Re-ask a dirty widget at its offer locally instead of deferring to its parent

A local redraw refused any widget whose given box was not as long as its
offer and marked its parent instead. Under the frame/extent protocol that
is nearly every widget beneath a self-sized container: a span hands its
children its own placement across itself, which is `FULL` while the span
is measured and its answer once it is placed, so the children's offer and
given frames differ on every such axis. A `many` frame at seed 13, depth
8 escalated 43 marks along chains up to seven levels and redrew 508 of
583 active widgets where e44dea3 redraws 159.

Retain the offer's frame beside the given one and ask the offer question
locally: the offer frame composed where the given one is, at the offer's
lengths and placement, then place at the given box where the two differ.
Seed 13 `many` goes from 4.73 ms to 1.28 ms against e44dea3's 0.90, and
294 distinct widgets a frame; size, scroll and repaint are unchanged.

Not sound yet: the suite, the debug oracle and the shrinker at 400 trees
of depth 5 pass, but the oracle at 1000 seeds of depth 6 diverges on seed
532 under reorder and seed 398 under every-size. Both reduce to a
self-sized container whose answer changes under a local redraw; the
reduced plans are in docs/HANDOFF.md of ai-app-2.

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

core/src/fixed.rs

@@ -109,6 +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 {
@@ -372,6 +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())
     }
@@ -488,6 +506,25 @@ mod tests {
         assert_eq!(Px::from_int(100) / Rel::from_f32(0.5), Px::from_int(200));
     }
 
+    /// The bound a greedy line break needs: the width it was measured at is
+    /// not on the grid, and the narrowest box the break still holds for is
+    /// the step at or above it, never the one below.
+    #[test]
+    fn a_ceiling_never_lands_below_the_number_it_came_from() {
+        let step = 1.0 / (1 << PX_SHIFT) as f32;
+        for n in 0..64 {
+            let v = 189.0 + n as f32 * step / 3.0;
+            let up = Px::ceil_from_f32(v);
+            assert!(up.to_f32() >= v, "{up:?} is below {v}");
+            assert!(
+                up.to_f32() - v < step,
+                "{up:?} is more than a step above {v}"
+            );
+        }
+        // An exact step is its own ceiling.
+        assert_eq!(Px::ceil_from_f32(189.5), Px::from_f32(189.5));
+    }
+
     #[test]
     fn a_number_from_outside_is_clamped_to_the_grid() {
         assert_eq!(Px::from_f32(1e12), Px::MAX);

core/src/layout_diagnostics.rs

@@ -54,10 +54,13 @@ pub(crate) enum Counter {
     TextShapes,
     TextBreaks,
     GlyphPlacements,
+    OutsidePlacement,
+    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 placement it was pinned to",
+        "reuse outside: a frame length",
+        "reuse outside: an extent length",
     ];
 }
 

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/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/ui/active.rs

@@ -1,6 +1,6 @@
 use crate::{
-    Holds, LayerId, LayoutLen, MaskIdx, MoveIdx, PrimitiveHandle, RegionAlign, Size, TextureHandle,
-    UiRegion, UiVec2, WidgetId,
+    DrawRegion, LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, RegionAlign, RetainedPrimitive,
+    Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -9,29 +9,32 @@ use crate::{
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
-    /// The box its drawing is in, in `parent_move`'s coordinates.
+    /// The box its parent gave it, in `parent_move`'s coordinates: what it
+    /// was asked about, and what a fraction under it is a fraction of. A
+    /// local redraw asks here.
     pub region: UiRegion,
-    /// The box its parent gave it, in the same coordinates: what it was
-    /// asked about, before its own answer placed its drawing inside it.
-    /// `region` is that placement, and a local redraw asks here.
-    pub given: UiRegion,
-    /// The same box as lengths of its parent's box, which is the one route
-    /// to a box in pixels: a draw threads these down a level at a time, and
-    /// [`crate::UiRenderState::redraw`] takes the same steps back up.
-    pub given_len: UiVec2,
+    /// Where its drawing sits inside that box, in the box's own coordinates.
+    pub placement: UiRegion,
+    /// The original frame in its parent widget's coordinates. Recomposition
+    /// and pixel-length evaluation both follow this chain.
+    pub given_region: UiRegion,
+    /// The frame it was first asked in, in the same coordinates: the offer's
+    /// frame, which its parent's placing draw may since have narrowed.
+    pub offer_region: UiRegion,
     /// The lengths of the box its parent first asked about it in, as
     /// lengths of the box the parent was itself offered. Any later box it
     /// was given was decided knowing its answer, so this is the question
     /// asked again -- and a chain of fractions has no frame in it, which is
     /// why a region node between two widgets cannot break it.
     pub offer_len: UiVec2,
-    /// What it answered there: the size and what that held for.
-    pub answer: (Size, [Holds; 2]),
+    pub offer_placement: [Option<crate::UiSpan>; 2],
+    /// 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 box, 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, extent and explicit placement 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
@@ -39,7 +42,13 @@ 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>,
+    pub primitives: Vec<RetainedPrimitive>,
+    pub mask_region: Option<DrawRegion>,
+    /// The children whose box is a part of this widget's extent rather than
+    /// of its frame, and which part each was given. Moving the extent
+    /// re-places them through that part, so the drawing need not depend on
+    /// where it sits.
+    pub(crate) extent_children: Vec<(WidgetId, ExtentPlacement)>,
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
@@ -70,8 +79,37 @@ pub struct ActiveData {
 }
 
