Keep the step 3/4 experiment as evidence

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

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

core/src/layout_diagnostics.rs

@@ -54,7 +54,7 @@ pub(crate) enum Counter {
     TextShapes,
     TextBreaks,
     GlyphPlacements,
-    OutsidePlacement,
+    OutsidePinnedLen,
     OutsideFrame,
     OutsideExtent,
 }
@@ -92,7 +92,7 @@ impl Counter {
         "text shapes",
         "text line breaks",
         "glyph placements",
-        "reuse outside: the placement it was pinned to",
+        "reuse outside: the length it was pinned to",
         "reuse outside: a frame length",
         "reuse outside: an extent length",
     ];

core/src/ui/active.rs

@@ -17,7 +17,7 @@ pub struct ActiveData {
     pub extent: UiRegion,
     /// That frame in its parent's frame coordinates, before composition:
     /// forwarded whole by a transparent container, narrowed by a declared
-    /// length or an inset. Its length is the same on every ask, which is what
+    /// length. Its length is the same on every ask, which is what
     /// a local redraw relies on to ask its parent's own question again.
     pub frame: UiRegion,
     /// What of its parent's extent the drawing was given, and what it was
@@ -77,6 +77,14 @@ pub struct ActiveData {
 }
 
 impl ActiveData {
+    /// What it answered when its parent measured it, where it has been
+    /// measured at all. Not `size`, which is what its last drawing reported:
+    /// a drawing made in the box that answer chose is answering a different
+    /// question.
+    pub fn measured(&self) -> Option<Size> {
+        self.answer.map(|(size, _)| size)
+    }
+
     /// Whether what it answered still stands for a frame of these pixel
     /// lengths. The answer was given in the box its parent first asked
     /// about, which is what it is checked against -- `holds` on the record

core/src/ui/layout_holds.rs

@@ -9,8 +9,9 @@ const AXES: [Axis; 2] = [Axis::X, Axis::Y];
 /// The symbolic length is a pin rather than a range: a container places its
 /// children as lengths of its frame measured from where its own box starts,
 /// so what it draws turns on that box's length and on nothing about where it
-/// is. It does not compose into the parent -- a widget pinned this way is
+/// is. It reaches the parent only where the box it pinned is the parent's
-/// checked when it is re-placed.
+/// own; anywhere else the parent chose that length itself, and a widget
+/// pinned this way is checked when it is re-placed.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct LayoutHolds {
     pub frame: [Holds; 2],

core/src/ui/painter.rs

@@ -20,7 +20,7 @@ pub struct Painter<'a> {
     /// What a fraction this widget declares or reports is a fraction of, in
     /// the coordinates of `move_idx`: forwarded from its parent unchanged
     /// through a span, a stack or a scroll, and narrowed only by what was
-    /// decided above it -- a declared length, an inset, the root. Its length
+    /// decided above it -- a declared length, or the root. Its length
     /// is the same on every ask of the widget, which is what keeps a fraction
     /// under it from being resolved twice.
     pub(super) frame: UiRegion,
@@ -32,6 +32,10 @@ pub struct Painter<'a> {
     /// drawing one that holds for that length alone -- the way reading a
     /// length in pixels makes it hold for that number of pixels.
     pub(super) extent_len: [Option<Len>; 2],
+    /// Symbolic box lengths read only to compute the answer. A container can
+    /// replace the provisional drawings used for that answer with drawings
+    /// in decided boxes, so this contract is independent of the final one.
+    pub(super) answer_extent_len: [Option<Len>; 2],
     /// The frame in pixels, which its children's frames are a length of:
     /// threaded down rather than composed back up the chain, so every length
     /// in layout is one multiply from its parent's and [`Holds::through`]
@@ -55,11 +59,15 @@ pub struct Painter<'a> {
     pub(super) size_deps: Vec<WidgetId>,
     /// What this draw itself read of its frame in pixels, per axis: every
     /// length until it reads one, then that one, unless it says otherwise.
-    pub(super) own: [Holds; 2],
+    pub(super) frame_own: [Holds; 2],
-    /// The same for its extent.
+    /// The same for its own box.
     pub(super) extent_own: [Holds; 2],
-    /// Dependencies of every child drawing, including unmeasured overlays.
+    /// Pixel-box dependencies used only to compute the answer. These do not
-    pub(super) under: LayoutHolds,
+    /// constrain a retained drawing placed inside that answer.
+    pub(super) answer_extent_own: [Holds; 2],
+    /// The final drawing kept for each child. Asking one child again replaces
+    /// its provisional drawing and therefore replaces this contract too.
+    pub(super) under: Vec<(WidgetId, LayoutHolds)>,
     /// The movable region this widget's primitives are positioned through:
     /// its own when opted in, otherwise the nearest ancestor's.
     pub(super) move_idx: MoveIdx,
@@ -161,8 +169,8 @@ impl<'a> Painter<'a> {
     /// [`UiRegion::FULL`] forwards this widget's frame, which is what a
     /// container that only divides room passes, so a fraction under it means
     /// the same wherever it sits and however deeply it is nested. Narrowing
-    /// it is for what is decided from above -- an inset's margins -- and a
+    /// it is for what is decided from above, and a declared length narrows
-    /// declared length narrows it here.
+    /// it here.
     ///
     /// `place` is where the drawing goes, per axis, as a part of this
     /// widget's extent: see [`Place`]. A narrowed frame is its own extent,
@@ -176,14 +184,12 @@ impl<'a> Painter<'a> {
         let region_node = self.rsc.widgets().is_region_node(id.id());
         let declared = self.declared_lens(id);
         let align = self.rsc.widgets().alignment(id.id());
-        let narrow = AXES.map(|axis| {
+        let (local, extent) = frame_and_extent(
-            let n = axis as usize;
+            frame,
-            // A rule's fraction is a fraction of the frame the child was
+            part_of(self.extent, place),
-            // given, which is the one length the rule can mean.
+            narrowed_by(declared, frame),
-            declared[n]
+            align,
-                .map(|len| Len::from_parts(len.rel, len.px).within_len(frame.axis(axis).len()))
+        );
-        });
-        let (local, extent) = frame_and_extent(frame, part_of(self.extent, place), narrow, align);
         let within = match local == UiRegion::FULL {
             true => self.frame,
             false => local.within(&self.frame),
@@ -224,17 +230,17 @@ impl<'a> Painter<'a> {
                 part: extent,
                 place,
                 offer_place,
-                // The question its parent measured it by, asked again: the
-                // same widget in the same place, however this draw came
-                // about.
-                offer: place == offer_place,
                 px,
             },
             None,
             self.rsc,
         );
         let compose = |holds| in_parent(holds, local, extent, place, declared);
-        self.under = self.under.and(compose(holds));
+        let holds = compose(holds);
+        match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
+            Some((_, kept)) => *kept = holds,
+            None => self.under.push((id.id(), holds)),
+        }
         DrawResult {
             child: id,
             painter: self,
@@ -248,6 +254,7 @@ 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.under.retain(|(child, _)| *child != id.id());
         self.state.undraw_rec(id.id(), self.rsc);
     }
 
