Retain layout sizes by pixel axis

src/widget/position/scroll.rs

@@ -11,13 +11,15 @@ pub struct Scroll {
 
 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));
-        // 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)
+        let output_len = painter.output_len(self.axis);
+        let container_len = UiScalar::abs(painter.px_len(self.axis));
+        // Draw in the whole container only when its scrolling-axis length is
+        // not already known, then place it at the scrolled offset.
+        let known_len = painter.known_len(&self.inner, self.axis, UiRegion::FULL);
+        let measured = known_len.is_none();
+        let child = measured.then(|| painter.place(&self.inner, UiRegion::FULL).size());
+        let content_len = known_len
+            .unwrap_or_else(|| child.unwrap().axis(self.axis))
             .apply_rest()
             .within_len(container_len)
             .to_abs(output_len);
@@ -31,8 +33,8 @@ impl Widget for Scroll {
 
         let mut region = UiRegion::FULL.offset(Vec2::from_axis(self.axis, -self.amt, 0.0));
         region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
-        painter.place(&self.inner, region);
-        child
+        let placed = painter.place(&self.inner, region).size();
+        child.unwrap_or(placed)
     }
 }