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/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/primitive/text.rs

@@ -176,6 +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())
     }

core/src/ui/active.rs

@@ -15,10 +15,12 @@ pub struct ActiveData {
     pub region: UiRegion,
     /// Where its drawing sits inside that box, in the box's own coordinates.
     pub placement: 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,
+    /// 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
@@ -42,7 +44,11 @@ pub struct ActiveData {
     pub textures: Vec<TextureHandle>,
     pub primitives: Vec<RetainedPrimitive>,
     pub mask_region: Option<DrawRegion>,
-    pub inherited_children: Vec<WidgetId>,
+    /// 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>,
@@ -88,3 +94,22 @@ impl ActiveData {
         })
     }
 }
+
+/// 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/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

@@ -15,6 +15,36 @@ impl LayoutHolds {
         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| {

core/src/ui/painter.rs

@@ -1,9 +1,9 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, DrawRegion, Holds, LayoutLen, Len, Px, PxVec2, RegionAlign, RenderedText,
-    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
-    UiRegion, UiRenderState, UiRsc, UiSpan, 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, PrimitiveInst, PrimitiveKind,
         TexturePrimitive,
@@ -40,9 +40,10 @@ pub struct Painter<'a> {
     pub(super) textures: Vec<TextureHandle>,
     pub(super) primitives: Vec<RetainedPrimitive>,
     pub(super) mask_region: Option<DrawRegion>,
-    pub(super) inherited_children: Vec<WidgetId>,
+    pub(super) extent_children: Vec<(WidgetId, ExtentPlacement)>,
     pub(super) extent_own: [Holds; 2],
-    pub(super) extent_under: [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.
@@ -59,8 +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,
@@ -80,6 +81,21 @@ impl<'a> Painter<'a> {
 
     /// Takes the kind, for a caller writing many of one primitive.
     fn write<P: Primitive>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: 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(
@@ -88,7 +104,7 @@ impl<'a> Painter<'a> {
                 kind,
                 id: self.id,
                 primitive,
-                region: region.resolve(self.region, self.placement),
+                region: resolved,
                 mask_idx: self.mask,
                 move_idx: self.move_idx,
             },
@@ -139,7 +155,13 @@ impl<'a> Painter<'a> {
     /// 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> {
         let own = self.placement;
-        self.widget_at_inner(id, UiRegion::FULL, [Some(own.x), Some(own.y)], true)
+        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
@@ -155,20 +177,36 @@ 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.inherited_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 region, and is the child's own region: what its declared lengths
-    /// and its report are fractions of. Where its drawing sits inside that is
-    /// its own answer placed by its alignment.
+    /// 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, [None; 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 `region`, saying where in it the drawing goes.
@@ -191,7 +229,7 @@ impl<'a> Painter<'a> {
         region: UiRegion,
         placement: [Option<UiSpan>; 2],
     ) -> DrawResult<'s, 'a, W> {
-        self.widget_at_inner(id, region, placement, false)
+        self.widget_at_inner(id, region, placement, None, false)
     }
 
     fn widget_at_inner<'s, W: ?Sized>(
@@ -199,14 +237,12 @@ impl<'a> Painter<'a> {
         id: &'s StrongWidget<W>,
         region: UiRegion,
         placement: [Option<UiSpan>; 2],
-        inherited: bool,
+        extent: Option<ExtentPlacement>,
+        measuring: bool,
     ) -> DrawResult<'s, 'a, W> {
-        if inherited {
-            if !self.inherited_children.contains(&id.id()) {
-                self.inherited_children.push(id.id());
-            }
-        } else {
-            self.inherited_children.retain(|child| *child != id.id());
+        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);
@@ -235,6 +271,14 @@ 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 {
@@ -248,7 +292,7 @@ impl<'a> Painter<'a> {
         // 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 {
@@ -258,7 +302,8 @@ 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,
@@ -266,30 +311,65 @@ impl<'a> Painter<'a> {
                 placement,
             },
             None,
+            measuring,
             self.rsc,
         );
-        // Whatever the child's answer holds for keeps this one to the boxes
-        // that give the child a length inside it.
-        for axis in AXES {
-            let n = axis as usize;
-            let frame = holds.frame[n].through(local.axis(axis).len());
-            self.under[n] = self.under[n].and(frame);
-            if inherited && declared[n].is_none() {
-                self.extent_under[n] = self.extent_under[n].and(holds.extent[n]);
-                self.reads_placement |= holds.placement.is_some();
-            } else {
-                let extent = placement[n].unwrap_or(UiSpan::FULL).len();
-                self.under[n] = self.under[n].and(
-                    holds.extent[n]
-                        .through(extent)
-                        .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.size(), declared),
+            answer_holds,
         }
     }
 