 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)
+    /// Whether what it answered still stands for a box of these pixel
+    /// lengths -- the box it was asked in, where `holds` is about the box its
+    /// answer then chose.
+    pub fn answers_at(&self, px: crate::PxVec2) -> bool {
+        self.answer.is_some_and(|(_, holds)| {
+            holds.contains(
+                px,
+                UiRegion {
+                    x: self.offer_placement[0].unwrap_or(crate::UiSpan::FULL),
+                    y: self.offer_placement[1].unwrap_or(crate::UiSpan::FULL),
+                },
+            )
+        })
+    }
+}
+
+/// What of a container's extent a child was given: the whole of it, for a
+/// wrapper whose box is its child's, or a part of it.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub(crate) enum ExtentPlacement {
+    Inherit,
+    Within(UiRegion),
+}
+
+impl ExtentPlacement {
+    /// The child's frame in the container's frame coordinates, and the slot
+    /// the container chose within it.
+    pub fn resolve(self, extent: UiRegion) -> (UiRegion, [Option<crate::UiSpan>; 2]) {
+        match self {
+            Self::Inherit => (UiRegion::FULL, [Some(extent.x), Some(extent.y)]),
+            Self::Within(part) => (part.within(&extent), [None; 2]),
+        }
     }
 }

core/src/ui/draw_region.rs

@@ -0,0 +1,36 @@
+use crate::{PrimitiveHandle, UiRegion};
+
+/// Retains which box geometry follows when only the extent changes.
+#[derive(Clone, Copy, Debug)]
+pub enum DrawRegion {
+    Frame(UiRegion),
+    Extent(UiRegion),
+}
+
+impl DrawRegion {
+    pub(crate) fn resolve(self, frame: UiRegion, extent: UiRegion) -> UiRegion {
+        match self {
+            Self::Frame(local) => local.within(&frame),
+            Self::Extent(local) => local.within(&extent).within(&frame),
+        }
+    }
+
+    pub(crate) fn map(self, f: impl FnOnce(UiRegion) -> UiRegion) -> Self {
+        match self {
+            Self::Frame(local) => Self::Frame(f(local)),
+            Self::Extent(local) => Self::Extent(f(local)),
+        }
+    }
+}
+
+impl From<UiRegion> for DrawRegion {
+    fn from(region: UiRegion) -> Self {
+        Self::Frame(region)
+    }
+}
+
+#[derive(Debug)]
+pub struct RetainedPrimitive {
+    pub handle: PrimitiveHandle,
+    pub region: DrawRegion,
+}

core/src/ui/holds.rs

@@ -52,6 +52,9 @@ impl Holds {
     /// 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;
@@ -92,6 +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.

core/src/ui/layout_holds.rs

@@ -0,0 +1,63 @@
+use crate::{Axis, Holds, PxVec2, UiRegion};
+
+/// Dependencies of one evaluation, before the frame and extent are composed.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct LayoutHolds {
+    pub frame: [Holds; 2],
+    pub extent: [Holds; 2],
+    pub placement: Option<UiRegion>,
+}
+
+impl LayoutHolds {
+    pub const ANY: Self = Self {
+        frame: [Holds::ANY; 2],
+        extent: [Holds::ANY; 2],
+        placement: None,
+    };
+
+    pub fn and(self, other: Self) -> Self {
+        debug_assert!(
+            self.placement.is_none()
+                || other.placement.is_none()
+                || self.placement == other.placement
+        );
+        Self {
+            frame: [
+                self.frame[0].and(other.frame[0]),
+                self.frame[1].and(other.frame[1]),
+            ],
+            extent: [
+                self.extent[0].and(other.extent[0]),
+                self.extent[1].and(other.extent[1]),
+            ],
+            placement: self.placement.or(other.placement),
+        }
+    }
+
+    pub fn covers(self, other: Self) -> bool {
+        self.placement
+            .is_none_or(|placement| other.placement == Some(placement))
+            && [0, 1].into_iter().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
+            })
+    }
+
+    pub fn contains(self, px: PxVec2, placement: UiRegion) -> bool {
+        self.placement.is_none_or(|old| old == placement)
+            && [Axis::X, Axis::Y].into_iter().all(|axis| {
+                self.frame[axis as usize].contains(px.axis(axis))
+                    && self.extent[axis as usize]
+                        .contains(placement.axis(axis).len().to_px(px.axis(axis)))
+            })
+    }
+
+    pub fn in_frame(self, placement: UiRegion) -> [Holds; 2] {
+        [Axis::X, Axis::Y].map(|axis| {
+            self.frame[axis as usize]
+                .and(self.extent[axis as usize].through(placement.axis(axis).len()))
+        })
+    }
+}

core/src/ui/mod.rs

@@ -10,12 +10,16 @@ use crate::{
 pub const CHAIN_LIMIT: u32 = 64;
 
 mod active;
+mod draw_region;
 mod holds;
+mod layout_holds;
 mod painter;
 mod render_state;
 
 pub use active::*;
+pub use draw_region::*;
 pub use holds::*;
+pub use layout_holds::*;
 pub use painter::{Painter, PrimitiveLike};
 pub use render_state::*;
 