@@ -362,9 +369,31 @@ impl<'a> Painter<'a> {
     pub fn extent_len(&mut self, axis: Axis) -> Len {
         let len = self.extent.axis(axis).len();
         self.extent_len[axis as usize] = Some(len);
+        self.answer_extent_len[axis as usize] = Some(len);
         len
     }
 
+    /// The symbolic length used to compute this widget's answer, where the
+    /// final drawing itself is rebuilt without depending on that length.
+    pub fn answer_extent_len(&mut self, axis: Axis) -> Len {
+        let len = self.extent.axis(axis).len();
+        self.answer_extent_len[axis as usize] = Some(len);
+        len
+    }
+
+    /// Says that the final drawing uses a symbolic length already read for
+    /// the answer.
+    pub fn drawing_uses_extent_len(&mut self, axis: Axis, len: Len) {
+        debug_assert_eq!(self.extent.axis(axis).len(), len);
+        self.extent_len[axis as usize] = Some(len);
+    }
+
+    /// A part of this widget's box, expressed in its frame coordinates so it
+    /// can be used as a child frame decided here.
+    pub fn extent_part(&self, axis: Axis, part: Part) -> UiSpan {
+        part.of(*self.extent.axis(axis))
+    }
+
     /// Where this widget sits in a box longer than the length it takes. A
     /// widget that positions its own content reads it to place that content
     /// the way the box around it would have placed the widget.
@@ -396,6 +425,14 @@ impl<'a> Painter<'a> {
         PxVec2::new(self.px_len(Axis::X), self.px_len(Axis::Y))
     }
 
+    /// This widget's own box in pixels, used only to compute its answer.
+    pub fn answer_px_size(&mut self) -> PxVec2 {
+        PxVec2::new(
+            self.answer_px_len(Axis::X),
+            self.answer_px_len(Axis::Y),
+        )
+    }
+
     /// One axis of this widget's own box in pixels. Prefer this to
     /// [`Self::px_size`] when the other axis cannot affect the drawing.
     pub fn px_len(&mut self, axis: Axis) -> Px {
@@ -408,6 +445,17 @@ impl<'a> Painter<'a> {
         len
     }
 
+    /// One pixel length used only to compute this widget's answer. The final
+    /// drawing may be retained when that answer is placed in another box.
+    pub fn answer_px_len(&mut self, axis: Axis) -> Px {
+        let len = self.extent.axis(axis).len().to_px(self.px.axis(axis));
+        let own = &mut self.answer_extent_own[axis as usize];
+        if *own == Holds::ANY {
+            *own = Holds::at(len);
+        }
+        len
+    }
+
     /// The lengths of this widget's own box on `axis` that what it is drawing
     /// holds for -- the same primitives, in the same fractions and offsets
     /// of the box, and the same reported size. A widget that read its length
@@ -429,7 +477,7 @@ impl<'a> Painter<'a> {
     /// children. Its own box is a part of this one.
     pub fn frame_px_len(&mut self, axis: Axis) -> Px {
         let len = self.px.axis(axis);
-        let own = &mut self.own[axis as usize];
+        let own = &mut self.frame_own[axis as usize];
         if *own == Holds::ANY {
             *own = Holds::at(len);
         }
@@ -447,7 +495,7 @@ impl<'a> Painter<'a> {
             self.label(),
             self.id
         );
-        self.own[axis as usize] = holds;
+        self.frame_own[axis as usize] = holds;
     }
 
     pub fn text_data(&mut self) -> &mut TextData {
@@ -561,7 +609,7 @@ pub(crate) fn in_parent(
             // own box, which is what lets that box move without a redraw. A
             // length it pinned is this widget's length wherever the part is
             // the whole of it, and pins the same way.
-            (Part::All, None) => {
+            (Part::All, None) if *frame.axis(axis) == UiSpan::FULL => {
                 result.extent[n] = holds.extent[n];
                 result.extent_len[n] = holds.extent_len[n];
             }
@@ -663,10 +711,8 @@ pub(crate) fn placed_extent(
     placed
 }
 