@@ -321,32 +401,34 @@ impl<'a> Painter<'a> {
         }
     }
 
-    /// A child's length in the region 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,
         placement: [Option<UiSpan>; 2],
-    ) -> Option<LayoutLen> {
+    ) -> LayoutLen {
+        let offered = placement;
         let declared = self.declared_lens(child);
         let align = self.rsc.widgets().alignment(child.id());
         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) = self.state.retained_size(
-            child.id(),
-            px,
-            placement,
-            self.move_idx,
-            self.rsc.widgets(),
-        )?;
+        let retained =
+            self.state
+                .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);
@@ -354,6 +436,7 @@ impl<'a> Painter<'a> {
             self.offered.push(child.id());
             let active = self.state.active.get_mut(&child.id()).unwrap();
             active.offer_len = local.size();
+            active.offer_region = local;
             active.offer_placement = placement;
         }
         let placement = UiRegion {
@@ -361,10 +444,10 @@ impl<'a> Painter<'a> {
             y: placement[1].unwrap_or(UiSpan::FULL),
         };
         let holds = holds.in_frame(placement);
-        for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
+        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.size(), 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
@@ -400,9 +483,12 @@ impl<'a> Painter<'a> {
     // TODO: merge the text methods into the primitive ones.
     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 region = origin.map(|mut region| {
+            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));
@@ -413,8 +499,8 @@ impl<'a> Painter<'a> {
                 region.x.end = region.x.start.offset(size.x);
                 region.y.end = region.y.start.offset(size.y);
                 region
-            });
-            self.write(
+            };
+            self.write_resolved(
                 kind,
                 GlyphPrimitive {
                     uv_min: glyph.entry.uv_min,
@@ -423,7 +509,8 @@ impl<'a> Painter<'a> {
                     color: text.color,
                     flags: glyph.entry.flags(),
                 },
-                region,
+                origin.map(place),
+                place(resolved),
             );
         }
     }
@@ -581,6 +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> {
@@ -591,6 +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
     }
 
@@ -712,16 +801,9 @@ pub(crate) fn placed_box(region: UiRegion, lens: UiVec2, align: RegionAlign) ->
     placed
 }
 
-/// A child's own region and where in it its drawing goes, from the box it is
-/// offered, the lengths its rules declare, and what its parent chose.
-///
-/// A rule gives the region its length outright -- that is what makes a rule
-/// win, and it is why a widget under one never learns of it -- and the region
-/// then sits where its parent placed it, or where its alignment says if its
-/// parent left the axis open. With no rule the region is the whole of what
-/// was offered, since that is the area a fraction under it is a fraction of,
-/// and what the parent chose is where in it the drawing goes. So the two
-/// coordinate spaces are the same one wherever a placement survives.
+/// 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],
@@ -736,10 +818,8 @@ pub(crate) fn ask_box(
         };
         let span = region.axis_mut(axis);
         let len = Len::from_parts(len.rel, len.px);
-        span.start = match chosen {
-            Some(chosen) => chosen.start,
-            None => 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, placed)

core/src/ui/render_state.rs

@@ -2,9 +2,9 @@
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::ui::painter::{ask_box, declared_lens, placed_box, placed_lens};
 use crate::{
-    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx,
-    Moves, Painter, PixelRegion, Px, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc, UiSpan,
-    UiVec2, Weight, WidgetId, Widgets,
+    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Moves,
+    Painter, PixelRegion, PxVec2, Size, StrongWidget, UiRegion, UiRsc, UiSpan, UiVec2, Weight,
+    WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
 
@@ -20,12 +20,12 @@ pub(super) struct DrawInfo {
     pub parent_move: MoveIdx,
     pub region_node: bool,
     pub mask: MaskIdx,
-    /// The box its parent gave it, as lengths of the parent's own box, and
-    /// the lengths of the box it was first asked about in the same form.
-    /// Both describe the box the *parent* stated, so the second, placing ask
-    /// carries them unchanged while its own region is the placement inside.
-    pub given_len: UiVec2,
+    /// The frame in the parent widget's coordinates, before composition.
+    pub given_region: UiRegion,
+    /// The original offer's lengths relative to the parent's own offer.
     pub offer_len: UiVec2,
+    /// The offer's frame in the parent widget's coordinates.
+    pub offer_region: UiRegion,
     pub offer_placement: [Option<UiSpan>; 2],
     /// This ask's box in pixels, and the offer's: one multiply from the
     /// parent's own, which is where every pixel length in layout comes from.
@@ -68,16 +68,12 @@ pub struct UiRenderState {
     /// A widget's move slot, which outlives any one `ActiveData`: a redraw
     /// replaces that while its children go on pointing at the slot.
     slots: HashMap<WidgetId, MoveIdx>,
-    /// Answers invalidated by a declared-length change below them. These are
-    /// replaced even when retained placement means the redraw is not at the
-    /// old offer.
-    answer_invalid: crate::util::HashSet<WidgetId>,
-    /// Whether this frame contains a declared-length change, so any dirty
-    /// dependent replaces its answer too.
-    replace_answers: bool,
     /// Widgets waiting for an ancestor to draw them, so the walk down the
     /// depths does not pick one up again at its own depth.
     deferred: crate::util::HashSet<WidgetId>,
+    /// What the walk has left to settle, deepest last. Ordered rather than
+    /// searched for, so finding the next one is not a pass over the marks.
+    pending: std::collections::BTreeSet<(usize, WidgetId)>,
     pub moves: Moves,
 }
 
