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()

src/widget/position/span.rs

@@ -12,14 +12,24 @@ impl Widget for Span {
         let axis = self.dir.axis;
         // A length for every child before any is placed: from its own hint
         // where it has one, and from drawing it where it does not.
-        let lens: Vec<Len> = self
-            .children
-            .iter()
-            .map(|child| match painter.size_hint(child, axis) {
+        let mut cursor = UiScalar::rel_min();
+        let mut lens = Vec::with_capacity(self.children.len());
+        for child in &self.children {
+            let len = match painter.size_hint(child, axis) {
                 Some(len) => len,
-                None => painter.place(child, UiRegion::FULL).len(axis),
-            })
-            .collect();
+                None => {
+                    let mut span = UiSpan::new(cursor, UiScalar::rel_max());
+                    if self.dir.sign == Sign::Neg {
+                        span.flip();
+                    }
+                    let region = UiRegion::from_axis(axis, span, UiSpan::FULL);
+                    painter.place(child, region).len(axis)
+                }
+            };
+            cursor.abs += len.abs + self.gap;
+            cursor.rel += len.rel;
+            lens.push(len);
+        }
 
         let gap = self.gap * self.children.len().saturating_sub(1) as f32;
         let total = lens.iter().fold(Len::abs(gap), |sum, len| sum + *len);