-/// The part of a widget's extent a `place` names, in the coordinates its
+/// The part of a widget's own box a `place` names, in the coordinates that
-/// extent is in: a span is measured in frame lengths from where the extent
+/// box is in.
-/// starts, so nothing under it depends on where that is, and an extent that
-/// moved re-places every child by re-adding its start.
 pub(crate) fn part_of(extent: UiRegion, place: [Place; 2]) -> UiRegion {
     let mut part = extent;
     for axis in AXES {
@@ -675,6 +721,16 @@ pub(crate) fn part_of(extent: UiRegion, place: [Place; 2]) -> UiRegion {
     part
 }
 
+/// The length a rule gives a child's frame, per axis: a fraction in it is a
+/// fraction of the frame the child was given, which is the one length the
+/// rule can mean.
+pub(crate) fn narrowed_by(declared: [Option<LayoutLen>; 2], frame: UiRegion) -> [Option<Len>; 2] {
+    AXES.map(|axis| {
+        declared[axis as usize]
+            .map(|len| Len::from_parts(len.rel, len.px).within_len(frame.axis(axis).len()))
+    })
+}
+
 /// The frame a child is asked in and the box its drawing goes in, both in
 /// the coordinates of the widget asking.
 ///

core/src/ui/render_state.rs

@@ -1,9 +1,9 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
-use crate::ui::painter::{declared_lens, frame_and_extent, part_of, placed_extent};
+use crate::ui::painter::{declared_lens, frame_and_extent, narrowed_by, part_of, placed_extent};
 use crate::{
-    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx,
+    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, Moves,
-    Moves, Painter, Part, PixelRegion, Place, PxVec2, Size, StrongWidget, UiRegion, UiRsc, Weight,
+    Painter, Part, PixelRegion, Place, PxVec2, Size, StrongWidget, UiRegion, UiRsc, Weight,
     WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
@@ -34,9 +34,6 @@ pub(super) struct DrawInfo {
     /// given at the parent's first ask of it. See [`Place`].
     pub place: [Place; 2],
     pub offer_place: [Place; 2],
-    /// Whether this ask is the one the widget's answer is kept from: the
-    /// first box its parent asked about, in the parent's own measuring draw.
-    pub offer: bool,
     /// The frame in pixels: one multiply from the parent's own, which is
     /// where every pixel length in layout comes from.
     pub px: PxVec2,
@@ -48,6 +45,20 @@ impl DrawInfo {
     fn fill(&self) -> [bool; 2] {
         self.place.map(Place::fills)
     }
+
+    /// Whether this ask is the one the widget's answer is kept from: the
+    /// same widget in the place its parent measured it by, however this draw
+    /// came about.
+    ///
+    /// **Open.** A place is a length from where the asking widget's own box
+    /// starts, so two drawings of that widget -- one in the box its parent
+    /// measured it in, one in the box its own answer chose -- ask their
+    /// children in the same places and different boxes, and this cannot tell
+    /// them apart. Shrinker seeds 2 (`repaint`) and 108 (`reorder`) at depth
+    /// 5 are where that shows.
+    fn offer(&self) -> bool {
+        self.place == self.offer_place
+    }
 }
 
 /// What a widget's children are placed in: its own box, the coordinates its
@@ -153,7 +164,6 @@ impl UiRenderState {
             part: UiRegion::FULL,
             place: [Place::Within(Part::All); 2],
             offer_place: [Place::Within(Part::All); 2],
-            offer: true,
             px,
         }
     }
@@ -209,9 +219,7 @@ impl UiRenderState {
     /// The root's frame: the window, narrowed by the root's own rules. Its
     /// extent is that frame, since nothing above it chose anything else.
     fn root_region(id: WidgetId, widgets: &Widgets) -> UiRegion {
-        let declared = declared_lens(widgets, id);
+        let narrow = narrowed_by(declared_lens(widgets, id), UiRegion::FULL);
-        let narrow =
-            AXES.map(|axis| declared[axis as usize].map(|len| Len::from_parts(len.rel, len.px)));
         frame_and_extent(
             UiRegion::FULL,
             UiRegion::FULL,
@@ -243,7 +251,7 @@ impl UiRenderState {
             true => None,
             false => self
                 .retained_answer(id, part, info)
-                .or_else(|| self.try_reuse(id, frame, part, info, rsc)),
+                .and_then(|answer| self.try_reuse(id, frame, part, info, rsc).map(|_| answer)),
         };
         let answer = retained.unwrap_or_else(|| {
             if old.is_none() {
@@ -257,49 +265,25 @@ impl UiRenderState {
         // axis the parent left open. The frame itself does not change, so
         // nothing under it resolves a fraction a second time.
         //
-        // From the answer it gave when its parent first asked, and not from
-        // what a placing evaluation reported: a drawing made in the box that
-        // answer chose is answering a different question, and placing it by
-        // that would move the box out from under itself.
-        let measured = match info.offer {
-            true => answer.0,
-            false => self.active[&id].answer.map_or(answer.0, |(size, _)| size),
-        };
         let extent = placed_extent(
             part,
-            measured,
+            answer.0,
             declared_lens(rsc.widgets(), id),
             info.fill(),
             align,
         );
         self.place(id, extent, info, rsc);
 