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, RenderedText, Size, StrongWidget,
-    TextAttrs, TextBuffer, TextData, TextureHandle, UiRegion, UiRenderState, UiRsc, UiVec2, Weight,
-    WidgetId, Widgets,
+    Axis, DrawRegion, ExtentPlacement, Holds, LayoutHolds, LayoutLen, Len, 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,
 };
@@ -17,8 +17,20 @@ 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`.
+    /// The box its parent gave it, in the coordinates of `move_idx`: what a
+    /// fraction of this widget's area is a fraction of, and what every region
+    /// it writes composes within. The same box on the ask that measures and
+    /// the ask that places, which is what keeps a fraction under it from
+    /// being resolved twice.
     pub(super) region: UiRegion,
+    /// Where this widget's drawing sits inside that box, in the box's own
+    /// coordinates: `FULL` while its answer is not yet known, and the box
+    /// its answer or its parent chose once one of them has.
+    pub(super) placement: UiRegion,
+    /// Whether this draw read its placement, which makes the drawing one
+    /// that holds for that placement alone -- the way reading a length in
+    /// pixels makes it hold for that length.
+    pub(super) reads_placement: bool,
     /// That box in pixels, which its children's are a length of: threaded
     /// down from the box this widget was given rather than composed back up
     /// the chain, so every length in layout is one multiply from its
@@ -26,7 +38,12 @@ pub struct Painter<'a> {
     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<DrawRegion>,
+    pub(super) extent_children: Vec<(WidgetId, ExtentPlacement)>,
+    pub(super) extent_own: [Holds; 2],
+    /// 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
     /// is the one recorded as its offer.
@@ -43,8 +60,8 @@ pub struct Painter<'a> {
     /// What this draw itself read of its box 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],
+    /// Dependencies of every child drawing, including unmeasured overlays.
+    pub(super) under: 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,
@@ -57,13 +74,28 @@ pub struct Painter<'a> {
 }
 
 impl<'a> Painter<'a> {
-    fn primitive_at<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
+    fn primitive_at<P: Primitive>(&mut self, primitive: P, region: DrawRegion) {
         let kind = self.rsc.ui_mut().primitives.kind::<P>();
         self.write(kind, primitive, region);
     }
 
     /// Takes the kind, for a caller writing many of one primitive.
-    fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: UiRegion) {
+    fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: DrawRegion) {
+        self.write_resolved(
+            kind,
+            primitive,
+            region,
+            region.resolve(self.region, self.placement),
+        );
+    }
+
+    fn write_resolved<P: Primitive>(
+        &mut self,
+        kind: PrimitiveKind<P>,
+        primitive: P,
+        region: DrawRegion,
+        resolved: UiRegion,
+    ) {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::PrimitiveWrites);
         let h = self.state.layers.write(
@@ -72,15 +104,15 @@ 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);
@@ -88,28 +120,48 @@ 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 at = DrawRegion::Extent(UiRegion::FULL);
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, self.region)
+        self.primitive_at(primitive, at)
     }
 
-    pub fn primitive_within(&mut self, primitive: impl PrimitiveLike, region: UiRegion) {
+    /// Writes in the frame by default. `DrawRegion::Extent` keeps the local
+    /// geometry attached to this widget's box without reading its placement.
+    pub fn primitive_within(
+        &mut self,
+        primitive: impl PrimitiveLike,
+        region: impl Into<DrawRegion>,
+    ) {
         let primitive = primitive.into_primitive(self);
-        self.primitive_at(primitive, region.within(&self.region));
+        self.primitive_at(primitive, region.into());
     }
 
-    pub fn set_mask(&mut self, region: UiRegion) {
+    /// Sets a mask in the selected frame or extent coordinates.
+    pub fn set_mask(&mut self, region: impl Into<DrawRegion>) {
+        let region = region.into();
+        self.mask_region = Some(region);
         assert!(self.mask == MaskIdx::NONE);
         self.mask = self.rsc.ui_mut().masks.push(Mask {
-            region,
+            region: region.resolve(self.region, self.placement),
             move_idx: self.move_idx,
         });
     }
 
-    /// Draws a widget within this widget's region.
+    /// Draws a widget in the whole of this widget's own box: it gets the
+    /// same region -- the same area for its fractions to be of -- and is put
+    /// where this widget was put. What a container that is only a wrapper
+    /// around one child wants, since its box is the child's.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
-        self.widget_within(id, UiRegion::FULL)
+        let own = self.placement;
+        self.widget_at_inner(
+            id,
+            UiRegion::FULL,
+            [Some(own.x), Some(own.y)],
+            Some(ExtentPlacement::Inherit),
+            false,
+        )
     }
 
     /// What a widget's rules declare its lengths to be, which whoever draws
@@ -125,54 +177,77 @@ impl<'a> Painter<'a> {
     /// 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.extent_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.
+    /// Draws a child in `region`, relative to this widget's frame. The child
+    /// resolves declared lengths and reports against that frame, then places
+    /// its drawing by its own alignment.
+    ///
+    /// `DrawRegion::Extent` gives a part of where this widget's drawing sits
+    /// instead, for a container whose children belong inside that rather than
+    /// inside the box it was offered. The part is what is kept, so moving the
+    /// extent re-places the child rather than drawing this widget again.
     pub fn widget_within<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
-        region: UiRegion,
+        region: impl Into<DrawRegion>,
     ) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, region, [false; 2])
+        match region.into() {
+            DrawRegion::Frame(region) => self.widget_at(id, region, [None; 2]),
+            DrawRegion::Extent(part) => {
+                let within = part.within(&self.placement);
+                self.widget_at_inner(
+                    id,
+                    within,
+                    [None; 2],
+                    Some(ExtentPlacement::Within(part)),
+                    false,
+                )
+            }
+        }
     }
 
