Settle dirty layout from the leaves upward

core/src/ui/render_state.rs

@@ -74,10 +74,10 @@ impl UiRenderState {
                 }
             }
         }
-        self.resized = false;
         if rsc.widgets().has_updates() {
             self.redraw_updates(rsc);
         }
+        self.resized = false;
     }
 
     fn redraw_all(&mut self, root: Option<&StrongWidget>, rsc: &mut dyn UiRsc) {
@@ -109,19 +109,18 @@ impl UiRenderState {
         parent_move: MoveIdx,
         slotted: bool,
         mask: MaskIdx,
-        old_active: Option<ActiveData>,
+        old_children: Option<Vec<WidgetId>>,
         rsc: &mut dyn UiRsc,
     ) -> Size {
-        let mut old_active = old_active;
+        let mut old_children = old_children.unwrap_or_default();
         if self.active.contains_key(&id) {
             if let Some(size) = self.try_reuse(id, region, parent_move, rsc) {
                 return size;
             }
             // if not, then maintain resize and track old children to remove unneeded
-            old_active = self.remove(id, false, rsc);
+            let active = self.remove(id, false, rsc).unwrap();
+            old_children = active.children;
         }
-        let previous_px = old_active.as_ref().map(|active| active.px);
-        let old_children = old_active.map(|active| active.children).unwrap_or_default();
 
         // draw widget
         let (move_idx, local) = match slotted {
@@ -134,7 +133,6 @@ impl UiRenderState {
             }
         };
         let px = self.px_of(move_idx, local);
-        let same_box = previous_px == Some(px);
         rsc.widgets_mut().needs_redraw.remove(&id);
         self.draw_started.insert(id);
 
@@ -149,7 +147,6 @@ impl UiRenderState {
             children: Vec::new(),
             size_deps: Vec::new(),
             reads_output: false,
-            same_box,
             move_idx,
             rsc,
         };
@@ -168,7 +165,6 @@ impl UiRenderState {
             children,
             size_deps,
             reads_output,
-            same_box: _,
             move_idx,
             layer,
             id,
@@ -401,12 +397,31 @@ impl UiRenderState {
     }
 
     pub fn redraw_updates(&mut self, rsc: &mut dyn UiRsc) {
-        while let Some(&id) = rsc.widgets().needs_redraw.iter().next() {
+        // A reader's answer is only valid after every dirty size it reads has
+        // settled. Equal-depth widgets are independent, so their order does
+        // not matter.
+        while let Some(id) = rsc
+            .widgets()
+            .needs_redraw
+            .iter()
+            .copied()
+            .max_by_key(|&id| self.depth(id))
+        {
             self.redraw(id, rsc);
         }
         rsc.free();
     }
 
+    fn depth(&self, id: WidgetId) -> usize {
+        let mut depth = 0;
+        let mut at = Some(id);
+        while let Some(id) = at {
+            at = self.active.get(&id).and_then(|active| active.parent);
+            depth += 1;
+        }
+        depth
+    }
+
     pub fn root_changed<'a>(&self, root: impl Into<Option<&'a StrongWidget>>) -> bool {
         root.into().map(|r| r.id()) != self.old_root
     }
@@ -462,7 +477,9 @@ impl UiRenderState {
             .active
             .get(&id)
             .is_some_and(|active| self.px_of(active.parent_move, active.region) != active.px);
-        if box_changed && let Some(top) = self.mark_readers(id, rsc) {
+        if (self.resized || box_changed)
+            && let Some(top) = self.mark_readers(id, rsc)
+        {
             self.redraw(top, rsc);
             rsc.widgets_mut().needs_redraw.remove(&id);
             return;
@@ -486,15 +503,20 @@ impl UiRenderState {
             active.parent_move,
             active.move_idx != active.parent_move,
             active.mask,
-            Some(active),
+            Some(active.children),
             rsc,
         );
 
         if size != was
-            && let Some(top) = self.mark_readers(id, rsc)
+            && let Some(parent) = self.active.get(&id).and_then(|active| active.parent)
+            && self
+                .active
+                .get(&parent)
+                .is_some_and(|active| active.size_deps.contains(&id))
         {
-            self.redraw(top, rsc);
-            rsc.widgets_mut().needs_redraw.remove(&id);
+            // Propagate one dependency edge at a time. If drawing the reader
+            // does not change its own size, nothing above it can observe this.
+            rsc.widgets_mut().needs_redraw.insert(parent);
         }
     }