-        // On axes the parent filled, measurement and drawing share an extent.
-        // Otherwise the answer fixes the final extent as a function of the
-        // frame, so pull that drawing's validity back through it.
         let drawing_holds = self.active[&id].holds;
-        let mut settled = answer;
-        for axis in AXES {
-            let n = axis as usize;
-            settled.1.frame[n] = settled.1.frame[n].and(drawing_holds.frame[n]);
-            if info.fill()[n] {
-                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(extent.axis(axis).len()));
-            }
-        }
         let active = self.active.get_mut(&id).unwrap();
         // Whoever asked owns how the boxes were reached: the frame it stated,
         // and what of its own box it gave the drawing. A local redraw asks
         // the same question again from these.
         active.frame_abs = frame;
         active.frame = info.frame;
-        if info.offer {
         active.answer = Some(answer);
         active.offer_place = info.offer_place;
         active.offer_part = part;
-        }
         active.place = info.place;
         active.own_align = align;
         // A subtree can be reused whole under a different parent -- same box,
@@ -314,7 +298,7 @@ impl UiRenderState {
         {
             old_parent.children.retain(|child| *child != id);
         }
-        (answer.0, answer.1, settled.1)
+        (answer.0, answer.1, drawing_holds)
     }
 
     /// Recompose retained geometry when the evaluation still holds at this extent.
@@ -325,6 +309,15 @@ impl UiRenderState {
         {
             return;
         }
+        assert_eq!(
+            extent,
+            info.part,
+            "'{}' ({id:?}) does not hold for the box its answer chose: part {:?}, wanted {extent:?}, retained {:?} with {:?}",
+            rsc.widgets().label(id),
+            info.part,
+            self.active.get(&id).map(|active| active.extent),
+            self.active.get(&id).map(|active| active.holds),
+        );
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::PlaceRedraws);
         let old = self.remove(id, false, rsc);
@@ -361,13 +354,14 @@ impl UiRenderState {
         // Only evaluation at the original offer establishes the children's
         // offers. A placing evaluation must not overwrite that question.
         let px = info.px;
-        let at_offer = info.offer;
+        let at_offer = info.offer();
 
         let mut painter = Painter {
             state: self,
             frame: local,
             extent,
             extent_len: [None; 2],
+            answer_extent_len: [None; 2],
             px,
             mask: info.mask,
             layer: info.layer,
@@ -380,9 +374,10 @@ impl UiRenderState {
             offered: Vec::new(),
             at_offer,
             size_deps: Vec::new(),
-            own: [Holds::ANY; 2],
+            frame_own: [Holds::ANY; 2],
-            under: LayoutHolds::ANY,
+            under: Vec::new(),
             extent_own: [Holds::ANY; 2],
+            answer_extent_own: [Holds::ANY; 2],
             answer_under: LayoutHolds::ANY,
             depth: info.depth,
             move_idx,
@@ -411,13 +406,15 @@ impl UiRenderState {
             primitives,
             mask_region,
             extent_own,
+            answer_extent_own,
             extent_len,
+            answer_extent_len,
             answer_under,
             children,
             offered: _,
             at_offer: _,
             size_deps,
-            own,
+            frame_own,
             under,
             move_idx,
             layer,
@@ -457,13 +454,23 @@ impl UiRenderState {
         if let Some(idx) = retired_move {
             self.moves.remove(idx);
         }
-        let own_holds = LayoutHolds {
+        let drawing_own = LayoutHolds {
-            frame: own,
+            frame: frame_own,
             extent: extent_own,
             extent_len,
         };
-        let answer_holds = own_holds.and(answer_under);
+        let answer_own = LayoutHolds {
-        let holds = answer_holds.and(under);
+            extent: [
+                drawing_own.extent[0].and(answer_extent_own[0]),
+                drawing_own.extent[1].and(answer_extent_own[1]),
+            ],
+            extent_len: answer_extent_len,
+            ..drawing_own
+        };
+        let answer_holds = answer_own.and(answer_under);
+        let holds = under
+            .into_iter()
+            .fold(drawing_own, |holds, (_, child)| holds.and(child));
         debug_assert!(
             holds.contains(px, extent),
             "'{}' ({id:?}) drew in {px:?}, outside the ranges it reported: {holds:?}",
@@ -488,7 +495,6 @@ impl UiRenderState {
                         part: UiRegion::FULL,
                         place: [Place::Within(Part::All); 2],
                         offer_place: [Place::Within(Part::All); 2],
-                        offer: false,
                         px,
                     },
                     rsc,
@@ -659,7 +665,7 @@ impl UiRenderState {
                 for axis in AXES {
                     let n = axis as usize;
                     if holds.extent_len[n].is_some_and(|pinned| pinned != extent.axis(axis).len()) {
-                        diag::bump(Counter::OutsidePlacement);
+                        diag::bump(Counter::OutsidePinnedLen);
                     }
                     if !holds.frame[n].contains(info.px.axis(axis)) {
                         diag::bump(Counter::OutsideFrame);
@@ -721,13 +727,9 @@ impl UiRenderState {
     fn place_child(&mut self, child: WidgetId, at: &Placing, rsc: &mut dyn UiRsc) {
         let active = &self.active[&child];
         let (frame, part) = Self::re_ask(active, at.extent, active.place);
-        // The answer it gave, and not what its last drawing reported: a
-        // drawing made in the box that answer chose is answering a different
-        // question.
-        let answer = active.answer.map_or(active.size, |(size, _)| size);
         let extent = placed_extent(
             part,
-            answer,
+            active.measured().unwrap_or(active.size),
             active.declared,
             active.place.map(Place::fills),
             active.own_align,
@@ -744,10 +746,6 @@ impl UiRenderState {
             part,
             place: active.place,
             offer_place: active.offer_place,
-            // Putting it back where it was asked about is that ask again, so
-            // what it answers there is the answer -- and putting it anywhere
-            // else is not, however the box was arrived at.
-            offer: active.place == active.offer_place,
             px: frame.size().to_px(at.px),
         };
         self.place(child, extent, info, rsc);
@@ -1180,10 +1178,10 @@ impl UiRenderState {
         };
         let px = self.asked_px(id);
         let (was_answer, was_holds) = (active.answer, active.holds);
-        // The boxes its parent gave it, then and now: its frame is the same
+        let needs_replacement = active.place != active.offer_place;
-        // on every ask, so the question its parent asked is the one this
+        // Re-ask the question its answer came from, not a later placement of
-        // asks again -- there is no box here that could be its parent's to
+        // that answer. A parent may move the retained drawing into a box the
-        // choose instead.
+        // answer chose without making that box a new question.
         let parent_extent = self.active[&parent].extent;
         let info = DrawInfo {
             layer: active.layer,
@@ -1194,51 +1192,16 @@ impl UiRenderState {
             mask: active.parent_mask,
             frame: active.frame,
             frame_abs: active.frame_abs,
-            part: Self::re_ask(active, parent_extent, active.place).1,
+            part: Self::re_ask(active, parent_extent, active.offer_place).1,
-            place: active.place,
+            place: active.offer_place,
             offer_place: active.offer_place,
-            offer: false,
             px,
         };
-        // The ask that measured it, asked again: the box it was measured in
-        // as its parent left it, rather than where that ask's place resolves
-        // to now -- a parent drawn again in the box its own answer chose
-        // gives its children boxes it never measured anything in.
-        let offered = DrawInfo {
-            place: info.offer_place,
-            part: active.offer_part,
-            offer: true,
-            ..info
-        };
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::LocalRedraws);
 