@@ -89,9 +85,8 @@ impl UiRenderState {
             output_size: PxVec2::ZERO,
             old_root: None,
             slots: Default::default(),
-            answer_invalid: Default::default(),
-            replace_answers: false,
             deferred: Default::default(),
+            pending: Default::default(),
             moves: Default::default(),
             resized: false,
         }
@@ -105,9 +100,8 @@ impl UiRenderState {
     /// retained entry at all.
     ///
     /// The root is the only widget a resize marks, and only where the new
-    /// output falls outside what its answer holds for: that range is the
-    /// intersection of everything under it, so admitting the new output says
-    /// the whole tree still stands. Where it does not, the ordinary walk
+    /// output invalidates its answer or its drawing. The latter includes
+    /// children whose size it never read. Where either fails, the ordinary walk
     /// draws the root, and each widget's own range decides how far down the
     /// new length reaches.
     pub fn resize(&mut self, size: impl Into<Vec2>, widgets: &mut Widgets) {
@@ -118,10 +112,10 @@ impl UiRenderState {
         self.output_size = size;
         self.resized = true;
         let Some(root) = self.old_root else { return };
-        let stands = self
-            .active
-            .get(&root)
-            .is_some_and(|active| active.answers_at(active.given_len.to_px(size)));
+        let stands = self.active.get(&root).is_some_and(|active| {
+            let px = active.given_region.size().to_px(size);
+            active.answers_at(px) && active.holds.contains(px, active.placement)
+        });
         if !stands {
             widgets.needs_redraw.insert(root);
         }
@@ -141,7 +135,8 @@ impl UiRenderState {
             parent_move: MoveIdx::NONE,
             region_node: false,
             mask: MaskIdx::NONE,
-            given_len: region.size(),
+            given_region: region,
+            offer_region: region,
             offer_len: UiVec2::FULL_SIZE,
             offer_placement: [None; 2],
             px,
@@ -184,7 +179,6 @@ impl UiRenderState {
         if rsc.widgets().has_updates() {
             self.redraw_updates(rsc);
         }
-        self.replace_answers = false;
         self.free(rsc);
     }
 
@@ -195,7 +189,7 @@ impl UiRenderState {
         if let Some(id) = root {
             let region = Self::root_region(id.id(), rsc.widgets());
             let info = self.root_info(region);
-            self.draw_inner(id.id(), region, info, None, rsc);
+            self.draw_inner(id.id(), region, info, None, false, rsc);
         }
     }
 
@@ -215,8 +209,9 @@ impl UiRenderState {
         region: UiRegion,
         info: DrawInfo,
         mut old: Option<ActiveData>,
+        measuring: bool,
         rsc: &mut dyn UiRsc,
-    ) -> (Size, LayoutHolds) {
+    ) -> (Size, LayoutHolds, LayoutHolds) {
         #[cfg(feature = "layout-diagnostics")]
         {
             diag::bump(Counter::DrawRequests);
@@ -227,8 +222,7 @@ impl UiRenderState {
         // one bottom-up walk, so anything deeper has settled or deferred to
         // its own parent, and a deferred one leaves that parent marked.
         let stale = rsc.widgets().needs_redraw.contains(&id);
-        let replace_answer = self.answer_invalid.remove(&id) || (self.replace_answers && stale);
-        let retained = match replace_answer || stale {
+        let retained = match stale {
             true => None,
             false => self
                 .retained_answer(id, info)
@@ -241,16 +235,20 @@ impl UiRenderState {
             self.draw_at(id, region, info.offered_placement(), info, old.take(), rsc)
         });
 
-        let declared = declared_lens(rsc.widgets(), id);
         // Where the drawing goes, in the region's own coordinates: what the
         // parent chose, and on any axis it left open, what the answer took of
         // the region placed by the widget's alignment. The region itself does
         // not change, so nothing under it resolves a fraction a second time.
-        let lens = placed_lens(answer.0, declared, info.decided());
-        let own = placed_box(UiRegion::FULL, lens, align);
-        let placement = UiRegion {
-            x: info.placement[0].unwrap_or(own.x),
-            y: info.placement[1].unwrap_or(own.y),
+        let placement = if measuring {
+            info.offered_placement()
+        } else {
+            let declared = declared_lens(rsc.widgets(), id);
+            let lens = placed_lens(answer.0, declared, info.decided());
+            let own = placed_box(UiRegion::FULL, lens, align);
+            UiRegion {
+                x: info.placement[0].unwrap_or(own.x),
+                y: info.placement[1].unwrap_or(own.y),
+            }
         };
         self.place(id, region, placement, info, rsc);
 
@@ -278,10 +276,11 @@ impl UiRenderState {
         // what of that box the answer then took. A local redraw asks the
         // same question again from these.
         active.region = region;
-        active.given_len = info.given_len;
+        active.given_region = info.given_region;
+        active.offer_region = info.offer_region;
         active.offer_len = info.offer_len;
         if info.placement == info.offer_placement && info.px == info.offered_px {
-            active.answer = Some(settled);
+            active.answer = Some(answer);
             active.offer_placement = info.offer_placement;
         }
         active.decided = info.decided();
@@ -297,9 +296,9 @@ impl UiRenderState {
             && let Some(old_parent) = self.active.get_mut(&old_parent)
         {
             old_parent.children.retain(|child| *child != id);
-            old_parent.inherited_children.retain(|child| *child != id);
+            old_parent.extent_children.retain(|(child, _)| *child != id);
         }
-        settled
+        (answer.0, answer.1, settled.1)
     }
 