-    /// Draws a widget in a box this widget chose from the widget's own
-    /// answer along the `decided` axes. On those the answer is not placed
-    /// inside the box again: it already is the box, and a fraction the
-    /// widget reported of its offer, taken of this box a second time, would
-    /// shrink it twice. A container uses this where it hands back exactly
-    /// what a child asked for -- a span placing a child at the length it
-    /// reported, a scroll giving its content the content's own length.
-    pub fn widget_decided<'s, W: ?Sized>(
+    /// Draws a widget in `region`, saying where in it the drawing goes.
+    ///
+    /// `region` is the child's own area: what a fraction it declares or
+    /// reports is a fraction of, and the coordinates the regions it writes
+    /// compose within. It is the same box on the ask that measures and the
+    /// ask that places, which is what stops a fraction under it being
+    /// resolved twice.
+    ///
+    /// `placement` is what of that region the child's drawing takes, per
+    /// axis, wherever this widget is choosing. `None` leaves the axis to the
+    /// child's own answer and alignment, which is what
+    /// [`Self::widget_within`] passes. A span passes the whole row as the
+    /// region, so `rel(0.5)` is half the row wherever the child sits in it,
+    /// and places the child by passing the slot along its axis.
+    pub fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
-        decided: [bool; 2],
+        placement: [Option<UiSpan>; 2],
     ) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, region, decided)
+        self.widget_at_inner(id, region, placement, None, false)
     }
 
-    fn widget_at<'s, W: ?Sized>(
+    fn widget_at_inner<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
         region: UiRegion,
-        decided: [bool; 2],
+        placement: [Option<UiSpan>; 2],
+        extent: Option<ExtentPlacement>,
+        measuring: bool,
     ) -> DrawResult<'s, 'a, W> {
+        self.extent_children.retain(|(child, _)| *child != id.id());
+        if let Some(extent) = extent {
+            self.extent_children.push((id.id(), extent));
+        }
         let region_node = self.rsc.widgets().is_region_node(id.id());
         let declared = self.declared_lens(id);
         let align = self.rsc.widgets().alignment(id.id());
-        // 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 (local, placement) = ask_box(region, declared, align, placement);
         let within = match local == UiRegion::FULL {
             true => self.region,
             false => local.within(&self.region),
@@ -196,15 +271,28 @@ impl<'a> Painter<'a> {
                 .get(&id.id())
                 .map_or(given_len, |a| a.offer_len),
         };
+        let offer_region = match first_ask {
+            true => local,
+            false => self
+                .state
+                .active
+                .get(&id.id())
+                .map_or(local, |a| a.offer_region),
+        };
+        let offer_placement = if first_ask {
+            placement
+        } else {
+            self.state
+                .active
+                .get(&id.id())
+                .map_or(placement, |a| a.offer_placement)
+        };
         let px = given_len.to_px(self.px);
         let offered_px = offer_len.to_px(self.offered_px);
-        // Whether this ask is the child's offer question, which is a question
-        // about lengths: the same lengths somewhere else is the same question.
-        let answers_offer = self.at_offer && px == offered_px;
         // 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
         // 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 {
@@ -214,27 +302,74 @@ impl<'a> Painter<'a> {
                 parent_move: self.move_idx,
                 region_node,
                 mask: self.mask,
-                given_len,
+                given_region: local,
+                offer_region,
                 offer_len,
+                offer_placement,
                 px,
                 offered_px,
-                decided,
+                placement,
             },
             None,
+            measuring,
             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 in_parent = |holds: LayoutHolds| {
+            let mut result = LayoutHolds::ANY;
+            for axis in AXES {
+                let n = axis as usize;
+                let chosen = placement[n].unwrap_or(UiSpan::FULL).len();
+                match extent {
+                    // Its box is this widget's own, so what its drawing holds
+                    // for is what this widget's extent holds for.
+                    Some(ExtentPlacement::Inherit) if declared[n].is_none() => {
+                        result.frame[n] = holds.frame[n].through(local.axis(axis).len());
+                        result.extent[n] = holds.extent[n];
+                        if holds.placement.is_some() {
+                            result.placement = Some(self.placement);
+                        }
+                    }
+                    // Its box is a part of this widget's extent, so what it
+                    // holds for is a range on that extent and none of it a
+                    // range on the frame. Only the part's length reaches it,
+                    // which is what lets the extent move without a redraw.
+                    Some(ExtentPlacement::Within(part)) if declared[n].is_none() => {
+                        result.extent[n] = holds.frame[n]
+                            .and(holds.extent[n].through(chosen))
+                            .through(part.axis(axis).len());
+                    }
+                    // Its box is a length of this widget's frame: an
+                    // ordinary ask, or a declared length, which is that
+                    // length wherever the box it sits in came from.
+                    _ => {
+                        result.frame[n] = holds.frame[n].through(local.axis(axis).len()).and(
+                            holds.extent[n]
+                                .through(chosen)
+                                .through(local.axis(axis).len()),
+                        );
+                    }
+                }
+            }
+            result
+        };
+        self.under = self.under.and(in_parent(holds));
+        let mut answer_holds = in_parent(answer_holds);
+        // What it reports is a fraction of the box it was given, which is a
+        // part of this widget's extent -- so the same fraction is a different
+        // length once that extent is, and pixels are not. The answer only:
+        // the drawing this holds is re-placed rather than made again.
+        if matches!(extent, Some(ExtentPlacement::Within(_)))
+            && AXES.into_iter().any(|axis| {
+                declared[axis as usize].is_none() && size.axis(axis).rel != crate::Rel::ZERO
+            })
+        {
+            answer_holds.placement = Some(self.placement);
         }
         DrawResult {
             child: id,
             painter: self,
-            size: in_parent_frame(size, local, declared),
+            size: in_parent_frame(size, local.size(), declared),
+            answer_holds,
         }
     }
 
