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).
+        let changed = self.active.get(&id).map(|a| a.size) != Some(old_size);
+        if changed {
             if let Some(pid) = parent {
-            let new_size = self.active.get(&id).map(|a| a.size);
+                self.redraw_and_settle(pid, rsc);
-            if new_size != Some(old_size) {
-                self.redraw(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);
         }
     }
 }

src/layout_tests.rs

@@ -1172,3 +1172,67 @@ fn a_span_of_padded_children_inside_a_span_draws_each_where_its_box_is() {
         );
     }
 }
+
+/// Growing an already-drawn row first measures its new child against the
+/// row's old height. That provisional box can end before it starts when the
+/// old trailing edge is above the new child's cursor. The size must bubble to
+/// `LazySpan` and the corrected allocation must travel back down before this
+/// update is presented; a later stream event is not a layout pass.
+#[test]
+fn a_new_child_in_a_growing_lazy_row_uses_its_final_box_immediately() {
+    const FIRST: f32 = 30.0;
+    const SECOND: f32 = 70.0;
+    const GAP: f32 = 8.0;
+
+    let mut rsc = TestRsc {
+        ui: UiData::default(),
+    };
+    let first = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
+    let first_id = first.id();
+    let first = rsc.ui.widgets.add_strong(Sized {
+        inner: first.any(),
+        x: None,
+        y: Some(Len::abs(FIRST)),
+    });
+    let mut contents = Span::empty(Dir::DOWN).gap(Len::abs(GAP));
+    contents.push(first.any());
+    let contents = rsc.ui.widgets.add_strong(contents);
+    let contents_w = contents.weak();
+    let row = rsc.ui.widgets.add_strong(Sized {
+        inner: contents.any(),
+        x: Some(Len::rest(1.0)),
+        y: None,
+    });
+    let mut list = LazySpan::new(Dir::DOWN, Pin::End);
+    list.push_back(LazyItem::new(0, row.any()));
+    let root = rsc.ui.widgets.add_strong(list).any();
+
+    let mut render = UiRenderState::new();
+    render.resize((200.0, 200.0));
+    render.update(&root, &mut rsc);
+
+    let second = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
+    let second_id = second.id();
+    let second = rsc.ui.widgets.add_strong(Sized {
+        inner: second.any(),
+        x: None,
+        y: Some(Len::abs(SECOND)),
+    });
+    rsc.ui
+        .widgets
+        .get_mut(&contents_w)
+        .unwrap()
+        .push(second.any());
+    render.update(&root, &mut rsc);
+
+    let first = render.primitive_corners(render.first_primitive(first_id).unwrap(), &rsc);
+    let second = render.primitive_corners(render.first_primitive(second_id).unwrap(), &rsc);
+    assert!(
+        (second.top_left.y - (first.bot_right.y + GAP)).abs() < 0.01,
+        "the new child should start after the old child and its gap: first={first:?} second={second:?}"
+    );
+    assert!(
+        (second.bot_right.y - second.top_left.y - SECOND).abs() < 0.01,
+        "the new child retained its provisional box: {second:?}"
+    );
+}

src/widget/position/sized.rs

@@ -31,4 +31,12 @@ impl Widget for Sized {
             Axis::Y => self.y,
         }
     }
+
+    fn requires_exact_region(&self) -> bool {
+        // `Sized` may be measured in a provisional box and then placed in
+        // the content-sized box it reported. Its own region can be corrected
+        // by a move, but its child consumed the original box and must see the
+        // final one too.
+        true
+    }
 }