     /// Recompose retained geometry when the evaluation still holds at this extent.
@@ -370,16 +369,16 @@ impl UiRenderState {
             textures: Vec::new(),
             primitives: Vec::new(),
             mask_region: None,
-            inherited_children: Vec::new(),
+            extent_children: Vec::new(),
             children: Vec::new(),
             offered: Vec::new(),
             offered_px: info.offered_px,
             at_offer,
             size_deps: Vec::new(),
             own: [Holds::ANY; 2],
-            under: [Holds::ANY; 2],
+            under: LayoutHolds::ANY,
             extent_own: [Holds::ANY; 2],
-            extent_under: [Holds::ANY; 2],
+            answer_under: LayoutHolds::ANY,
             depth: info.depth,
             move_idx,
             rsc,
@@ -407,9 +406,9 @@ impl UiRenderState {
             textures,
             primitives,
             mask_region,
-            inherited_children,
+            extent_children,
             extent_own,
-            extent_under,
+            answer_under,
             children,
             offered: _,
             offered_px: _,
@@ -447,14 +446,13 @@ impl UiRenderState {
             "'{}' ({id:?}) clips to {px:?} and reports {size}",
             rsc.widgets().label(id),
         );
-        let holds = LayoutHolds {
-            frame: [own[0].and(under[0]), own[1].and(under[1])],
-            extent: [
-                extent_own[0].and(extent_under[0]),
-                extent_own[1].and(extent_under[1]),
-            ],
+        let own_holds = LayoutHolds {
+            frame: own,
+            extent: extent_own,
             placement: reads_placement.then_some(placement),
         };
+        let answer_holds = own_holds.and(answer_under);
+        let holds = answer_holds.and(under);
         debug_assert!(
             holds.contains(px, placement),
             "'{}' ({id:?}) drew in {px:?}, outside the ranges it reported: {holds:?}",
@@ -483,7 +481,8 @@ impl UiRenderState {
                         parent_move: move_idx,
                         region_node: false,
                         mask,
-                        given_len: UiVec2::FULL_SIZE,
+                        given_region: UiRegion::FULL,
+                        offer_region: UiRegion::FULL,
                         offer_len: UiVec2::FULL_SIZE,
                         offer_placement: [None; 2],
                         px,
@@ -500,7 +499,8 @@ impl UiRenderState {
             id,
             region,
             placement,
-            given_len: info.given_len,
+            given_region: info.given_region,
+            offer_region: info.offer_region,
             offer_len: info.offer_len,
             offer_placement: info.offer_placement,
             // Whoever asked writes the answer, if this was the asking.
@@ -513,7 +513,7 @@ impl UiRenderState {
             textures,
             primitives,
             mask_region,
-            inherited_children,
+            extent_children,
             children,
             size_deps,
             declared: declared_lens(rsc.widgets(), id),
@@ -527,7 +527,7 @@ impl UiRenderState {
         };
         rsc.on_draw(&active);
         self.active.insert(id, active);
-        (size, holds)
+        (size, answer_holds)
     }
 
