Retain frame and extent dependencies independently

Keep the original measurement placement separate from the assigned slot.
Validate frame and extent lengths before reusing an answer or drawing, and
represent hint-only records as having no measured answer.

Retain primitive and mask coordinates with their frame/extent reference.
Forwarded children follow a reused wrapper's placement without rerunning
valid draw bodies. Keep the single Widget::draw API.

Restore the eight failing suite cases from the region/placement prototype,
with regressions for mixed coordinate references, a changed inherited
extent, the sizing-stack fraction, and an undrawn share becoming visible.

This remains experimental: nested container updates do substantially more
work than e44dea3 despite restoring the leaf and wrapper reuse guarantees.
Do not merge it as a performance improvement.

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,
+    DrawRegion, LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, RegionAlign, RetainedPrimitive,
-    UiRegion, UiVec2, WidgetId,
+    Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
 /// What is kept of a widget its parent has asked about. `drawn` says whether
@@ -15,10 +15,6 @@ pub struct ActiveData {
     pub region: UiRegion,
     /// Where its drawing sits inside that box, in the box's own coordinates.
     pub placement: UiRegion,
-    /// Whether its drawing read that placement, which is what says whether
-    /// moving it within the region is a redraw or only a different claim on
-    /// the same drawing.
-    pub reads_placement: bool,
     /// 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.
@@ -29,13 +25,14 @@ pub struct ActiveData {
     /// 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 offer_placement: [Option<crate::UiSpan>; 2],
-    pub answer: (Size, [Holds; 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`
+    /// The frame, extent and explicit placement reads that this drawing holds for.
-    /// hold for.
+    pub holds: LayoutHolds,
-    pub holds: [Holds; 2],
     pub drawn: bool,
     pub parent: Option<WidgetId>,
     /// How far down the tree it was drawn, the root being 1. Carried down a
@@ -43,7 +40,9 @@ 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>,
+    pub inherited_children: Vec<WidgetId>,
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
@@ -74,16 +73,18 @@ 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 {
-        let (_, holds) = self.answer;
+        self.answer.is_some_and(|(_, holds)| {
-        holds[0].contains(px.x) && holds[1].contains(px.y)
+            holds.contains(
+                px,
+                UiRegion {
+                    x: self.offer_placement[0].unwrap_or(crate::UiSpan::FULL),
+                    y: self.offer_placement[1].unwrap_or(crate::UiSpan::FULL),
+                },
+            )
+        })
     }
 }

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/layout_holds.rs

@@ -0,0 +1,33 @@
+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 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,
+    Axis, DrawRegion, Holds, LayoutLen, Len, Px, PxVec2, RegionAlign, RenderedText,
-    TextAttrs, TextBuffer, TextData, TextureHandle, UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2,
+    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
-    Weight, WidgetId, Widgets,
+    UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
     render::{
-        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveHandle, PrimitiveInst,
+        GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
-        PrimitiveKind, TexturePrimitive,
+        TexturePrimitive,
     },
     ui::render_state::DrawInfo,
 };
@@ -38,7 +38,11 @@ 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) inherited_children: Vec<WidgetId>,
+    pub(super) extent_own: [Holds; 2],
+    pub(super) extent_under: [Holds; 2],
     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.
@@ -69,13 +73,13 @@ 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) {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::PrimitiveWrites);
         let h = self.state.layers.write(
@@ -84,15 +88,15 @@ impl<'a> Painter<'a> {
                 kind,
                 id: self.id,
                 primitive,
-                region,
+                region: region.resolve(self.region, self.placement),
                 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);
@@ -102,24 +106,29 @@ impl<'a> Painter<'a> {
 
     /// Writes a primitive over the whole of this widget's own box.
     pub fn primitive(&mut self, primitive: impl PrimitiveLike) {
-        let at = self.placed();
+        let at = DrawRegion::Extent(UiRegion::FULL);
         let primitive = primitive.into_primitive(self);
         self.primitive_at(primitive, at)
     }
 
-    /// Writes one somewhere in this widget's region. A widget that wants its
+    /// Writes in the frame by default. `DrawRegion::Extent` keeps the local
-    /// own box rather than the region it was given composes through
+    /// geometry attached to this widget's box without reading its placement.
-    /// [`Self::placement`] first.
+    pub fn primitive_within(
-    pub fn primitive_within(&mut self, primitive: impl PrimitiveLike, region: UiRegion) {
+        &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());
     }
 