-        // 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 one it was
-        // measured in. 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.
-        //
-        // The frame is the same on every ask now, so this is about the box
-        // the drawing goes in alone. Removing it -- asking the measuring
-        // question here and placing the answer afterwards -- is what the
-        // transparent-frames plan asks for next, and it does not hold yet:
-        // seeds 104 (align) and 210 (reorder) at depth 5 settle differently
-        // warm and cold without it.
-        if info.part.size() != offered.part.size() {
-            self.mark(id, rsc.widgets_mut());
-            self.mark(parent, rsc.widgets_mut());
-            return false;
-        }
         let old = self.remove(id, false, rsc);
-        // The original measurement is refreshed before the assigned slot is
+        let answer = self.draw_inner(id, info, old, rsc);
-        // restored: its lengths may differ even though the frame is
-        // unchanged.
-        let answer = self.draw_inner(id, offered, old, rsc);
-        if info.place != info.offer_place || info.part != offered.part {
-            self.draw_inner(id, info, None, rsc);
-        }
         let active = self.active.get_mut(&id).unwrap();
         // A wider contract does not invalidate the guarantee the parent kept.
         // Retain that guarantee so widening and narrowing back do not churn it.
@@ -1251,7 +1214,7 @@ impl UiRenderState {
         if active.holds.covers(was_holds) && was_holds.contains(px, active.extent) {
             active.holds = was_holds;
         }
-        if active.answer != was_answer || active.holds != was_holds {
+        if needs_replacement || 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")]

src/random.rs

@@ -124,7 +124,7 @@ impl Widget for Branch {
             .len(Axis::X);
         let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
 
-        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
+        let below = Place::Within(Part::Of(UiSpan::new(cut, Len::FULL)));
         let place = [Place::Within(Part::All), below];
         match px > Px::from_f32(self.threshold) {
             true => painter.widget_at(&self.wide, UiRegion::FULL, place),

src/widget/position/pad.rs

@@ -16,12 +16,10 @@ impl Widget for Pad {
         //
         // The padding goes around what it pads: the frame passes through, so
         // the inner's fractions mean what they would without it, and only
-        // the box it draws in is moved in by the pixels.
+        // the box it draws in is moved in by the pixels. Said as a part of
-        // A part of this widget's own box, in that box's own lengths: the
+        // this widget's own box in that box's own lengths, so nothing here
-        // padding is pixels, which are the same pixels wherever the box
+        // reads how long the box is -- and a box chosen from this widget's
-        // lands, so nothing here reads how long the box is -- and a box
+        // own answer therefore does not feed back into that answer.
-        // chosen from this widget's own answer therefore does not feed back
-        // into that answer.
         let inset = |lead: Px, trail: Px| {
             Place::Within(Part::Of(UiSpan::new(
                 Len::from_parts(Rel::ZERO, lead),