     /// Keeps a region node's entry across redraws because descendants retain
@@ -616,7 +616,7 @@ impl UiRenderState {
             Some(parent) => self.asked_px(parent.id),
             None => (self.output_size, self.output_size),
         };
-        let px = active.given_len.to_px(parent_px);
+        let px = active.given_region.size().to_px(parent_px);
         let mut offered = active.offer_len.to_px(parent_offer);
         for axis in AXES {
             // A declared length is resolved by whoever drew the widget, in
@@ -690,6 +690,24 @@ impl UiRenderState {
         if !active.holds.contains(info.px, placement) {
             #[cfg(feature = "layout-diagnostics")]
             {
+                // Which of the three said no, so a frame that redraws more
+                // than it should says where to look. They overlap: a drawing
+                // can be outside two of them at once.
+                let holds = active.holds;
+                if holds.placement.is_some_and(|pinned| pinned != placement) {
+                    diag::bump(Counter::OutsidePlacement);
+                }
+                for axis in AXES {
+                    let n = axis as usize;
+                    if !holds.frame[n].contains(info.px.axis(axis)) {
+                        diag::bump(Counter::OutsideFrame);
+                    }
+                    if !holds.extent[n]
+                        .contains(placement.axis(axis).len().to_px(info.px.axis(axis)))
+                    {
+                        diag::bump(Counter::OutsideExtent);
+                    }
+                }
                 diag::bump(Counter::ReuseOutside);
                 diag::reuse(id, ReuseOutcome::Outside);
             }
@@ -697,14 +715,12 @@ impl UiRenderState {
         }
         let extent_moved = active.placement != placement;
         let moved = active.region != region;
-        let (answer, old_region, slot) =
-            ((active.size, active.holds), active.region, active.move_idx);
+        let (answer, slot) = ((active.size, active.holds), active.move_idx);
         if moved {
             if has_region_node {
                 self.moves.set(slot, region);
             } else {
-                let remap = RegionRemap::new(old_region, region)?;
-                self.remap_subtree(id, &remap, info.parent_move, rsc);
+                self.recompose_subtree(id, region, info.parent_move, rsc);
             }
         }
         if extent_moved {
@@ -713,7 +729,8 @@ impl UiRenderState {
         self.redepth(id, info.depth);
         let active = self.active.get_mut(&id).unwrap();
         active.region = region;
-        active.given_len = info.given_len;
+        active.given_region = info.given_region;
+        active.offer_region = info.offer_region;
         active.offer_len = info.offer_len;
         #[cfg(feature = "layout-diagnostics")]
         {
@@ -764,19 +781,18 @@ impl UiRenderState {
         }
         let parent_move = active.move_idx;
         let mask = active.mask;
-        let children = active.inherited_children.len();
+        let children = active.extent_children.len();
         for index in 0..children {
-            let child = self.active[&id].inherited_children[index];
+            let (child, extent) = self.active[&id].extent_children[index];
+            let (part, slot) = extent.resolve(placement);
             let active = &self.active[&child];
-            let (child_local, chosen) = ask_box(
-                UiRegion::FULL,
-                active.declared,
-                active.own_align,
-                [Some(placement.x), Some(placement.y)],
-            );
+            let (child_local, chosen) = ask_box(part, active.declared, active.own_align, slot);
+            // What it took of that box is its own answer, which this move did
+            // not ask again: keep the placement it has on any axis this
+            // widget is not the one choosing.
             let child_placement = UiRegion {
-                x: chosen[0].unwrap_or(UiSpan::FULL),
-                y: chosen[1].unwrap_or(UiSpan::FULL),
+                x: chosen[0].unwrap_or(active.placement.x),
+                y: chosen[1].unwrap_or(active.placement.y),
             };
             let child_info = DrawInfo {
                 layer: active.layer,
@@ -785,7 +801,8 @@ impl UiRenderState {
                 parent_move,
                 region_node: active.move_idx != active.parent_move,
                 mask,
-                given_len: child_local.size(),
+                given_region: child_local,
+                offer_region: active.offer_region,
                 offer_len: active.offer_len,
                 offer_placement: active.offer_placement,
                 px: child_local.size().to_px(info.px),
@@ -820,44 +837,35 @@ impl UiRenderState {
         }
     }
 
-    /// Re-expresses an ordinary retained subtree in a new parent region.
-    /// An independently movable descendant needs only its own region changed;
-    /// its contents stay in that region's coordinate space.
-    fn remap_subtree(
+    /// Replays the original local compositions, including their rounding order.
+    /// A region node terminates the walk because its contents name its slot.
+    fn recompose_subtree(
         &mut self,
         id: WidgetId,
-        remap: &RegionRemap,
+        region: UiRegion,
         parent_move: MoveIdx,
         rsc: &mut dyn UiRsc,
     ) {
         let active = self.active.get_mut(&id).unwrap();
+        active.region = region;
         if active.move_idx != parent_move {
-            let region = remap.apply(active.region);
-            active.region = region;
             self.moves.set(active.move_idx, region);
             return;
         }
-        active.region = remap.apply(active.region);
         for primitive in &active.primitives {
             let handle = &primitive.handle;
             *self.layers[handle.layer].region_mut(handle) =
-                primitive.region.resolve(active.region, active.placement);
+                primitive.region.resolve(region, active.placement);
+        }
+        if let Some(local) = active.mask_region {
+            rsc.ui_mut().masks.get_mut(active.mask).region =
+                local.resolve(region, active.placement);
         }
-        let own_mask = (active.mask != active.parent_mask).then_some(active.mask);
         let children = active.children.len();
-        // A mask the widget set itself moves with it; one it inherited
-        // belongs to the widget that set it, and moves there or not at all.
-        if let Some(idx) = own_mask {
-            let mask = rsc.ui_mut().masks.get_mut(idx);
-            debug_assert_eq!(mask.move_idx, parent_move);
-            mask.region = active
-                .mask_region
-                .unwrap()
-                .resolve(active.region, active.placement);
-        }
         for index in 0..children {
             let child = self.active[&id].children[index];
-            self.remap_subtree(child, remap, parent_move, rsc);
+            let local = self.active[&child].given_region;
+            self.recompose_subtree(child, local.within(&region), parent_move, rsc);
         }
     }
 
