Measure a dirty widget where its parent asked, not in a box its answer decided

A local redraw drew a dirty widget in the box it was placed in. When a reader
decided that box from the widget's own answer -- an aligned span sized to its
children, a text at the tail of a row, a scroll's content -- the old answer is a
fixed point of measuring there whatever the content now says, so the layout had
two stable answers and which one it reached depended on the tree's history.
`tests/unsettled.rs` has the two shrunk cases: the four-widget aligned span,
and a scroll placing a pass-through `SetSize` in a box the content decided,
where the span under it was placed once and nothing at its own edge said so.

`ActiveData::offered_px` keeps the pixel size of the box the parent first asked
about the child in, whether through `known_len` or a first `place`, beside `px`,
the box it drew against. A dirty widget whose size reads an axis on which some
reader up its chain gave what it read a box other than the one it asked in is
not drawn locally: the chain is marked and the parent of the highest such
placement draws, since above it every box is a constraint rather than an
answer. The walk goes up the whole reader chain because a pass-through hands a
derived box down unchanged.

`Scroll` read its box's length for the clamp through `px_len`, which records
the reported size as depending on it, and it does not: its size is its
content's. That made every scroll tick a size question asked in a derived box,
at 34x the instructions. `Painter::px_len_for_draw` is the read that records
nothing. Instructions per frame on the depth-8 rig against the previous head:
`many` at 32 dirty 0.66M to 0.74M, at 130 dirty 27.7M to 26.5M, `resize` 15.8M
to 15.0M, `scroll`, `repaint` and `size` unchanged. The shrinking fuzzer passes
200 trees at depth 7 in all four cases, the hundred-seed sweep passes, and the
five reference renders and the resize render are byte-identical.

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

core/src/ui/painter.rs

@@ -21,6 +21,9 @@ pub struct Painter<'a> {
     pub(super) textures: Vec<TextureHandle>,
     pub(super) primitives: Vec<PrimitiveHandle>,
     pub(super) children: Vec<WidgetId>,
+    /// The children asked about so far, so the first box each was asked
+    /// about is the one recorded as its offer.
+    pub(super) offered: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
     /// Offered pixel axes which can affect the size this draw reports.
@@ -141,6 +144,7 @@ impl<'a> Painter<'a> {
             None,
             self.rsc,
         );
+        self.offer(id.id(), region);
         DrawResult {
             child: id,
             painter: self,
@@ -182,6 +186,8 @@ impl<'a> Painter<'a> {
         axis: Axis,
         region: UiRegion,
     ) -> Option<Len> {
+        let region = region.within(&self.region);
+        self.offer(child.id(), region);
         if let Some(hint) = self.size_hint(child, axis) {
             return Some(hint);
         }
@@ -189,12 +195,12 @@ impl<'a> Painter<'a> {
             .map(|size| size.axis(axis))
     }
 
+    /// `region` in this widget's own coordinates.
     fn retained_size<W: ?Sized>(
         &mut self,
         child: &StrongWidget<W>,
         region: UiRegion,
     ) -> Option<Size> {
-        let region = region.within(&self.region);
         let (size, box_inputs, output_inputs) =
             self.state
                 .retained_size(child.id(), region, self.move_idx, self.rsc.widgets())?;
@@ -204,6 +210,20 @@ impl<'a> Painter<'a> {
         Some(size)
     }
 
+    /// Records the box a child was first asked about in this draw. Any later
+    /// box this draw gives it was decided knowing its answer, so a size the
+    /// child measures there is not an answer to this widget's question.
+    fn offer(&mut self, child: WidgetId, region: UiRegion) {
+        if self.offered.contains(&child) {
+            return;
+        }
+        self.offered.push(child);
+        let px = self.state.px_of(self.move_idx, region);
+        if let Some(active) = self.state.active.get_mut(&child) {
+            active.offered_px = px;
+        }
+    }
+
     /// Depends on a length the child gave without being drawn. A hint is
     /// context-free, so this depends on the child but on no pixel axis.
     fn depend_on_hint<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
@@ -309,6 +329,13 @@ impl<'a> Painter<'a> {
     /// [`Self::px_size`] when the other axis cannot affect the reported size.
     pub fn px_len(&mut self, axis: Axis) -> f32 {
         self.size_box_inputs[axis as usize] = true;
+        self.px_len_for_draw(axis)
+    }
+
+    /// One axis of this widget's box in pixels, for a draw whose reported
+    /// size does not follow from it -- a clamp or a position. Nothing records
+    /// the read, so a size that does depend on it would go stale.
+    pub fn px_len_for_draw(&self, axis: Axis) -> f32 {
         let region = self.state.moves.resolve(self.move_idx, self.region);
         region
             .size()