Avoid speculative layout when retained answers suffice

src/widget/position/scroll.rs

@@ -13,9 +13,13 @@ 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));
-        // Drawn in the whole container to learn its length, then placed at
-        // the scrolled offset.
-        let child = painter.place(&self.inner, UiRegion::FULL).size();
+        // 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(),
+        };
         let content_len = child
             .axis(self.axis)
             .apply_rest()