Settle dirty layout from the leaves upward

src/widget/position/scroll.rs

@@ -13,13 +13,9 @@ impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let output_len = painter.output_size().axis(self.axis);
         let container_len = UiScalar::abs(painter.px_size().axis(self.axis));
-        // Its last measured size stays valid while neither the child nor this
-        // container's constraints changed. Otherwise draw it in the whole
-        // container to learn its length, then place it at the scrolled offset.
-        let child = match painter.retained_size(&self.inner) {
-            Some(size) => size,
-            None => painter.place(&self.inner, UiRegion::FULL).size(),
-        };
+        // Draw in the whole container to learn the content's length, then
+        // place it at the scrolled offset.
+        let child = painter.place(&self.inner, UiRegion::FULL).size();
         let content_len = child
             .axis(self.axis)
             .apply_rest()