@@ -933,7 +941,8 @@ impl UiRenderState {
                 id,
                 region: UiRegion::FULL,
                 placement: UiRegion::FULL,
-                given_len: UiVec2::FULL_SIZE,
+                given_region: UiRegion::FULL,
+                offer_region: UiRegion::FULL,
                 offer_len: UiVec2::FULL_SIZE,
                 offer_placement: [None; 2],
                 answer: None,
@@ -945,7 +954,7 @@ impl UiRenderState {
                 textures: Vec::new(),
                 primitives: Vec::new(),
                 mask_region: None,
-                inherited_children: Vec::new(),
+                extent_children: Vec::new(),
                 children: Vec::new(),
                 size_deps: Vec::new(),
                 move_idx: info.parent_move,
@@ -967,8 +976,6 @@ impl UiRenderState {
             }
         }
         self.slots.clear();
-        self.answer_invalid.clear();
-        self.replace_answers = false;
         self.moves.clear();
         self.layers.clear();
         rsc.widgets_mut().needs_redraw.clear();
@@ -982,7 +989,6 @@ impl UiRenderState {
             rsc.on_remove(id);
             self.remove(id, true, rsc);
             self.drop_slot(id);
-            self.answer_invalid.remove(&id);
         }
         rsc.ui_mut().textures.free();
     }
@@ -1001,24 +1007,52 @@ impl UiRenderState {
         // something below is about to change it -- which is the whole class
         // of defect where a widget settles inside its parent's draw, clears
         // its mark there, and tells nobody its answer moved.
+        // The queue is that set, ordered: a mark made while the walk runs
+        // queues itself through `mark`. What ends the walk is still the set
+        // being spent, not the queue, so a mark that reached it another way
+        // cannot be left for the next frame.
         loop {
-            let next = rsc
-                .widgets()
-                .needs_redraw
-                .iter()
-                .copied()
-                .filter(|id| !self.deferred.contains(id))
-                .max_by_key(|&id| self.depth(id));
-            let Some(id) = next else { break };
-            #[cfg(feature = "layout-diagnostics")]
-            diag::bump(Counter::QueuePops);
-            if !self.redraw(id, rsc) {
-                self.deferred.insert(id);
+            for &id in rsc.widgets().needs_redraw.iter() {
+                if !self.deferred.contains(&id) {
+                    let depth = self.depth(id);
+                    self.pending.insert((depth, id));
+                }
+            }
+            if self.pending.is_empty() {
+                break;
+            }
+            while let Some((depth, id)) = self.pending.pop_last() {
+                // Settled inside an ancestor's draw, or deferred to one,
+                // since the mark that queued it.
+                if self.deferred.contains(&id) || !rsc.widgets().needs_redraw.contains(&id) {
+                    continue;
+                }
+                // A subtree that changed hands takes its descendants' depths
+                // with it, so an entry queued before that move names the
+                // depth it had under the parent it left.
+                let now = self.depth(id);
+                if now != depth {
+                    self.pending.insert((now, id));
+                    continue;
+                }
+                #[cfg(feature = "layout-diagnostics")]
+                diag::bump(Counter::QueuePops);
+                if !self.redraw(id, rsc) {
+                    self.deferred.insert(id);
+                }
             }
         }
         self.deferred.clear();
     }
 
+    /// Marks a widget for the walk to settle, and queues it at its depth.
+    fn mark(&mut self, id: WidgetId, widgets: &mut Widgets) {
+        if widgets.needs_redraw.insert(id) && !self.deferred.contains(&id) {
+            let depth = self.depth(id);
+            self.pending.insert((depth, id));
+        }
+    }
+
     fn depth(&self, id: WidgetId) -> usize {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::DepthReads);
@@ -1119,20 +1153,11 @@ impl UiRenderState {
         if let Some(parent) = active.parent
             && (declared_changed || alignment_changed || !active.drawn || active.answer.is_none())
         {
-            if declared_changed {
-                self.replace_answers = true;
-                let mut at = Some(id);
-                while let Some(next) = at {
-                    self.answer_invalid.insert(next);
-                    rsc.widgets_mut().needs_redraw.insert(next);
-                    at = self.active[&next].parent;
-                }
-            }
             // Both stay marked: the parent because it has this to draw, and
             // this because the parent must draw it rather than keep what it
             // has. The mark comes off in `draw_at`, where the parent draws.
-            rsc.widgets_mut().needs_redraw.insert(id);
-            rsc.widgets_mut().needs_redraw.insert(parent);
+            self.mark(id, rsc.widgets_mut());
+            self.mark(parent, rsc.widgets_mut());
             return false;
         }
         if !active.drawn {
@@ -1150,20 +1175,10 @@ impl UiRenderState {
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::LocalRedraws);
             let old = self.remove(id, false, rsc);
-            self.draw_inner(id, region, info, old, rsc);
+            self.draw_inner(id, region, info, old, false, rsc);
             return true;
         };
         let (given_px, offered_px) = self.asked_px(id);
-        // Asked again in the box its parent gave it, which is the question
-        // its parent asked only while that box is as long as the offer. Any
-        // other box is a different question, so the parent asks it, with the
-        // mark left on. Lengths and not whole boxes: what a drawing depends
-        // on is its lengths, so the same lengths elsewhere is one question.
-        if given_px != offered_px {
-            rsc.widgets_mut().needs_redraw.insert(id);
-            rsc.widgets_mut().needs_redraw.insert(parent);
-            return false;
-        }
         let info = DrawInfo {
             layer: active.layer,
             parent: active.parent,
@@ -1171,7 +1186,8 @@ impl UiRenderState {
             parent_move: active.parent_move,
             region_node: rsc.widgets().is_region_node(id),
             mask: active.parent_mask,
-            given_len: active.given_len,
+            given_region: active.given_region,
+            offer_region: active.offer_region,
             offer_len: active.offer_len,
             offer_placement: active.offer_placement,
             px: given_px,
@@ -1181,30 +1197,61 @@ impl UiRenderState {
             placement: AXES
                 .map(|axis| active.decided[axis as usize].then(|| *active.placement.axis(axis))),
         };
-        let (given, was_answer) = (active.region, active.answer);
+        let (given, was_answer, was_holds) = (active.region, active.answer, active.holds);
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::LocalRedraws);
 