@@ -266,40 +401,53 @@ 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>(
+    /// Measures a child's length from its hint, a retained answer, or `draw`.
+    /// A fresh draw evaluates the offer without placing its answer. The caller
+    /// must later place or undraw the child.
+    pub fn measure_len<W: ?Sized>(
         &mut self,
         child: &StrongWidget<W>,
         axis: Axis,
         region: UiRegion,
-    ) -> Option<LayoutLen> {
+        placement: [Option<UiSpan>; 2],
+    ) -> LayoutLen {
+        let offered = placement;
         let declared = self.declared_lens(child);
         let align = self.rsc.widgets().alignment(child.id());
-        let local = declared_box(region, declared, align);
-        let first_ask = self.offer(child.id());
-        if first_ask && let Some(active) = self.state.active.get_mut(&child.id()) {
-            active.offer_len = local.size();
-        }
+        let (local, placement) = ask_box(region, declared, align, placement);
+        let first_ask = self.at_offer && !self.offered.contains(&child.id());
+
         if let Some(hint) = self.size_hint(child, axis) {
-            return Some(hint);
+            return hint;
         }
         let px = local.size().to_px(self.px);
-        let (size, holds) =
+        let retained =
             self.state
-                .retained_size(child.id(), px, self.move_idx, self.rsc.widgets())?;
+                .retained_size(child.id(), px, placement, self.move_idx, self.rsc.widgets());
+        let Some((size, holds)) = retained else {
+            return self
+                .widget_at_inner(child, region, offered, None, true)
+                .len(axis);
+        };
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::RetainedSizeHits);
         self.depend_on(child);
         if first_ask {
+            self.offered.push(child.id());
             let active = self.state.active.get_mut(&child.id()).unwrap();
-            active.answer = (size, holds);
+            active.offer_len = local.size();
+            active.offer_region = local;
+            active.offer_placement = placement;
         }
-        for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
+        let placement = UiRegion {
+            x: placement[0].unwrap_or(UiSpan::FULL),
+            y: placement[1].unwrap_or(UiSpan::FULL),
+        };
+        let holds = holds.in_frame(placement);
+        for (axis, under) in AXES.into_iter().zip(self.answer_under.frame.iter_mut()) {
             *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
         }
-        Some(in_parent_frame(size, local, declared).axis(axis))
+        in_parent_frame(size, local.size(), declared).axis(axis)
     }
 
     /// Whether this is the first box a child is asked about in during a draw
@@ -331,21 +479,28 @@ 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) {
+    pub fn glyphs(&mut self, text: &RenderedText, origin: impl Into<DrawRegion>) {
+        let origin = origin.into();
+        // Glyph offsets and sizes are pixels, which compose additively.
+        // Only the shared origin needs the frame/extent composition.
+        let resolved = origin.resolve(self.region, self.placement);
         let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {
-            let mut region = origin;
-            region.x.end = region.x.start;
-            region.y.end = region.y.start;
-            let mut region = region.offset(UiVec2::from_px(glyph.offset));
-            let size = PxVec2::new(
-                Px::from_int(glyph.entry.width as i32),
-                Px::from_int(glyph.entry.height as i32),
-            );
-            region.x.end = region.x.start.offset(size.x);
-            region.y.end = region.y.start.offset(size.y);
-            self.write(
+            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));
+                let size = PxVec2::new(
+                    Px::from_int(glyph.entry.width as i32),
+                    Px::from_int(glyph.entry.height as i32),
+                );
+                region.x.end = region.x.start.offset(size.x);
+                region.y.end = region.y.start.offset(size.y);
+                region
+            };
+            self.write_resolved(
                 kind,
                 GlyphPrimitive {
                     uv_min: glyph.entry.uv_min,
@@ -354,17 +509,32 @@ impl<'a> Painter<'a> {
                     color: text.color,
                     flags: glyph.entry.flags(),
                 },
-                region,
+                origin.map(place),
+                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.
+    /// The box this widget's parent gave it, in the coordinates its own
+    /// primitives are written in -- so a region composed `within` it may be
+    /// drawn directly. Its own box is [`Self::placement`] of this one.
     pub fn region(&self) -> UiRegion {
         self.region
     }
 