src/widget/position/scroll.rs

@@ -17,9 +17,9 @@ impl Widget for Scroll {
         let answer_len = painter
             .widget_at(&self.inner, whole, [Place::Fill(Part::All); 2])
             .len(self.axis);
-        let content = answer_len.apply_leftover();
+        let fixed = Len::from_parts(answer_len.rel, answer_len.px).to_px(container_len);
         self.container_len = container_len;
-        self.content_len = content.to_px(container_len);
+        self.content_len = fixed.max(container_len);
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
@@ -32,9 +32,9 @@ impl Widget for Scroll {
         // the drawing holds for that length alone. One scrolled part way sits
         // where it is until the box shrinks past what is left of it. Kept to
         // the end, it moves with every length.
-        let fixed_len = content.rel == Rel::ZERO;
+        let fixed_len = answer_len.rel == Rel::ZERO && answer_len.leftover == Weight::ZERO;
         if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
-            painter.holds(self.axis, self.content_len..=Px::MAX);
+            painter.holds(self.axis, fixed..=Px::MAX);
         } else if fixed_len && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, Px::MIN..=left);

src/widget/position/span.rs

@@ -13,8 +13,8 @@ impl Widget for Span {
         // The row: this span's own box, as a length of the frame its children
         // are laid out against. Its start is nothing's business -- a slot is
         // a length from it -- so what this reads is the length alone.
-        let far = painter.extent_len(axis);
+        let far = painter.answer_extent_len(axis);
-        let along = |from: Len, to: Len| match self.dir.sign {
+        let measure_along = |from: Len, to: Len| match self.dir.sign {
             Sign::Pos => UiSpan::new(from, to),
             Sign::Neg => UiSpan::new(far - to, far - from),
         };
@@ -30,7 +30,7 @@ impl Widget for Span {
         let mut cursor = Len::rel_min();
         let mut lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let room = Place::Within(Part::From(along(cursor, far)));
+            let room = Place::Fill(Part::From(measure_along(cursor, far)));
             let len = painter
                 .widget_at(child, UiRegion::FULL, axis.pair(room, across))
                 .len(axis);
@@ -50,9 +50,10 @@ impl Widget for Span {
             |sum, len| sum + *len,
         );
 
+        let fixed_total = Len::from_parts(total.rel, total.px);
         // What is left for the shares to divide: the row less everything
         // fixed, as a length of the frame rather than a number of pixels.
-        let room = far - Len::from_parts(total.rel, total.px);
+        let room = far - fixed_total;
         // Whether anything is left over is a question in pixels: `rel(0.5)`
         // beside 300 px is full at 600 and overfull at 400. Asked of `room`
         // itself, and answered back through the same expression, so the
@@ -72,6 +73,18 @@ impl Widget for Span {
             painter.frame_holds(axis, holds.through(room));
         }
 
+        if shares {
+            painter.drawing_uses_extent_len(axis, far);
+        }
+        let drawing_far = match shares {
+            true => far,
+            false => fixed_total,
+        };
+        let along = |from: Len, to: Len| match self.dir.sign {
+            Sign::Pos => UiSpan::new(from, to),
+            Sign::Neg => UiSpan::new(drawing_far - to, drawing_far - from),
+        };
+
         // Across itself a span is as long as its longest child -- unless a
         // rule beside it gives that length outright, and then reading them
         // answers nothing and makes its size depend on theirs for it. A rule
@@ -108,8 +121,19 @@ impl Widget for Span {
             // Along the row the span says where the child goes, and that slot
             // is the drawing's box outright rather than something to place an
             // answer inside again.
-            let slot = Place::Fill(Part::From(along(from, start)));
+            let span = along(from, start);
-            let placed = painter.widget_at(child, UiRegion::FULL, axis.pair(slot, across));
+            let (frame, slot) = match len.leftover > Weight::ZERO && shares {
+                true => (
+                    UiRegion::from_axis(
+                        axis,
+                        painter.extent_part(axis, Part::From(span)),
+                        UiSpan::FULL,
+                    ),
+                    Place::Fill(Part::All),
+                ),
+                false => (UiRegion::FULL, Place::Fill(Part::From(span))),
+            };
+            let placed = painter.widget_at(child, frame, axis.pair(slot, across));
             if shrinks {
                 let used = placed.len(!axis);
                 // Choosing between a fixed and a relative length from the

src/widget/position/stack.rs

@@ -33,9 +33,15 @@ impl Widget for Stack {
                 continue;
             }
             painter.child_layer_at(i);
-            // Every other child is drawn in the stack's own box, and where it
+            let place = [Axis::X, Axis::Y].map(|axis| {
-            // sits in one bigger than itself is its own business.
+                let len = size.axis(axis);
-            painter.widget(child);
+                let part = match len.leftover > Weight::ZERO {
+                    true => Part::All,
+                    false => Part::From(UiSpan::new(Len::ZERO, Len::from_parts(len.rel, len.px))),
+                };
+                Place::Within(part)
+            });
+            painter.widget_at(child, UiRegion::FULL, place);
         }
         size
     }

tests/cases/retained.rs

@@ -12,6 +12,7 @@ struct Counted {
     draws: Rc<Cell<usize>>,
     size: Size,
     reads_box: bool,
+    reads_answer_box: bool,
 }
 
 impl Widget for Counted {
@@ -19,6 +20,8 @@ impl Widget for Counted {
         self.draws.set(self.draws.get() + 1);
         if self.reads_box {
             painter.px_size();
+        } else if self.reads_answer_box {
+            painter.answer_px_size();
         }
         self.size
     }
@@ -38,11 +41,18 @@ fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>
         draws: draws.clone(),
         size,
         reads_box,
+        reads_answer_box: false,
     }
     .add(&mut h.rsc);
     (id, Counts(draws))
 }
 