-    /// `region` is in this widget's own region, as every region it writes is.
+    /// Sets a mask in the selected frame or extent coordinates.
-    pub fn set_mask(&mut self, region: UiRegion) {
+    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.within(&self.region),
+            region: region.resolve(self.region, self.placement),
             move_idx: self.move_idx,
         });
     }
@@ -129,8 +138,8 @@ impl<'a> Painter<'a> {
     /// 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> {
-        let own = self.placement();
+        let own = self.placement;
-        self.widget_at(id, UiRegion::FULL, [Some(own.x), Some(own.y)])
+        self.widget_at_inner(id, UiRegion::FULL, [Some(own.x), Some(own.y)], true)
     }
 
     /// What a widget's rules declare its lengths to be, which whoever draws
@@ -146,6 +155,7 @@ 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.state.undraw_rec(id.id(), self.rsc);
     }
 
@@ -181,6 +191,23 @@ impl<'a> Painter<'a> {
         region: UiRegion,
         placement: [Option<UiSpan>; 2],
     ) -> DrawResult<'s, 'a, W> {
+        self.widget_at_inner(id, region, placement, false)
+    }
+
+    fn widget_at_inner<'s, W: ?Sized>(
+        &'s mut self,
+        id: &'s StrongWidget<W>,
+        region: UiRegion,
+        placement: [Option<UiSpan>; 2],
+        inherited: 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());
+        }
         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());
@@ -208,11 +235,16 @@ impl<'a> Painter<'a> {
                 .get(&id.id())
                 .map_or(given_len, |a| a.offer_len),
         };
+        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.
@@ -228,6 +260,7 @@ impl<'a> Painter<'a> {
                 mask: self.mask,
                 given_len,
                 offer_len,
+                offer_placement,
                 px,
                 offered_px,
                 placement,
@@ -235,13 +268,23 @@ impl<'a> Painter<'a> {
             None,
             self.rsc,
         );
-        if answers_offer {
-            self.state.active.get_mut(&id.id()).unwrap().answer = (size, holds);
-        }
         // Whatever the child's answer holds for keeps this one to the boxes
         // that give the child a length inside it.
-        for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
+        for axis in AXES {
-            *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
+            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()),
+                );
+            }
         }
         DrawResult {
             child: id,
@@ -286,28 +329,38 @@ impl<'a> Painter<'a> {
         child: &StrongWidget<W>,
         axis: Axis,
         region: UiRegion,
+        placement: [Option<UiSpan>; 2],
     ) -> Option<LayoutLen> {
         let declared = self.declared_lens(child);
         let align = self.rsc.widgets().alignment(child.id());
-        let (local, _) = ask_box(region, declared, align, [None; 2]);
+        let (local, placement) = ask_box(region, declared, align, placement);
-        let first_ask = self.offer(child.id());
+        let first_ask = self.at_offer && !self.offered.contains(&child.id());
-        if first_ask && let Some(active) = self.state.active.get_mut(&child.id()) {
+
-            active.offer_len = local.size();
-        }
         if let Some(hint) = self.size_hint(child, axis) {
             return Some(hint);
         }
         let px = local.size().to_px(self.px);
-        let (size, holds) =
+        let (size, holds) = self.state.retained_size(
-            self.state
+            child.id(),
-                .retained_size(child.id(), px, self.move_idx, self.rsc.widgets())?;
+            px,
+            placement,
+            self.move_idx,
+            self.rsc.widgets(),
+        )?;
         #[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_placement = placement;
         }
+        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.under.iter_mut()) {
             *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
         }
@@ -343,13 +396,13 @@ impl<'a> Painter<'a> {
         ui.text.render(buffer, attrs, width)
     }
 
-    /// `origin` is in this widget's own region, as every region it writes is.
+    /// 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.within(&self.region);
+        let origin = origin.into();
         let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {
-            let mut region = origin;
+            let region = origin.map(|mut region| {
                 region.x.end = region.x.start;
                 region.y.end = region.y.start;
                 let mut region = region.offset(UiVec2::from_px(glyph.offset));
@@ -359,6 +412,8 @@ 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(
                 kind,
                 GlyphPrimitive {
@@ -393,15 +448,6 @@ impl<'a> Painter<'a> {
         self.placement
     }
 
