Add retained paints and shared text selection

src/widget/position/scroll_area.rs

@@ -98,7 +98,7 @@ mod tests {
         let mut rsc = TestRsc {
             ui: UiData::default(),
         };
-        let fill = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE)).any();
+        let fill = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE)).any();
         let id = fill.id();
         let long = Some(Len::abs(1000.0));
         let tall = rsc.ui.widgets.add_strong(Sized {