Settle dirty layout from the leaves upward

src/random.rs

@@ -148,6 +148,22 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
         }
     }
 
+    fn align(&mut self) -> Align {
+        let mut axis = || match self.rng.below(4) {
+            0 => None,
+            1 => Some(AxisAlign::Neg),
+            2 => Some(AxisAlign::Center),
+            _ => Some(AxisAlign::Pos),
+        };
+        let (mut x, y) = (axis(), axis());
+        // Aligning on neither axis is just another transparent wrapper and
+        // would leave this branch unexercised.
+        if x.is_none() && y.is_none() {
+            x = Some(AxisAlign::Center);
+        }
+        Align { x, y }
+    }
+
     /// A declared size over half the tree, kept where a test can change it.
     fn sized(&mut self, inner: StrongWidget) -> StrongWidget {
         if !self.rng.chance() {
@@ -171,7 +187,8 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
         if depth == 0 {
             return self.leaf();
         }
-        if self.rng.below(6) == 0 {
+        let positioned = self.rng.below(6);
+        if positioned == 0 {
             // Scrolling reads the pixel length of its box, which nothing
             // else here does, and gives its child a box longer than its own.
             let inner = self.node(depth - 1);
@@ -182,6 +199,17 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
             self.tree.ids.push(id.id());
             return id.add_strong(self.rsc);
         }
+        if positioned == 1 {
+            let inner = self.node(depth - 1);
+            let inner = self.sized(inner);
+            let id = Aligned {
+                inner,
+                align: self.align(),
+            }
+            .add_strong(self.rsc);
+            self.tree.ids.push(id.id());
+            return id;
+        }
         if self.rng.below(4) == 0 {
             let inner = self.node(depth - 1);
             let inner = self.sized(inner);

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()