Move LazySpan rows through one retained offset

src/layout_tests.rs

@@ -37,6 +37,67 @@ impl Widget for FixedRect {
     }
 }
 
+struct ChildOffset {
+    child: StrongWidget,
+    offset: Vec2,
+}
+
+impl Widget for ChildOffset {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        painter.set_child_offset(self.offset);
+        painter.widget(&self.child)
+    }
+}
+
+#[test]
+fn a_child_coordinate_offset_moves_only_the_child_subtree() {
+    let mut rsc = TestRsc {
+        ui: UiData::default(),
+    };
+    let child = rsc.ui.widgets.add_strong(FixedRect(40.0));
+    let child_weak = child.weak();
+    let parent = rsc.ui.widgets.add_strong(ChildOffset {
+        child: child.any(),
+        offset: vec2(0.0, 15.0),
+    });
+    let parent_weak = parent.weak();
+    // Keep the coordinate-owning widget below the root: a nested widget is
+    // normally redrawn when its parent visits it, which takes a different
+    // retained-state path from `UiRenderState::redraw` on the root itself.
+    let outer = rsc.ui.widgets.add_strong(Sized {
+        inner: parent.any(),
+        x: None,
+        y: None,
+    });
+    let outer_weak = outer.weak();
+    let root = outer.any();
+    let mut render = UiRenderState::new();
+    render.resize((100.0, 100.0));
+    render.update(&root, &mut rsc);
+    render.take_counters();
+
+    let parent_before = render.window_region(&parent_weak, &rsc).unwrap();
+    let child_before = render.window_region(&child_weak, &rsc).unwrap();
+    assert!((parent_before.top_left.y - 0.0).abs() < 0.01);
+    assert!((child_before.top_left.y - 15.0).abs() < 0.01);
+
+    rsc.ui.widgets.get_mut(&parent_weak).unwrap().offset.y = 35.0;
+    // Force the ordinary ancestor-redraw path rather than letting the
+    // renderer visit only the dirty descendant directly.
+    rsc.ui.widgets.get_mut(&outer_weak).unwrap().x = None;
+    render.update(&root, &mut rsc);
+    let (draws, rewrites, moves, _shapes) = render.take_counters();
+    let parent_after = render.window_region(&parent_weak, &rsc).unwrap();
+    let child_after = render.window_region(&child_weak, &rsc).unwrap();
+
+    assert_eq!((draws, rewrites, moves), (2, 0, 1));
+    assert_eq!(
+        parent_after, parent_before,
+        "the container itself stays put"
+    );
+    assert!((child_after.top_left.y - 35.0).abs() < 0.01);
+}
+
 #[test]
 fn a_hinted_rest_draws_once_and_only_moves_the_fixed_child_after_it() {
     let mut rsc = TestRsc {