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/render_state.rs

@@ -203,7 +203,7 @@ impl UiRenderState {
         parent_move: MoveIdx,
         slotted: bool,
         mask: MaskIdx,
-        old_children: Option<Vec<WidgetId>>,
+        mut old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
     ) -> Size {
         #[cfg(feature = "layout-diagnostics")]
@@ -211,14 +211,12 @@ impl UiRenderState {
             diag::bump(Counter::DrawRequests);
             diag::draw_request(id, parent, region, self.px_of(parent_move, region), slotted);
         }
-        let mut old_children = old_children.unwrap_or_default();
         if self.active.contains_key(&id) {
             if let Some(size) = self.try_reuse(id, region, depth, parent_move, rsc) {
                 return size;
             }
             // if not, then maintain resize and track old children to remove unneeded
-            let active = self.remove(id, false, rsc).unwrap();
-            old_children = active.children;
+            old = self.remove(id, false, rsc);
         }
 
         // draw widget
@@ -232,6 +230,12 @@ impl UiRenderState {
             }
         };
         let px = self.px_of(move_idx, local);
+        // Drawn again in a box its parent already decided: the offer is the
+        // one recorded when the parent first asked, not this box.
+        let (old_children, offered_px) = match old {
+            Some(old) => (old.children, old.offered_px),
+            None => (Vec::new(), px),
+        };
         rsc.widgets_mut().needs_redraw.remove(&id);
         self.draw_started.insert(id);
 
@@ -244,6 +248,7 @@ impl UiRenderState {
             textures: Vec::new(),
             primitives: Vec::new(),
             children: Vec::new(),
+            offered: Vec::new(),
             size_deps: Vec::new(),
             depth,
             size_box_inputs: [false; 2],
@@ -271,6 +276,7 @@ impl UiRenderState {
             textures,
             primitives,
             children,
+            offered: _,
             size_deps,
             size_box_inputs,
             size_output_inputs,
@@ -292,6 +298,7 @@ impl UiRenderState {
             region,
             size,
             px,
+            offered_px,
             parent,
             depth,
             textures,
@@ -347,7 +354,7 @@ impl UiRenderState {
     }
 
     /// The pixel size of a region held in `slot`'s coordinates.
-    fn px_of(&self, slot: MoveIdx, region: UiRegion) -> Vec2 {
+    pub(super) fn px_of(&self, slot: MoveIdx, region: UiRegion) -> Vec2 {
         self.moves
             .resolve(slot, region)
             .size()
@@ -736,16 +743,25 @@ impl UiRenderState {
         // A widget can only answer whether its size changed by drawing in the
         // box its parent chose. If that box changed in pixels, its retained
         // placement is stale and the highest size reader must choose the new
-        // box first. Otherwise the widget can draw locally, and its readers
-        // only matter if the returned size actually changed.
+        // box first. The same holds when the box was decided from the
+        // widget's own answer: measuring there again can only repeat it,
+        // whatever the content now says. Otherwise the widget can draw
+        // locally, and its readers only matter if the returned size actually
+        // changed.
         let box_changed = self.active.get(&id).is_some_and(|active| {
             let px = self.px_of(active.parent_move, active.region);
             AXES.into_iter()
                 .any(|axis| pixel_len_changed(active.px.axis(axis), px.axis(axis)))
         });
-        if box_changed && let Some(top) = self.mark_readers(id, rsc) {
+        let top = match box_changed {
+            true => self.top_reader(id),
+            false => None,
+        }
+        .or_else(|| self.derived_box_reader(id));
+        if let Some(top) = top {
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::EagerReaderRedraws);
+            self.mark_below(id, top, rsc);
             self.redraw(top, rsc);
             rsc.widgets_mut().needs_redraw.remove(&id);
             return;
@@ -772,7 +788,7 @@ impl UiRenderState {
             active.parent_move,
             active.move_idx != active.parent_move,
             active.mask,
-            Some(active.children),
+            Some(active),
             rsc,
         );
 
@@ -795,24 +811,56 @@ impl UiRenderState {
         }
     }
 
-    /// The furthest ancestor that read this widget's size, directly or through
-    /// widgets that did the same, marking everything below it on the way.
-    fn mark_readers(&self, id: WidgetId, rsc: &mut dyn UiRsc) -> Option<WidgetId> {
+    /// The highest reader up the chain that gave what it read a box other
+    /// than the one it asked in, on an axis this widget's size reads. Above
+    /// it every box is a constraint rather than an answer. It is the highest
+    /// and not the nearest because a pass-through hands a derived box down
+    /// unchanged.
+    fn derived_box_reader(&self, id: WidgetId) -> Option<WidgetId> {
+        let reads = self.active.get(&id)?.size_box_inputs;
         let mut top = None;
-        let mut at = id;
-        while let Some(active) = self.active.get(&at)
-            && let Some(parent) = active.parent
-            && self
-                .active
-                .get(&parent)
-                .is_some_and(|p| p.size_deps.contains(&at))
-        {
-            rsc.widgets_mut().needs_redraw.insert(at);
-            top = Some(parent);
-            at = parent;
+        for (active, parent) in self.reader_chain(id) {
+            let px = self.px_of(active.parent_move, active.region);
+            if AXES.into_iter().zip(reads).any(|(axis, r)| {
+                r && pixel_len_changed(active.offered_px.axis(axis), px.axis(axis))
+            }) {
+                top = Some(parent);
+            }
         }
         top
     }
+
+    /// The furthest ancestor that read this widget's size, directly or through
+    /// widgets that did the same.
+    fn top_reader(&self, id: WidgetId) -> Option<WidgetId> {
+        self.reader_chain(id).last().map(|(_, parent)| parent)
+    }
+
+    /// Each widget from `id` upward whose parent read its size, with that
+    /// parent.
+    fn reader_chain(&self, id: WidgetId) -> impl Iterator<Item = (&ActiveData, WidgetId)> {
+        let mut at = Some(id);
+        std::iter::from_fn(move || {
+            let active = self.active.get(&at?)?;
+            let parent = active.parent?;
+            let read = self.active.get(&parent)?.size_deps.contains(&active.id);
+            at = read.then_some(parent);
+            read.then_some((active, parent))
+        })
+    }
+
+    /// Marks everything from `id` up to, and not including, `top`, so that
+    /// drawing `top` draws each of them rather than reusing it.
+    fn mark_below(&self, id: WidgetId, top: WidgetId, rsc: &mut dyn UiRsc) {
+        let mut at = id;
+        while at != top {
+            rsc.widgets_mut().needs_redraw.insert(at);
+            let Some(parent) = self.active.get(&at).and_then(|active| active.parent) else {
+                return;
+            };
+            at = parent;
+        }
+    }
 }
 
 impl Default for UiRenderState {