Replace placement calls with region nodes

src/widget/trait_fns.rs

@@ -31,6 +31,14 @@ widget_trait! {
         }
     }
 
+    fn region_node(self) -> impl WidgetIdFn<Rsc, WL::Widget> {
+        |state| {
+            let id = self.add(state);
+            state.ui_mut().widgets.set_region_node(id, true);
+            id
+        }
+    }
+
     fn sized(self, size: impl Into<Size>) -> impl WidgetFn<Rsc, SetSize> {
         let size = size.into();
         move |state| SetSize {
@@ -85,7 +93,9 @@ widget_trait! {
 
     fn scrollable(self) -> impl WidgetIdFn<Rsc, Scroll> where Rsc: HasEvents {
         move |state| {
-            Scroll::new(self.add_strong(state), Axis::Y)
+            let inner = self.add(state);
+            state.ui_mut().widgets.set_region_node(inner, true);
+            Scroll::new(inner.upgrade(state), Axis::Y)
                 .on(CursorSense::Scroll, |ctx, rsc| {
                     let delta = ctx.data.scroll_delta.y * 50.0;
                     ctx.widget(rsc).scroll(delta);