-    /// This widget's own box in the coordinates its primitives are written
-    /// in: its placement composed through the region it was given.
-    fn placed(&mut self) -> UiRegion {
-        match self.placement() == UiRegion::FULL {
-            true => self.region,
-            false => self.placement.within(&self.region),
-        }
-    }
-
     /// 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.
@@ -444,11 +490,11 @@ impl<'a> Painter<'a> {
     /// 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 part = self.placement.axis(axis).len();
         let len = part.to_px(self.px.axis(axis));
-        let own = &mut self.own[axis as usize];
+        let own = &mut self.extent_own[axis as usize];
         if *own == Holds::ANY {
-            *own = Holds::at(len).through(part);
+            *own = Holds::at(len);
         }
         len
     }
@@ -458,7 +504,7 @@ impl<'a> Painter<'a> {
     /// 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 part = self.placement.axis(axis).len();
         let holds = holds.into();
         debug_assert!(
             holds.contains(part.to_px(self.px.axis(axis))),
@@ -466,10 +512,7 @@ impl<'a> Painter<'a> {
             self.label(),
             self.id
         );
-        // Kept as a range of the region's lengths, which is the one variable
+        self.extent_own[axis as usize] = holds;
-        // every range here is about: its own box is a part of that box, and
-        // `through` is the exact preimage of taking the part.
-        self.own[axis as usize] = holds.through(part);
     }
 
     /// One axis of the box this widget's parent gave it, in pixels -- what a

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, LayoutLen, Len, MaskIdx, MoveIdx, Moves, Painter,
+    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx,
-    PixelRegion, Px, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc, UiSpan, UiVec2, Weight,
+    Moves, Painter, PixelRegion, Px, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc, UiSpan,
-    WidgetId, Widgets,
+    UiVec2, Weight, WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
 
@@ -26,6 +26,7 @@ pub(super) struct DrawInfo {
     /// carries them unchanged while its own region is the placement inside.
     pub given_len: UiVec2,
     pub offer_len: UiVec2,
+    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.
     pub px: PxVec2,
@@ -142,6 +143,7 @@ impl UiRenderState {
             mask: MaskIdx::NONE,
             given_len: region.size(),
             offer_len: UiVec2::FULL_SIZE,
+            offer_placement: [None; 2],
             px,
             offered_px: px,
             placement: [None; 2],
@@ -214,7 +216,7 @@ impl UiRenderState {
         info: DrawInfo,
         mut old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
-    ) -> (Size, [Holds; 2]) {
+    ) -> (Size, LayoutHolds) {
         #[cfg(feature = "layout-diagnostics")]
         {
             diag::bump(Counter::DrawRequests);
@@ -230,7 +232,7 @@ impl UiRenderState {
             true => None,
             false => self
                 .retained_answer(id, info)
-                .or_else(|| self.try_reuse(id, region, info, rsc)),
+                .or_else(|| self.try_reuse(id, region, info.offered_placement(), info, rsc)),
         };
         let answer = retained.unwrap_or_else(|| {
             if old.is_none() {
@@ -252,14 +254,23 @@ impl UiRenderState {
         };
         self.place(id, region, placement, info, rsc);
 
-        // The answer is only reusable while both parts of the operation are:
+        // On axes chosen by the parent, measurement and drawing share an
-        // what the widget reported, and what it drew once placed. Both are
+        // extent. Otherwise the answer fixes the final extent as a function
-        // ranges of the region's own lengths, since that is the box neither
+        // of the frame, so pull that drawing's validity back through it.
-        // ask changes.
         let drawing_holds = self.active[&id].holds;
         let mut settled = answer;
         for axis in AXES {
-            settled.1[axis as usize] = settled.1[axis as usize].and(drawing_holds[axis as usize]);
+            let n = axis as usize;
+            settled.1.frame[n] = settled.1.frame[n].and(drawing_holds.frame[n]);
+            if info.placement[n].is_some() {
+                settled.1.extent[n] = settled.1.extent[n].and(drawing_holds.extent[n]);
+            } else {
+                settled.1.frame[n] = settled.1.frame[n]
+                    .and(drawing_holds.extent[n].through(placement.axis(axis).len()));
+            }
+        }
+        if drawing_holds.placement.is_some() && info.placement.iter().any(Option::is_some) {
+            settled.1.placement = Some(info.offered_placement());
         }
 
