Track retained layout validity explicitly

src/widget/position/scroll.rs

@@ -11,31 +11,40 @@ pub struct Scroll {
 
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let output_len = painter.output_len(self.axis);
-        // Its size is its content's, whatever box that is scrolled within.
-        let container_len = UiScalar::px(painter.px_len_for_draw(self.axis));
+        let container_len = 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_leftover()
-            .within_len(container_len)
-            .to_px(output_len);
-        self.container_len = container_len.to_px(output_len);
-        self.content_len = content_len;
+        let (answer_len, measured) = match painter.known_len(&self.inner, self.axis, UiRegion::FULL)
+        {
+            Some(len) => (len, None),
+            None => {
+                let size = painter.place(&self.inner, UiRegion::FULL).size();
+                (size.axis(self.axis), Some(size))
+            }
+        };
+        let content = answer_len.apply_leftover();
+        self.container_len = container_len;
+        self.content_len = content.to_px(container_len);
 
         if self.snap_end {
             self.amt = self.content_len - self.container_len;
         }
         self.update_amt();
+        // Content of a fixed length that fits sits at the start of any box
+        // it fits in; one scrolled part way sits where it is until the box
+        // shrinks past what is left of it. Kept to the end, it moves with
+        // every length.
+        if content.rel == 0.0 && self.content_len <= self.container_len {
+            painter.holds(self.axis, self.content_len..=f32::INFINITY);
+        } else if content.rel == 0.0 && !self.snap_end {
+            let left = self.content_len - self.amt;
+            painter.holds(self.axis, f32::NEG_INFINITY..=left);
+        }
 
         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);
         let placed = painter.place(&self.inner, region).size();
-        child.unwrap_or(placed)
+        measured.unwrap_or_else(|| Size::from_axis(self.axis, answer_len, placed.axis(!self.axis)))
     }
 }