+    /// Where this widget's drawing goes inside the box it was given, in that
+    /// box's coordinates: what its own answer took of it, or what its parent
+    /// chose for it. `FULL` on the ask that measures, since nothing has been
+    /// placed yet.
+    ///
+    /// Reading it is what says the drawing depends on it, so a widget that
+    /// positions its own content reads it and is drawn again once its box is
+    /// known, and one that fills whatever it is given never is.
+    pub fn placement(&mut self) -> UiRegion {
+        self.reads_placement = true;
+        self.placement
+    }
+
     /// Where this widget sits in a box longer than the length it takes. A
     /// widget that positions its own content reads it to place that content
     /// the way the box around it would have placed the widget.
@@ -398,20 +568,44 @@ impl<'a> Painter<'a> {
         placed_box(UiRegion::FULL, lens, RegionAlign::NEAR)
     }
 
-    /// 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 {
-        for (own, len) in self.own.iter_mut().zip([self.px.x, self.px.y]) {
-            if *own == Holds::ANY {
-                *own = Holds::at(len);
-            }
-        }
-        self.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
+    /// 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 part = self.placement.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 own box on `axis` that what it is drawing
+    /// holds for -- the same primitives, in the same fractions and offsets
+    /// of the box, and the same reported size. A widget that read its length
+    /// in pixels holds for that one alone until it says otherwise.
+    pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+        let part = self.placement.axis(axis).len();
+        let holds = holds.into();
+        debug_assert!(
+            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.extent_own[axis as usize] = holds;
+    }
+
+    /// One axis of the box this widget's parent gave it, 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 region_px_len(&mut self, axis: Axis) -> Px {
         let len = self.px.axis(axis);
         let own = &mut self.own[axis as usize];
         if *own == Holds::ANY {
@@ -420,15 +614,14 @@ impl<'a> Painter<'a> {
         len
     }
 
-    /// The lengths of this widget's box on `axis` that what it is drawing
-    /// holds for -- the same primitives, in the same fractions and offsets
-    /// of the box, and the same reported size. A widget that read its
-    /// length in pixels holds for that one alone until it says otherwise.
-    pub fn holds(&mut self, axis: Axis, holds: impl Into<Holds>) {
+    /// [`Self::holds`] stated about the region rather than about this
+    /// widget's own box, for a container whose drawing turns on the box it
+    /// was given rather than on the part of it it took.
+    pub fn region_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 own box",
+            "'{}' ({:?}) says its drawing holds for lengths that leave out its region",
             self.label(),
             self.id
         );
@@ -475,6 +668,7 @@ 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> {
@@ -485,6 +679,7 @@ 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
     }
 
@@ -518,18 +713,16 @@ impl PrimitiveLike for &TextureHandle {
     }
 }
 
-/// A child's answer as lengths of the box it was asked from. A widget reports
-/// a fraction of the box it was given, and the widget that gave it wants the
-/// same length as a fraction of its own: one composition apart wherever the
-/// offer was not the whole of the parent's extent, as a span's is after a
-/// relative child. A declared axis is already the parent's: it resolved the
-/// rule in its own box, and the rule is what the report says.
-fn in_parent_frame(size: Size, local: UiRegion, declared: [Option<LayoutLen>; 2]) -> Size {
+/// 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() {
-            let len = local.axis(axis).len();
-            *size.axis_mut(axis) = size.axis(axis).within_len(len);
+            *size.axis_mut(axis) = size.axis(axis).within_len(of.axis(axis));
         }
     }
     size
@@ -558,9 +751,9 @@ pub(crate) fn declared_lens(widgets: &Widgets, id: WidgetId) -> [Option<LayoutLe
 /// 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: `declared_box`
-/// already placed it, in the parent's box, and the rule's length is what the
-/// widget reports there. And an axis the parent decided from the answer is
+/// 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
@@ -608,21 +801,26 @@ pub(crate) fn placed_box(region: UiRegion, lens: UiVec2, align: RegionAlign) ->
     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(
+/// A declared axis gets a frame of that length, aligned within the parent's
+/// slot (or the offer). Undeclared axes keep the offered frame and chosen
+/// placement, so their reported fractions retain that reference.
+pub(crate) fn ask_box(
     mut region: UiRegion,
     declared: [Option<LayoutLen>; 2],
     align: RegionAlign,
-) -> UiRegion {
-    for (axis, len) in AXES.into_iter().zip(declared) {
-        let Some(len) = len else { continue };
+    placement: [Option<UiSpan>; 2],
+) -> (UiRegion, [Option<UiSpan>; 2]) {
+    let mut placed = [None; 2];
+    for (axis, (len, chosen)) in AXES.into_iter().zip(declared.into_iter().zip(placement)) {
+        let Some(len) = len else {
+            placed[axis as usize] = chosen;
+            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());
+        let slot = chosen.unwrap_or(*span);
+        span.start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
         span.end = span.start + len;
     }
-    region
+    (region, placed)
 }

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/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/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(DrawRegion::Extent(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/position/pad.rs

@@ -13,9 +13,8 @@ impl Widget for Pad {
         // 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.
-        let inner = painter
-            .widget_within(&self.inner, self.padding.region())
-            .size();
+        let inside = DrawRegion::Extent(self.padding.region());
+        let inner = painter.widget_within(&self.inner, inside).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,
@@ -53,14 +52,18 @@ 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