         let active = self.active.get_mut(&id).unwrap();
@@ -269,7 +280,10 @@ impl UiRenderState {
         active.region = region;
         active.given_len = info.given_len;
         active.offer_len = info.offer_len;
-        active.answer = settled;
+        if info.placement == info.offer_placement && info.px == info.offered_px {
+            active.answer = Some(settled);
+            active.offer_placement = info.offer_placement;
+        }
         active.decided = info.decided();
         active.own_align = align;
         // A subtree can be reused whole under a different parent -- same box,
@@ -283,14 +297,12 @@ 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);
         }
         settled
     }
 
-    /// Puts the drawing where the answer says it goes. A drawing that never
+    /// Recompose retained geometry when the evaluation still holds at this extent.
-    /// read its placement is the same drawing wherever it is put, so all that
-    /// changes is what box the widget is recorded as occupying; one that read
-    /// it is drawn again, and only where the placement it read has moved.
     fn place(
         &mut self,
         id: WidgetId,
@@ -299,15 +311,7 @@ impl UiRenderState {
         info: DrawInfo,
         rsc: &mut dyn UiRsc,
     ) {
-        let active = &self.active[&id];
+        if self.try_reuse(id, region, placement, info, rsc).is_some() {
-        // A drawing that never read its placement is the same drawing
-        // wherever it is put, so only what the widget is recorded as
-        // occupying changes; one that read it stands only for the placement
-        // it read. Either way the drawing still has to be where the region
-        // now is, which is what a retained answer on its own does not do.
-        let stands = !active.reads_placement || active.placement == placement;
-        if stands && self.try_reuse(id, region, info, rsc).is_some() {
-            self.active.get_mut(&id).unwrap().placement = placement;
             return;
         }
         #[cfg(feature = "layout-diagnostics")]
@@ -325,7 +329,7 @@ impl UiRenderState {
         info: DrawInfo,
         old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
-    ) -> (Size, [Holds; 2]) {
+    ) -> (Size, LayoutHolds) {
         let (move_idx, local, retired_move) = match info.region_node {
             // Its box becomes its movable region, so it draws in that
             // region's coordinates and its box is one entry to rewrite.
@@ -339,16 +343,19 @@ impl UiRenderState {
             false => (info.parent_move, region, self.slots.remove(&id)),
         };
         let (old_children, old_answer) = match old {
-            Some(old) => (old.children, Some(old.answer)),
+            Some(old) => (old.children, old.answer),
             None => (Vec::new(), None),
         };
         rsc.widgets_mut().needs_redraw.remove(&id);
-        // A box of the offered lengths asks the offer's question wherever it
+        // Only evaluation at the original offer establishes the children's
-        // sits, since what a drawing depends on is its lengths -- and
+        // offers. A placing evaluation must not overwrite that question.
-        // equality is the comparison, these being counts of a step rather
-        // than floats to be compared for nearness.
         let px = info.px;
-        let at_offer = px == info.offered_px;
+        let at_offer = px == info.offered_px
+            && placement
+                == UiRegion {
+                    x: info.offer_placement[0].unwrap_or(UiSpan::FULL),
+                    y: info.offer_placement[1].unwrap_or(UiSpan::FULL),
+                };
 
