}
-
-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)
 }
 
 #[test]
 #[ignore = "a fuzzer; run it once the ordinary tests pass"]
 fn no_grown_tree_lays_out_differently_warm_than_cold() {
-    let seeds: u64 = env("SHRINK_SEEDS", 400);
     let depth: usize = env("SHRINK_DEPTH", 5);
-    let case = match env("SHRINK_CASE", String::from("resize")).as_str() {
-        "repaint" => Case::Repaint,
-        "resize-repaint" => Case::ResizeRepaint,
-        "reorder" => Case::Reorder,
-        "size-change" => Case::SizeChange,
-        _ => Case::Resize,
+    let cases = cases();
+    let seeds: Vec<u64> = match std::env::var("SHRINK_SEED")
+        .ok()
+        .and_then(|v| v.parse().ok())
+    {
+        Some(seed) => vec![seed],
+        None => (1..=env("SHRINK_SEEDS", 400_u64)).collect(),
     };
+    let count = seeds.len();
 
-    over_seeds((1..=seeds).collect(), |seed| {
-        let node = grow(&mut Rng::new(seed), depth);
-        let Some(how) = diverges(&node, case) else {
-            return;
+    over_seeds(seeds, |seed| {
+        let grown = plan(seed, depth, &Edits::default());
+        for &case in &cases {
+            let Some(how) = diverges(&grown, case, seed) else {
+                continue;
             };
-        let small = shrink(node.clone(), case);
+            let small = shrink(grown.clone(), case, seed);
             println!(
-            "seed {seed}: {how}\ngrown {} widgets, shrank to {}\n{small:#?}",
-            node.size(),
+                "seed {seed} case {}: {how}\ngrown {} widgets, shrank to {}\n{small:#?}",
+                case.name(),
+                grown.size(),
                 small.size()
             );
-        panic!("seed {seed} lays out differently warm than cold");
+            panic!(
+                "seed {seed} lays out differently warm than cold after {}",
+                case.name()
+            );
+        }
     });
-    let sizes: Vec<usize> = (1..=seeds)
-        .map(|seed| grow(&mut Rng::new(seed), depth).size())
+
+    let sizes: Vec<usize> = (1..=count as u64)
+        .map(|seed| plan(seed, depth, &Edits::default()).size())
         .collect();
-    let total: usize = sizes.iter().sum();
     println!(
-        "{seeds} trees at depth {depth} agree: {} widgets total, largest {}",
-        total,
+        "{count} trees at depth {depth} agree over {} case(s): {} widgets total, largest {}",
+        cases.len(),
+        sizes.iter().sum::<usize>(),
         sizes.iter().max().copied().unwrap_or(0)
     );
 }

tests/suite.rs

@@ -16,6 +16,8 @@ mod drift;
 mod idempotence;
 #[path = "cases/layout.rs"]
 mod layout;
+#[path = "cases/plan.rs"]
+mod plan;
 #[path = "cases/pointer.rs"]
 mod pointer;
 #[path = "cases/pointer_routing.rs"]