@@ -14,10 +14,10 @@ impl Widget for Scroll {
         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 whole = UiRegion::FULL;
+        let own = painter.placement();
+        let answer_len =
+            painter.measure_len(&self.inner, self.axis, whole, [Some(own.x), Some(own.y)]);
         let content = answer_len.apply_leftover();
         self.container_len = container_len;
         self.content_len = content.to_px(container_len);
@@ -63,7 +63,11 @@ impl Widget for Scroll {
             region = region.offset(offset);
             region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
         }
-        painter.widget_decided(&self.inner, region, [true; 2]);
+        // The viewport is the inner's region, 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 put is the content
+        // box, scrolled.
+        painter.widget_at(&self.inner, whole, [Some(region.x), Some(region.y)]);
         // 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 +10,34 @@ pub struct Span {
 impl Widget for Span {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
+        // The row: this span's own box, as a span of the region it was given.
+        // Its children are laid out along it, and what they declare or report
+        // is a fraction of the region -- the area this span was told it has,
+        // which it passes on unchanged.
+        let own = painter.placement();
+        let row = *own.axis(axis);
+        // Across itself the span's own box is the child's region: a span is
+        // what contains its children there, and nothing divides that axis.
+        // Along it the whole region is, so a fraction means the same thing
+        // for every child however much of the row is left when it is asked.
+        let region = UiRegion::from_axis(axis, UiSpan::FULL, *own.axis(!axis));
+        let along = |from: Len, to: Len| match self.dir.sign {
+            Sign::Pos => UiSpan::new(row.start + from, row.start + to),
+            Sign::Neg => UiSpan::new(row.end - to, row.end - from),
+        };
+        let far = row.len();
         // A length for every child before their final boxes are chosen: from
         // a hint where one exists, and from drawing otherwise.
         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),
-            };
+            // The whole region is the child's, 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 placed in is the room left
+            // from the cursor, because a text has to wrap at the width
+            // actually there.
+            let room = axis.pair(Some(along(cursor, far)), None);
+            let len = painter.measure_len(child, axis, region, room);
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
             lens.push(len);
@@ -40,43 +54,26 @@ impl Widget for Span {
             |sum, len| sum + *len,
         );
 
+        // What is left for the shares to divide: the row less everything
+        // fixed, as a length of the region rather than a number of pixels.
+        let room = far - Len::from_parts(total.rel, total.px);
         // 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.region_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.region_holds(axis, holds.through(room));
         }
 
         // Across itself a span is as long as its longest child -- unless a
@@ -93,7 +90,6 @@ 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) {
@@ -106,23 +102,19 @@ 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);
-            }
-            // Along the row this box is the child's own answer, so the answer
-            // is not placed in it again; across it the child sits where its
-            // alignment says.
-            let placed = painter.widget_decided(child, region, [axis == Axis::X, axis == Axis::Y]);
+            // Along the row the span says where the child goes; across it the
+            // child sits where its own alignment says. Its region is the
+            // whole of what this span was given either way, which is what its
+            // fractions are of.
+            let placed =
+                painter.widget_at(child, region, axis.pair(Some(along(from, start)), None));
             if shrinks {
                 let used = placed.len(!axis);
                 // Choosing between a fixed and a relative length from the

src/widget/position/stack.rs

@@ -13,9 +13,10 @@ 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 keeps the stack's whole region as
+        // its own -- the stack is the length that child asked for, so taking
+        // the fraction of the stack's box again would take it twice -- and is
+        // put where the stack itself is put.
         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
@@ -26,18 +27,15 @@ impl Widget for Stack {
             }
             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);
-            // The sizing child placed its own content in the box its answer
-            // decided, and this box was derived from that answer, so applying
-            // its alignment again here would place it twice. Every other
-            // child is handed a box that owes nothing to its own answer, and
-            // where it sits in one bigger than itself is its own business.
-            match sizing == Some(i) {
-                true => painter.widget_decided(child, region, [true; 2]),
-                false => painter.widget_within(child, region),
-            };
+            // Every other child has the stack's own box for its region, since
+            // the stack is what contains it, and where it sits in one bigger
+            // than itself is its own business.
+            painter.widget_within(child, DrawRegion::Extent(UiRegion::FULL));
         }
         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, DrawRegion::Extent(region));
         (region, size)
     }
 

tests/cases/layout.rs

@@ -20,13 +20,14 @@ fn a_span_gives_each_child_the_width_it_asked_for() {
     assert_corners!(h, right, (100, 0), (400, 200));
 }
 
-/// A drawn child reports a fraction of the box it was given, and a span
-/// offers each child what is left after the one before. So a nested span
-/// that takes half of the half it was offered has taken a quarter of the row,
-/// and what follows starts three quarters along -- not at the end, which is
-/// where adding its report straight into the cursor put it.
+/// 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_what_it_offered() {
+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);
@@ -34,12 +35,52 @@ fn a_span_reads_a_child_report_as_a_fraction_of_what_it_offered() {
     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 drawn there
-    // once more; half of that final box is what its child takes, packed at
-    // the nested span's own start.
-    assert_corners!(h, nested, (200, 0), (300, 100));
-    assert_corners!(h, inner, (200, 0), (250, 100));
-    assert_corners!(h, tail, (300, 0), (400, 100));
+    // 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: its inset is the whole box less the
@@ -658,3 +699,41 @@ fn equal_shares_differ_by_at_most_two_steps_and_fill_the_row() {
         }
     }
 }
+
+#[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/retained.rs

@@ -156,16 +156,9 @@ 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"
-    );
+    // Only the available length changes: positioning the final slot does
+    // not invalidate a numeric size read.
+    assert_eq!(asked_draws.get(), 2);
 }
 