         let mut painter = Painter {
             state: self,
@@ -362,6 +369,8 @@ impl UiRenderState {
             id,
             textures: Vec::new(),
             primitives: Vec::new(),
+            mask_region: None,
+            inherited_children: Vec::new(),
             children: Vec::new(),
             offered: Vec::new(),
             offered_px: info.offered_px,
@@ -369,6 +378,8 @@ impl UiRenderState {
             size_deps: Vec::new(),
             own: [Holds::ANY; 2],
             under: [Holds::ANY; 2],
+            extent_own: [Holds::ANY; 2],
+            extent_under: [Holds::ANY; 2],
             depth: info.depth,
             move_idx,
             rsc,
@@ -395,6 +406,10 @@ impl UiRenderState {
             mask,
             textures,
             primitives,
+            mask_region,
+            inherited_children,
+            extent_own,
+            extent_under,
             children,
             offered: _,
             offered_px: _,
@@ -432,9 +447,16 @@ impl UiRenderState {
             "'{}' ({id:?}) clips to {px:?} and reports {size}",
             rsc.widgets().label(id),
         );
-        let holds = [own[0].and(under[0]), own[1].and(under[1])];
+        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]),
+            ],
+            placement: reads_placement.then_some(placement),
+        };
         debug_assert!(
-            holds[0].contains(px.x) && holds[1].contains(px.y),
+            holds.contains(px, placement),
             "'{}' ({id:?}) drew in {px:?}, outside the ranges it reported: {holds:?}",
             rsc.widgets().label(id),
         );
@@ -463,6 +485,7 @@ impl UiRenderState {
                         mask,
                         given_len: UiVec2::FULL_SIZE,
                         offer_len: UiVec2::FULL_SIZE,
+                        offer_placement: [None; 2],
                         px,
                         offered_px: px,
                         placement: [None; 2],
@@ -477,11 +500,11 @@ impl UiRenderState {
             id,
             region,
             placement,
-            reads_placement,
             given_len: info.given_len,
             offer_len: info.offer_len,
+            offer_placement: info.offer_placement,
             // Whoever asked writes the answer, if this was the asking.
-            answer: old_answer.unwrap_or((size, holds)),
+            answer: old_answer,
             size,
             holds,
             drawn: true,
@@ -489,6 +512,8 @@ impl UiRenderState {
             depth: info.depth,
             textures,
             primitives,
+            mask_region,
+            inherited_children,
             children,
             size_deps,
             declared: declared_lens(rsc.widgets(), id),
@@ -532,18 +557,24 @@ impl UiRenderState {
         &self,
         id: WidgetId,
         px: PxVec2,
+        placement: [Option<UiSpan>; 2],
         parent_move: MoveIdx,
         widgets: &Widgets,
-    ) -> Option<(Size, [Holds; 2])> {
+    ) -> Option<(Size, LayoutHolds)> {
         if widgets.needs_redraw.contains(&id) {
             return None;
         }
         let active = self.active.get(&id)?;
-        let (size, holds) = active.answer;
+        let (size, holds) = active.answer?;
         let valid = active.drawn
             && active.parent_move == parent_move
-            && holds[0].contains(px.x)
+            && holds.contains(
-            && holds[1].contains(px.y);
+                px,
+                UiRegion {
+                    x: placement[0].unwrap_or(UiSpan::FULL),
+                    y: placement[1].unwrap_or(UiSpan::FULL),
+                },
+            );
         valid.then_some((size, holds))
     }
 
@@ -551,7 +582,7 @@ impl UiRenderState {
     /// drawing ended up. Alignment is exactly that case: the first box is the
     /// question and the smaller placed box holds the drawing. Whether the
     /// answer is stale at all is its caller's question, asked once there.
-    fn retained_answer(&self, id: WidgetId, info: DrawInfo) -> Option<(Size, [Holds; 2])> {
+    fn retained_answer(&self, id: WidgetId, info: DrawInfo) -> Option<(Size, LayoutHolds)> {
         let active = self.active.get(&id)?;
         let has_region_node = active.move_idx != active.parent_move;
         if !active.drawn
@@ -560,7 +591,11 @@ impl UiRenderState {
         {
             return None;
         }
-        active.answers_at(info.px).then_some(active.answer)
+        let answer = active.answer?;
+        answer
+            .1
+            .contains(info.px, info.offered_placement())
+            .then_some(answer)
     }
 