+fn answer_counted(h: &mut Harness, size: Size) -> (WeakWidget<Counted>, Counts) {
+    let (id, draws) = counted(h, size, false);
+    h.rsc[id].reads_answer_box = true;
+    (id, draws)
+}
+
 struct Layered {
     children: [StrongWidget<Rect>; 2],
     _revision: usize,
@@ -156,8 +166,8 @@ 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);
-    // Only the available length changes: positioning the final slot does
+    // Its final slot is a parent decision, so it is evaluated there after
-    // not invalidate a numeric size read.
+    // the provisional ask established its length.
     assert_eq!(asked_draws.get(), 2);
 }
 
@@ -240,7 +250,7 @@ fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
     assert_corners!(h, inner, (0, 0), (400, 120));
 }
 
-/// Reads its box's size, which nothing but its own draw can put right.
+/// Reads its box's size to compute its answer.
 struct ReadsBox {
     draws: Rc<Cell<usize>>,
 }
@@ -248,26 +258,36 @@ struct ReadsBox {
 impl Widget for ReadsBox {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
-        Size::from_px(painter.px_size().div_int(4))
+        Size::from_px(painter.answer_px_size().div_int(4))
     }
 }
 
 /// Reads its box across one axis only, so its drawing holds for a taller
 /// box on its own and only a wider one is worth a draw.
 ///
-/// Both of these report a quarter of what they read, without saying that the
+/// Both report a quarter of what they read. Their empty drawings are
-/// drawing holds there too, so each length they are asked at costs two draws:
+/// independent of that read, so a changed question costs one draw.
-/// one to answer, and one in the quarter-sized box that answer places them
-/// in. The counts below are in those pairs.
 struct ReadsWidth {
     draws: Rc<Cell<usize>>,
 }
 
+struct ReadsDrawingWidth {
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for ReadsDrawingWidth {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.px_len(Axis::X);
+        Size::LEFTOVER
+    }
+}
+
 impl Widget for ReadsWidth {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         self.draws.set(self.draws.get() + 1);
         Size::from_px(PxVec2::new(
-            painter.px_len(Axis::X).div_int(4),
+            painter.answer_px_len(Axis::X).div_int(4),
             Px::from_int(20),
         ))
     }
@@ -323,6 +343,7 @@ fn a_row_moves_what_follows_a_child_that_grew_rather_than_drawing_it() {
             draws: ruled.clone(),
             size: Size::LEFTOVER,
             reads_box: false,
+            reads_answer_box: false,
         };
         let second = match declared {
             true => second.width(rel(0.25)).add(&mut h.rsc),
@@ -384,7 +405,7 @@ fn a_resize_redraws_what_read_its_box() {
     h.resize((800, 100));
     h.frame();
 
-    assert_eq!(draws.get(), settled + 2);
+    assert_eq!(draws.get(), settled + 1);
 }
 
 #[test]
@@ -404,7 +425,7 @@ fn a_resize_only_redraws_read_axes() {
 
     h.resize((800, 300));
     h.frame();
-    assert_eq!(draws.get(), settled + 2, "width changes its answer");
+    assert_eq!(draws.get(), settled + 1, "width changes its answer");
 }
 
 /// A window is measured onto the grid like everything else, so a resize too
@@ -431,7 +452,7 @@ fn a_resize_within_one_step_is_not_a_resize() {
 
     h.resize((400.0 + step, 200.0));
     h.frame();
-    assert_eq!(draws.get(), settled + 2);
+    assert_eq!(draws.get(), settled + 1);
 }
 
 /// The same for a box that changes because a sibling did: what is compared
@@ -500,7 +521,7 @@ fn a_change_two_levels_under_its_reader_still_reaches_it() {
     // Every wrapper up to the outer pad read the size below it, so the outer
     // pad is what draws again -- and the span it hands the box to is the same
     // size as before, which is what lets a draw reuse its way past the leaf.
-    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
+    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), false);
     let padded = leaf.pad(10).add(&mut h.rsc);
     let below = rect(Color::RED).add(&mut h.rsc);
     h.set_root((padded, below).span(Dir::DOWN).pad(12));
@@ -1066,7 +1087,7 @@ fn a_declared_size_change_stops_at_an_independent_parent() {
 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 {
+    let leaf = ReadsDrawingWidth {
         draws: draws.clone(),
     }
     .add(&mut h.rsc);
@@ -1077,7 +1098,7 @@ fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_resize() {
     h.frame();
 
     assert!(draws.get() > settled);
-    assert_corners!(h, leaf, (300, 90), (500, 110));
+    assert_corners!(h, leaf, (0, 0), (800, 200));
 }
 
 #[test]
@@ -1212,7 +1233,7 @@ 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);
+            let width = painter.answer_px_len(Axis::X);
             painter.primitive(RectPrimitive::color(Color::BLUE));
             Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
         }
@@ -1312,7 +1333,10 @@ fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
                 } else {
                     Size::from((80, 27))
                 };
-                let (leaf, _) = counted(h, size, !fractional);
+                let (leaf, _) = match fractional {
+                    true => counted(h, size, false),
+                    false => answer_counted(h, size),
+                };
                 let child = Container {
                     child: leaf.add_strong(&mut h.rsc),
                     region,

tests/cases/scroll.rs

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

tests/cases/unsettled.rs

@@ -14,6 +14,180 @@ use iris::harness::Harness;
 use iris::prelude::*;
 use iris::random::Branch;
 
