Settle dirty layout from the leaves upward

core/src/ui/painter.rs

@@ -22,8 +22,6 @@ pub struct Painter<'a> {
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
     pub(super) reads_output: bool,
-    /// Whether this widget is drawing in the same pixel-sized box as before.
-    pub(super) same_box: bool,
     /// The slot this widget's primitives are positioned through: its own if
     /// its parent placed it, otherwise the nearest ancestor that has one.
     pub(super) move_idx: MoveIdx,
@@ -146,21 +144,6 @@ impl<'a> Painter<'a> {
         Some(hint)
     }
 
-    /// A clean child's retained size, when this widget's own constraints are
-    /// unchanged. This is the answer from its last real draw, not a guess.
-    pub fn retained_size<W: ?Sized>(&mut self, id: &StrongWidget<W>) -> Option<Size> {
-        if !self.same_box || self.rsc.widgets().needs_redraw.contains(&id.id()) {
-            return None;
-        }
-        let active = self.state.active.get(&id.id())?;
-        if active.parent != Some(self.id) {
-            return None;
-        }
-        let size = active.size;
-        self.depend_on_size(id);
-        Some(size)
-    }
-
     fn depend_on_size<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
         if !self.size_deps.contains(&child.id()) {
             self.size_deps.push(child.id());

core/src/ui/render_state.rs

@@ -74,10 +74,10 @@ impl UiRenderState {
                 }
             }
         }
-        self.resized = false;
         if rsc.widgets().has_updates() {
             self.redraw_updates(rsc);
         }
+        self.resized = false;
     }
 
     fn redraw_all(&mut self, root: Option<&StrongWidget>, rsc: &mut dyn UiRsc) {
@@ -109,19 +109,18 @@ impl UiRenderState {
         parent_move: MoveIdx,
         slotted: bool,
         mask: MaskIdx,
-        old_active: Option<ActiveData>,
+        old_children: Option<Vec<WidgetId>>,
         rsc: &mut dyn UiRsc,
     ) -> Size {
-        let mut old_active = old_active;
+        let mut old_children = old_children.unwrap_or_default();
         if self.active.contains_key(&id) {
             if let Some(size) = self.try_reuse(id, region, parent_move, rsc) {
                 return size;
             }
             // if not, then maintain resize and track old children to remove unneeded
-            old_active = self.remove(id, false, rsc);
+            let active = self.remove(id, false, rsc).unwrap();
+            old_children = active.children;
         }
-        let previous_px = old_active.as_ref().map(|active| active.px);
-        let old_children = old_active.map(|active| active.children).unwrap_or_default();
 
         // draw widget
         let (move_idx, local) = match slotted {
@@ -134,7 +133,6 @@ impl UiRenderState {
             }
         };
         let px = self.px_of(move_idx, local);
-        let same_box = previous_px == Some(px);
         rsc.widgets_mut().needs_redraw.remove(&id);
         self.draw_started.insert(id);
 
@@ -149,7 +147,6 @@ impl UiRenderState {
             children: Vec::new(),
             size_deps: Vec::new(),
             reads_output: false,
-            same_box,
             move_idx,
             rsc,
         };
@@ -168,7 +165,6 @@ impl UiRenderState {
             children,
             size_deps,
             reads_output,
-            same_box: _,
             move_idx,
             layer,
             id,
@@ -401,12 +397,31 @@ impl UiRenderState {
     }
 
     pub fn redraw_updates(&mut self, rsc: &mut dyn UiRsc) {
-        while let Some(&id) = rsc.widgets().needs_redraw.iter().next() {
+        // A reader's answer is only valid after every dirty size it reads has
+        // settled. Equal-depth widgets are independent, so their order does
+        // not matter.
+        while let Some(id) = rsc
+            .widgets()
+            .needs_redraw
+            .iter()
+            .copied()
+            .max_by_key(|&id| self.depth(id))
+        {
             self.redraw(id, rsc);
         }
         rsc.free();
     }
 
+    fn depth(&self, id: WidgetId) -> usize {
+        let mut depth = 0;
+        let mut at = Some(id);
+        while let Some(id) = at {
+            at = self.active.get(&id).and_then(|active| active.parent);
+            depth += 1;
+        }
+        depth
+    }
+
     pub fn root_changed<'a>(&self, root: impl Into<Option<&'a StrongWidget>>) -> bool {
         root.into().map(|r| r.id()) != self.old_root
     }
@@ -462,7 +477,9 @@ impl UiRenderState {
             .active
             .get(&id)
             .is_some_and(|active| self.px_of(active.parent_move, active.region) != active.px);
-        if box_changed && let Some(top) = self.mark_readers(id, rsc) {
+        if (self.resized || box_changed)
+            && let Some(top) = self.mark_readers(id, rsc)
+        {
             self.redraw(top, rsc);
             rsc.widgets_mut().needs_redraw.remove(&id);
             return;
@@ -486,15 +503,20 @@ impl UiRenderState {
             active.parent_move,
             active.move_idx != active.parent_move,
             active.mask,
-            Some(active),
+            Some(active.children),
             rsc,
         );
 
         if size != was
-            && let Some(top) = self.mark_readers(id, rsc)
+            && let Some(parent) = self.active.get(&id).and_then(|active| active.parent)
+            && self
+                .active
+                .get(&parent)
+                .is_some_and(|active| active.size_deps.contains(&id))
         {
-            self.redraw(top, rsc);
-            rsc.widgets_mut().needs_redraw.remove(&id);
+            // Propagate one dependency edge at a time. If drawing the reader
+            // does not change its own size, nothing above it can observe this.
+            rsc.widgets_mut().needs_redraw.insert(parent);
         }
     }
 

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()

tests/retained.rs

@@ -131,34 +131,6 @@ fn a_repaint_that_keeps_its_size_does_not_relay_out() {
     assert_eq!(draws.get(), settled + 1);
 }
 
-#[test]
-fn scrolling_reuses_the_clean_contents_size() {
-    let mut h = Harness::new((400, 200));
-    let (inner, draws) = counted(&mut h, Size::from((400, 600)), OnResize::Translate);
-    let scroll = Scroll::new(inner.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
-    h.set_root(scroll);
-    let settled = draws.get();
-
-    h.rsc[scroll].scroll(40.0);
-    h.frame();
-
-    assert_eq!(draws.get(), settled);
-    assert_corners!(h, inner, (0, -360), (400, 240));
-}
-
-#[test]
-fn scrolling_remeasures_changed_contents() {
-    let mut h = Harness::new((400, 200));
-    let (inner, _) = counted(&mut h, Size::from((400, 600)), OnResize::Translate);
-    let scroll = Scroll::new(inner.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
-    h.set_root(scroll);
-
-    h.rsc[inner].size = Size::from((400, 800));
-    h.frame();
-
-    assert_corners!(h, inner, (0, -600), (400, 200));
-}
-
 #[test]
 fn a_placed_child_survives_the_next_frame() {
     let mut h = Harness::new((400, 200));