     /// The pixel lengths of the box a widget was given and of the box it was
@@ -600,9 +635,10 @@ impl UiRenderState {
         &mut self,
         id: WidgetId,
         region: UiRegion,
+        placement: UiRegion,
         info: DrawInfo,
         rsc: &mut dyn UiRsc,
-    ) -> Option<(Size, [Holds; 2])> {
+    ) -> Option<(Size, LayoutHolds)> {
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::ReuseAttempts);
         if rsc.widgets().needs_redraw.contains(&id) {
@@ -651,7 +687,7 @@ impl UiRenderState {
         // In pixels, because `region` is a fraction of the box its parent
         // drew in and that box may be what changed -- an unchanged fraction
         // of a box half the size is half the widget.
-        if !active.holds_at(info.px) {
+        if !active.holds.contains(info.px, placement) {
             #[cfg(feature = "layout-diagnostics")]
             {
                 diag::bump(Counter::ReuseOutside);
@@ -659,6 +695,7 @@ impl UiRenderState {
             }
             return None;
         }
+        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);
@@ -670,6 +707,9 @@ impl UiRenderState {
                 self.remap_subtree(id, &remap, info.parent_move, rsc);
             }
         }
+        if extent_moved {
+            self.reposition(id, region, placement, info, rsc);
+        }
         self.redepth(id, info.depth);
         let active = self.active.get_mut(&id).unwrap();
         active.region = region;
@@ -698,6 +738,70 @@ impl UiRenderState {
         Some(answer)
     }
 
+    fn reposition(
+        &mut self,
+        id: WidgetId,
+        region: UiRegion,
+        placement: UiRegion,
+        info: DrawInfo,
+        rsc: &mut dyn UiRsc,
+    ) {
+        let active = self.active.get_mut(&id).unwrap();
+        active.region = region;
+        active.placement = placement;
+        let local = if info.region_node {
+            UiRegion::FULL
+        } else {
+            region
+        };
+        for primitive in &active.primitives {
+            let handle = &primitive.handle;
+            *self.layers[handle.layer].region_mut(handle) =
+                primitive.region.resolve(local, placement);
+        }
+        if let Some(mask_region) = active.mask_region {
+            rsc.ui_mut().masks.get_mut(active.mask).region = mask_region.resolve(local, placement);
+        }
+        let parent_move = active.move_idx;
+        let mask = active.mask;
+        let children = active.inherited_children.len();
+        for index in 0..children {
+            let child = self.active[&id].inherited_children[index];
+            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_placement = UiRegion {
+                x: chosen[0].unwrap_or(UiSpan::FULL),
+                y: chosen[1].unwrap_or(UiSpan::FULL),
+            };
+            let child_info = DrawInfo {
+                layer: active.layer,
+                parent: Some(id),
+                depth: info.depth + 1,
+                parent_move,
+                region_node: active.move_idx != active.parent_move,
+                mask,
+                given_len: child_local.size(),
+                offer_len: active.offer_len,
+                offer_placement: active.offer_placement,
+                px: child_local.size().to_px(info.px),
+                offered_px: active.offer_len.to_px(info.offered_px),
+                placement: chosen,
+            };
+            self.place(
+                child,
+                child_local.within(&local),
+                child_placement,
+                child_info,
+                rsc,
+            );
+        }
+    }
+
     /// A reused subtree keeps its shape, so every widget in it moves by the
     /// same amount -- and where the top of it did not move, none of it did,
     /// which is what makes this free in the ordinary case.
@@ -733,11 +837,12 @@ impl UiRenderState {
             self.moves.set(active.move_idx, region);
             return;
         }
-        for handle in &active.primitives {
-            let region = self.layers[handle.layer].region_mut(handle);
-            *region = remap.apply(*region);
-        }
         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);
+        }
         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
@@ -745,7 +850,10 @@ impl UiRenderState {
         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 = remap.apply(mask.region);
+            mask.region = active
+                .mask_region
+                .unwrap()
+                .resolve(active.region, active.placement);
         }
         for index in 0..children {
             let child = self.active[&id].children[index];
@@ -768,8 +876,8 @@ impl UiRenderState {
     fn remove(&mut self, id: WidgetId, undraw: bool, rsc: &mut dyn UiRsc) -> Option<ActiveData> {
         let mut active = self.active.remove(&id);
         if let Some(active) = &mut active {
-            for h in &active.primitives {
+            for primitive in &active.primitives {
-                let mask = self.layers.free(h);
+                let mask = self.layers.free(&primitive.handle);
                 if mask != MaskIdx::NONE {
                     rsc.ui_mut().masks.remove(mask);
                 }
@@ -825,17 +933,19 @@ impl UiRenderState {
                 id,
                 region: UiRegion::FULL,
                 placement: UiRegion::FULL,
-                reads_placement: false,
                 given_len: UiVec2::FULL_SIZE,
                 offer_len: UiVec2::FULL_SIZE,
-                answer: (size, [Holds::ANY; 2]),
+                offer_placement: [None; 2],
+                answer: None,
                 size,
-                holds: [Holds::ANY; 2],
+                holds: LayoutHolds::ANY,
                 drawn: false,
                 parent: info.parent,
                 depth: info.depth,
                 textures: Vec::new(),
                 primitives: Vec::new(),
+                mask_region: None,
+                inherited_children: Vec::new(),
                 children: Vec::new(),
                 size_deps: Vec::new(),
                 move_idx: info.parent_move,
@@ -1007,7 +1117,7 @@ impl UiRenderState {
         let declared_changed = declared_lens(rsc.widgets(), id) != active.declared;
         let alignment_changed = rsc.widgets().alignment(id) != active.own_align;
         if let Some(parent) = active.parent
-            && (declared_changed || alignment_changed || !active.drawn)
+            && (declared_changed || alignment_changed || !active.drawn || active.answer.is_none())
         {
             if declared_changed {
                 self.replace_answers = true;
@@ -1063,6 +1173,7 @@ impl UiRenderState {
             mask: active.parent_mask,
             given_len: active.given_len,
             offer_len: active.offer_len,
+            offer_placement: active.offer_placement,
             px: given_px,
             offered_px,
             // The same question its parent asked: the axes its parent chose
@@ -1075,10 +1186,17 @@ impl UiRenderState {
         diag::bump(Counter::LocalRedraws);
 
