Settle growing layout branches in one frame

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
+    }
 }