 #[test]
@@ -628,3 +621,717 @@ fn a_masked_widget_redrawn_on_its_own_sets_its_mask_again() {
     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 frame_geometry_and_extent_geometry_keep_their_references() {
+    struct Both(Rc<Cell<usize>>);
+    impl Widget for Both {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.0.set(self.0.get() + 1);
+            painter.primitive_within(RectPrimitive::color(Color::RED), UiRegion::FULL);
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::LEFTOVER
+        }
+    }
+    for node in [false, true] {
+        let mut h = Harness::new((400, 200));
+        let first = rect(Color::GREEN).width(100).add(&mut h.rsc);
+        let draws = Rc::new(Cell::new(0));
+        let both = Both(draws.clone()).add(&mut h.rsc);
+        h.rsc.widgets_mut().set_region_node(both, node);
+        h.set_root((first, both).span(Dir::RIGHT));
+        let count = draws.get();
+        h.set_len(first, Axis::X, 200);
+        h.frame();
+        assert_eq!(draws.get(), count);
+        let bounds = primitive_bounds(&h, both.id());
+        assert_eq!(bounds[0].top_left.x, Px::ZERO);
+        assert_eq!(bounds[0].bot_right.x, Px::from_int(400));
+        assert_eq!(bounds[1].top_left.x, Px::from_int(200));
+        assert_eq!(bounds[1].bot_right.x, Px::from_int(400));
+    }
+}
+
+#[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_within(&self.child, self.region);
+            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(DrawRegion::Extent(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 a_span_does_not_place_its_measurement_before_assigning_the_childs_slot() {
+    struct MeasuredBox(Rc<Cell<usize>>);
+
+    impl Widget for MeasuredBox {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.0.set(self.0.get() + 1);
+            painter.px_size();
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::from((100, 50))
+        }
+    }
+
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = MeasuredBox(draws.clone()).add(&mut h.rsc);
+    h.set_root((leaf,).span(Dir::RIGHT).width(rel(1.0)).height(rel(1.0)));
+
+    assert_eq!(draws.get(), 3);
+    assert_corners!(h, leaf, (0, 75), (100, 125));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf.id()).unwrap()]
+    );
+    h.frame();
+    assert_eq!(draws.get(), 3);
+
+    h.resize((600, 300));
+    h.frame();
+    assert_eq!(draws.get(), 6);
+    assert_corners!(h, leaf, (0, 125), (100, 175));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf.id()).unwrap()]
+    );
+}
+
+#[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, DrawRegion::Frame(origin));
+            painter.glyphs(text, DrawRegion::Extent(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,
+                [Some(self.extent.x), Some(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 = ReadsWidth {
+        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, (300, 90), (500, 110));
+}
+
+#[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_frames_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,
+                [Some(self.extent.x), Some(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)
+        };
+        let mut warm = Harness::new((403, 211));
+        let (root, leaf, fixed, draws) = plant(&mut warm, UiRegion::FULL);
+        for (start, end) in [(0.13, 0.83), (-0.17, 1.23), (0.31, 0.67)] {
+            let extent = UiRegion::new(
+                UiSpan::new(Len::rel(start) + Len::px(3.125), Len::rel(end)),
+                UiSpan::new(Len::px(11.25), Len::rel(end)),
+            );
+            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.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,
+                [
+                    Some(UiSpan::new(
+                        Len::px(self.start),
+                        Len::px(self.start + 200.0),
+                    )),
+                    Some(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_within(&self.child, DrawRegion::Extent(self.region))
+                .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,
+                        [Some(self.extent.x), Some(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, _) = counted(h, size, !fractional);
+                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/unsettled.rs

@@ -7,7 +7,8 @@
 //! 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.
+//! 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::*;
@@ -554,3 +555,120 @@ fn a_widget_under_a_region_node_is_asked_in_the_box_that_node_was_offered() {
     }
     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/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

@@ -42,21 +42,6 @@ const OUTER: (f32, f32) = (1920.0, 1200.0);
 const INNER: (f32, f32) = (640.0, 900.0);
 const STILL: (f32, f32) = (900.0, 1200.0);
 
-/// The same box, to two steps of the grid between the two ways of reaching
-/// it. A move, a repaint, a row of shares and every length in pixels land on
-/// the same number. What needs the slack is a position: a box centred in a
-/// fraction of its parent against the same box centred in its own pixels,
-/// and a box re-expressed as a fraction of a parent that changed length.
-/// A step is a thousandth of a pixel, where this was a twentieth of one
-/// before any of it was on a grid.
-///
-/// **One step is not enough**, tried 2026-09-17 once a length in pixels
-/// stopped being composed: it passes the 100-seed oracle and fails the
-/// 400-seed shrinker on `resize-size`, seeds 384 and 162, by 0.002 px. So
-/// what is left here is the resize path's own rounding rather than a length
-/// reached two ways.
-const AGREE_STEPS: i32 = 2;
-
 /// 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.
@@ -399,20 +384,6 @@ fn describe_widget(id: WidgetId, h: &Harness) -> String {
     label
 }
 
-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,
-    }
-}
-
 /// 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.
@@ -439,7 +410,7 @@ pub fn diverges(plan: &Plan, case: Case, seed: u64) -> Option<String> {
     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 same_region(got, want) {
+        if got == want {
             continue;
         }
         // Where two trees disagree is rarely where the cause is, so the