         let old = self.remove(id, false, rsc);
-        // `draw_inner` places the answer inside that box itself, which is the
+        // Refresh the original measurement before restoring the assigned slot.
-        // ask that leaves the widget where its parent put it.
+        // Its lengths may differ even though the fraction reference is unchanged.
-        let answer = self.draw_inner(id, given, info, old, rsc);
+        let offered = DrawInfo {
-        if answer != was_answer {
+            placement: info.offer_placement,
+            ..info
+        };
+        let answer = self.draw_inner(id, given, offered, old, rsc);
+        if info.placement != offered.placement {
+            self.draw_inner(id, given, info, None, 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.
             #[cfg(feature = "layout-diagnostics")]

src/widget/mask.rs

@@ -6,8 +6,7 @@ pub struct Masked {
 
 impl Widget for Masked {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let own = painter.placement();
+        painter.set_mask(DrawRegion::Extent(UiRegion::FULL));
-        painter.set_mask(own);
         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/scroll.rs

@@ -15,7 +15,9 @@ impl Widget for Scroll {
         // Draw in the whole container only when its scrolling-axis length is
         // not already known, then draw it at the scrolled offset.
         let whole = UiRegion::FULL;
-        let answer_len = match painter.known_len(&self.inner, self.axis, whole) {
+        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),
             };

src/widget/position/span.rs

@@ -37,7 +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) {
+            let len = match painter.known_len(child, axis, region, room) {
                 Some(len) => len,
                 None => painter.widget_at(child, region, room).len(axis),
             };

src/widget/text/mod.rs

@@ -89,8 +89,7 @@ impl TextView {
         // 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));
-        let within = region.within(&painter.placement());
+        painter.glyphs(tex, DrawRegion::Extent(region));
-        painter.glyphs(tex, within);
         (region, size)
     }
 

tests/cases/layout.rs

@@ -699,3 +699,20 @@ 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));
+}

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,
+    // Only the available length changes: positioning the final slot does
-    // and it reports less than that box: so it is drawn again in the box its
+    // not invalidate a numeric size read.
-    // answer places it in, and once more in the final box the span chooses.
+    assert_eq!(asked_draws.get(), 2);
-    // 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"
-    );
 }
 
 #[test]
@@ -737,3 +730,81 @@ fn a_subtree_that_changed_parents_settles_at_the_depth_it_moved_to() {
         "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())
+    );
+}

tests/cases/unsettled.rs

@@ -614,3 +614,61 @@ fn a_text_is_given_back_a_box_the_line_it_measured_fits_in() {
 
     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));
+}