         let old = self.remove(id, false, rsc);
-        // Refresh the original measurement before restoring the assigned slot.
-        // Its lengths may differ even though the fraction reference is unchanged.
+        // Asked again where its parent asked: the offer's frame, composed
+        // where the given one is, at the offer's lengths and placement. That
+        // is the question its answer came from, whatever box the parent then
+        // chose from the answer -- which is often a different frame, since a
+        // span hands its children its own placement across itself. The
+        // parent draws in its own frame, or in `FULL` where it is a region
+        // node.
+        let parent_frame = match self.active.get(&parent) {
+            Some(p) if p.move_idx == p.parent_move => p.region,
+            _ => UiRegion::FULL,
+        };
+        let offer_frame = match info.offer_region == UiRegion::FULL {
+            true => parent_frame,
+            false => info.offer_region.within(&parent_frame),
+        };
         let offered = DrawInfo {
             placement: info.offer_placement,
+            given_region: info.offer_region,
+            px: offered_px,
             ..info
         };
-        let answer = self.draw_inner(id, given, offered, old, rsc);
-        if info.placement != offered.placement {
-            self.draw_inner(id, given, info, None, rsc);
+        // Where the given differs from the offer, the first draw is only the
+        // measurement and the second puts the drawing where the parent did.
+        let placed_apart =
+            info.placement != offered.placement || info.px != offered.px || given != offer_frame;
+        let answer = self.draw_inner(id, offer_frame, offered, old, placed_apart, rsc);
+        if placed_apart {
+            self.draw_inner(id, given, info, None, false, rsc);
         }
-        if Some(answer) != was_answer {
-            // Its parent chose its box knowing the old answer, so it lays out
-            // again and chooses the box the new one asks for.
+        let active = self.active.get_mut(&id).unwrap();
+        // A wider contract does not invalidate the guarantee the parent kept.
+        // Retain that guarantee so widening and narrowing back do not churn it.
+        if let Some((size, holds)) = was_answer
+            && answer.0 == size
+            && answer.1.covers(holds)
+        {
+            active.answer = was_answer;
+        }
+        if active.holds.covers(was_holds) && was_holds.contains(given_px, active.placement) {
+            active.holds = was_holds;
+        }
+        if active.answer != was_answer || active.holds != was_holds {
+            // The parent retains both the answer and the drawing's validity;
+            // even an unchanged size can narrow the range safe for a resize.
             #[cfg(feature = "layout-diagnostics")]
             {
                 diag::bump(Counter::SizeChanges);
                 diag::bump(Counter::ReaderEdges);
             }
-            rsc.widgets_mut().needs_redraw.insert(parent);
+            self.mark(parent, rsc.widgets_mut());
         }
         true
     }
@@ -1219,118 +1266,6 @@ fn within_box(size: Size, px: PxVec2, axis: Axis) -> bool {
     len.leftover != Weight::ZERO || box_len.mul(len.rel) + len.px <= box_len
 }
 
