Settle growing layout branches in one frame

core/src/ui/render_state.rs

@@ -1207,6 +1207,16 @@ impl UiRenderState {
 
     /// redraws a widget that's currently active (drawn)
     pub fn redraw(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) {
+        self.redraw_and_settle(id, rsc);
+    }
+
+    /// Measure a changed branch toward the root, then revisit each widget
+    /// whose reported size changed on the way back down. The upward pass gives
+    /// every parent the new child size; the downward pass is what lets those
+    /// children draw inside the final boxes their parents chose. Without it a
+    /// newly grown subtree can retain the provisional (even inverted) region
+    /// it was measured in until an unrelated later update redraws it.
+    fn redraw_and_settle(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) {
         rsc.widgets_mut().needs_redraw.remove(&id);
         // An ancestor is drawing this widget right now, and that draw is
         // about to write fresh primitives for it. Drawing it a second time
@@ -1248,11 +1258,15 @@ impl UiRenderState {
         // relay out too. Checked after the real draw, not before it --
         // there is no query left that answers "what size would this be"
         // without actually drawing (LAYOUT.md section 5).
-        if let Some(pid) = parent {
-            let new_size = self.active.get(&id).map(|a| a.size);
-            if new_size != Some(old_size) {
-                self.redraw(pid, rsc);
+        let changed = self.active.get(&id).map(|a| a.size) != Some(old_size);
+        if changed {
+            if let Some(pid) = parent {
+                self.redraw_and_settle(pid, rsc);
             }
+            // The parent pass above has now placed this widget in its final
+            // region. Draw it once more there; unchanged descendants still
+            // take draw_inner's retained fast path.
+            self.redraw_and_settle(id, rsc);
         }
     }
 }