+fn assert_same_regions(
+    warm: &Harness,
+    warm_ids: &[WidgetId],
+    cold: &Harness,
+    cold_ids: &[WidgetId],
+) {
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in warm_ids.iter().zip(cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Ten widgets, shrunk from seed 2 at depth 5. The stack is as tall as its
+/// first child, so its other children belong in that one-line box. A cold
+/// layout used to keep the span's answer from the larger measuring box while
+/// a repaint asked it in the stack's final box.
+fn plant_stack_in_its_sizing_childs_box(h: &mut Harness) -> Vec<WidgetId> {
+    let sizing = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let filler = rect(Color::CYAN.alpha(252)).add(&mut h.rsc);
+    let plain = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let span = (filler, plain).span(Dir::DOWN).add(&mut h.rsc);
+    let pad = Pad {
+        padding: Padding::ZERO,
+        inner: span.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    let probe = rect(Color::RED).add(&mut h.rsc);
+    let wide = rect(Color::YELLOW.alpha(252)).add(&mut h.rsc);
+    let narrow = rect(Color::RED).add(&mut h.rsc);
+    let branch = Branch {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 55.0,
+    }
+    .add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![
+            sizing.add_strong(&mut h.rsc),
+            pad.add_strong(&mut h.rsc),
+            branch.add_strong(&mut h.rsc),
+        ],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_size_rules(stack.id(), Some(LayoutLen::LEFTOVER), None);
+    h.set_root(stack);
+    vec![
+        sizing.id(),
+        filler.id(),
+        plain.id(),
+        span.id(),
+        pad.id(),
+        probe.id(),
+        wide.id(),
+        narrow.id(),
+        branch.id(),
+        stack.id(),
+    ]
+}
+
+#[test]
+fn repainting_a_stack_uses_the_box_its_sizing_child_decided() {
+    let mut warm = Harness::new((900, 1200));
+    let ids = plant_stack_in_its_sizing_childs_box(&mut warm);
+    for &id in &ids {
+        warm.rsc.widgets_mut().get_dyn_mut(id);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let cold_ids = plant_stack_in_its_sizing_childs_box(&mut cold);
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
+/// Ten widgets, shrunk from seed 108 at depth 5. The nested reverse spans
+/// evaluate the branch in successively narrower boxes. The answer from the
+/// final, decided box must be the one retained after every span is reordered.
+fn plant_branch_in_nested_reverse_spans(
+    h: &mut Harness,
+    reordered: bool,
+) -> (Vec<WidgetId>, [WeakWidget<Span>; 3]) {
+    let pair = |first: StrongWidget, second: StrongWidget| match reordered {
+        true => vec![second, first],
+        false => vec![first, second],
+    };
+    let probe = rect(Color::RED.alpha(63)).add(&mut h.rsc);
+    let wide = rect(Color::RED).add(&mut h.rsc);
+    let narrow = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let branch = Branch {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold: 483.0,
+    }
+    .add(&mut h.rsc);
+    let wrapped = wtext(PARAGRAPH).size(16).wrap(true).add(&mut h.rsc);
+    let down = Span {
+        children: pair(
+            branch.add_strong(&mut h.rsc),
+            wrapped.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::DOWN,
+        gap: Px::ZERO,
+    }
+    .add(&mut h.rsc);
+    let inner_filler = rect(Color::CYAN.alpha(63)).add(&mut h.rsc);
+    let inner = Span {
+        children: pair(
+            down.add_strong(&mut h.rsc),
+            inner_filler.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::LEFT,
+        gap: Px::ZERO,
+    }
+    .height(LayoutLen::rel(1.0))
+    .add(&mut h.rsc);
+    let outer_filler = rect(Color::GREEN.alpha(63)).add(&mut h.rsc);
+    let outer = Span {
+        children: pair(
+            inner.add_strong(&mut h.rsc),
+            outer_filler.add_strong(&mut h.rsc),
+        ),
+        dir: Dir::LEFT,
+        gap: Px::ZERO,
+    }
+    .height(LayoutLen::rel(1.0))
+    .add(&mut h.rsc);
+    h.set_root(outer);
+    (
+        vec![
+            probe.id(),
+            wide.id(),
+            narrow.id(),
+            branch.id(),
+            wrapped.id(),
+            down.id(),
+            inner_filler.id(),
+            inner.id(),
+            outer_filler.id(),
+            outer.id(),
+        ],
+        [down, inner, outer],
+    )
+}
+
+#[test]
+fn reordering_nested_spans_keeps_the_answer_from_the_decided_box() {
+    let mut warm = Harness::new((900, 1200));
+    let (ids, spans) = plant_branch_in_nested_reverse_spans(&mut warm, false);
+    for span in spans {
+        warm.rsc[span].children.rotate_left(1);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_ids, _) = plant_branch_in_nested_reverse_spans(&mut cold, true);
+
+    assert_same_regions(&warm, &ids, &cold, &cold_ids);
+}
+
 /// Six widgets, shrunk from a 402-widget tree the fuzzer found. Nothing about
 /// the tree changes -- every widget is marked for redraw and the frame is
 /// taken again -- so no box may move, and a warm frame has to land where a