-/// A retained region rewritten from one parent box into another. A fixed
-/// source extent can be translated but cannot recover fractions for a resize.
-#[derive(Clone, Copy)]
-struct RegionRemap {
-    axes: [AxisRemap; 2],
-}
-
-/// Moving one axis of a box into another, worked out once for the whole
-/// subtree that moves with it. Every part of that subtree is divided by the
-/// same extent and placed between the same two ends, so the ends and the
-/// divisor belong here rather than in each part's arithmetic.
-#[derive(Clone, Copy)]
-enum AxisRemap {
-    /// A box that kept its length carries its parts by moving them, which is
-    /// exact. Dividing to find the fraction each sits at and multiplying to
-    /// place it again are two roundings, and they land a step from where
-    /// growing the tree that way does.
-    Translate(Len),
-    /// A box that changed length has to re-express each part as a fraction of
-    /// the new one, which is what a part of a box means.
-    Scale(AxisScale),
-}
-
-#[derive(Clone, Copy)]
-struct AxisScale {
-    /// What the fraction is measured from, and what divides it. `whole` is
-    /// the common case of a box spanning the whole of its parent's, where
-    /// dividing by one is the expensive way to write a subtraction.
-    start_rel: Rel,
-    extent: Rel,
-    whole: bool,
-    /// `lerp` is `a + (b - a) * fraction`, and both ends are the same for
-    /// every part, so each is kept as its near end and its span.
-    from_px: Px,
-    from_px_span: Px,
-    to_rel: Rel,
-    to_rel_span: Rel,
-    to_px: Px,
-    to_px_span: Px,
-}
-
-impl RegionRemap {
-    fn new(from: UiRegion, to: UiRegion) -> Option<Self> {
-        Some(Self {
-            axes: [AxisRemap::new(from.x, to.x)?, AxisRemap::new(from.y, to.y)?],
-        })
-    }
-
-    fn apply(&self, region: UiRegion) -> UiRegion {
-        // A box that only moved carries every part of itself by the same two
-        // amounts, and that is the common move. Asking it once for the whole
-        // region is what lets it be eight adds in a row rather than four
-        // sequences with a branch each -- measured, it is where the time in a
-        // move goes.
-        if let [AxisRemap::Translate(x), AxisRemap::Translate(y)] = self.axes {
-            return region.translated(x, y);
-        }
-        UiRegion {
-            x: self.axes[0].apply_span(region.x),
-            y: self.axes[1].apply_span(region.y),
-        }
-    }
-}
-
-impl AxisRemap {
-    fn new(from: UiSpan, to: UiSpan) -> Option<Self> {
-        if from.len() == to.len() {
-            return Some(Self::Translate(to.start - from.start));
-        }
-        let extent = from.end.rel - from.start.rel;
-        // Without a relative extent there is no fraction to re-express: a box
-        // of fixed length cannot say where its parts sit in a different one.
-        if extent == Rel::ZERO {
-            return None;
-        }
-        Some(Self::Scale(AxisScale {
-            start_rel: from.start.rel,
-            extent,
-            whole: extent == Rel::ONE,
-            from_px: from.start.px,
-            from_px_span: from.end.px - from.start.px,
-            to_rel: to.start.rel,
-            to_rel_span: to.end.rel - to.start.rel,
-            to_px: to.start.px,
-            to_px_span: to.end.px - to.start.px,
-        }))
-    }
-
-    fn apply_span(&self, span: UiSpan) -> UiSpan {
-        UiSpan {
-            start: self.apply_scalar(span.start),
-            end: self.apply_scalar(span.end),
-        }
-    }
-
-    fn apply_scalar(&self, scalar: Len) -> Len {
-        let scale = match self {
-            Self::Translate(by) => return scalar + *by,
-            Self::Scale(scale) => scale,
-        };
-        let offset = scalar.rel - scale.start_rel;
-        let fraction = match scale.whole {
-            true => offset,
-            false => offset / scale.extent,
-        };
-        let from_px = scale.from_px + scale.from_px_span.mul(fraction);
-        let to_rel = scale.to_rel + scale.to_rel_span.mul(fraction);
-        let to_px = scale.to_px + scale.to_px_span.mul(fraction);
-        Len::from_parts(to_rel, scalar.px - from_px + to_px)
-    }
-}
-
 impl Default for UiRenderState {
     fn default() -> Self {
         Self::new()

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,

src/widget/position/pad.rs

@@ -13,7 +13,7 @@ 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 inside = self.padding.region_of(painter.placement());
+        let inside = DrawRegion::Extent(self.padding.region());
         let inner = painter.widget_within(&self.inner, inside).size();
         Size {
             x: LayoutLen {

src/widget/position/scroll.rs

@@ -17,10 +17,7 @@ impl Widget for Scroll {
         let whole = UiRegion::FULL;
         let own = painter.placement();
         let answer_len =
-            match painter.known_len(&self.inner, self.axis, whole, [Some(own.x), Some(own.y)]) {
-                Some(len) => len,
-                None => painter.widget(&self.inner).size().axis(self.axis),
-            };
+            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);

src/widget/position/span.rs

@@ -37,10 +37,7 @@ impl Widget for Span {
             // 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 = match painter.known_len(child, axis, region, room) {
-                Some(len) => len,
-                None => painter.widget_at(child, region, room).len(axis),
-            };
+            let len = painter.measure_len(child, axis, region, room);
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
             lens.push(len);

src/widget/position/stack.rs

@@ -13,8 +13,6 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
-        // This stack's own box, which is `FULL` until its answer is known.
-        let placement = painter.placement();
         // 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
@@ -25,13 +23,7 @@ impl Widget for Stack {
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter
-                    .widget_at(
-                        child,
-                        UiRegion::FULL,
-                        [Some(placement.x), Some(placement.y)],
-                    )
-                    .size()
+                painter.widget(child).size()
             }
             None => Size::LEFTOVER,
         };
@@ -43,7 +35,7 @@ impl Widget for Stack {
             // 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, placement);
+            painter.widget_within(child, DrawRegion::Extent(UiRegion::FULL));
         }
         size
     }

src/widget/text/mod.rs

@@ -50,20 +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.
-        //
-        // The step at or above that longest line rather than the nearest
-        // one, since the shaper measures in floats: the nearest step is
-        // under the line half the time, and a range starting there admits a
-        // box the line does not fit in, where the break is not this one.
-        if let Some(width) = width {
-            painter.holds(Axis::X, Px::ceil_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> {

tests/cases/layout.rs

@@ -716,3 +716,24 @@ fn a_stack_sized_by_a_child_does_not_take_that_childs_fraction_twice() {
     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

@@ -808,3 +808,530 @@ fn changing_an_inherited_extent_keeps_the_original_measurement_offer() {
         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/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