Avoid speculative layout when retained answers suffice

core/src/ui/painter.rs

@@ -22,6 +22,8 @@ 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,
@@ -144,6 +146,21 @@ 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

@@ -109,18 +109,19 @@ impl UiRenderState {
         parent_move: MoveIdx,
         slotted: bool,
         mask: MaskIdx,
-        old_children: Option<Vec<WidgetId>>,
+        old_active: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
     ) -> Size {
-        let mut old_children = old_children.unwrap_or_default();
+        let mut old_active = old_active;
         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
-            let active = self.remove(id, false, rsc).unwrap();
-            old_children = active.children;
+            old_active = self.remove(id, false, rsc);
         }
+        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 {
@@ -133,6 +134,7 @@ 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);
 
@@ -147,6 +149,7 @@ impl UiRenderState {
             children: Vec::new(),
             size_deps: Vec::new(),
             reads_output: false,
+            same_box,
             move_idx,
             rsc,
         };
@@ -165,6 +168,7 @@ impl UiRenderState {
             children,
             size_deps,
             reads_output,
+            same_box: _,
             move_idx,
             layer,
             id,
@@ -449,14 +453,17 @@ impl UiRenderState {
     /// redraws a widget that's currently active (drawn)
     pub fn redraw(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) {
         self.draw_started.remove(&id);
-        // Whoever read this widget's size may be a different size now, so the
-        // highest reader is what draws. Everything between the two is marked
-        // as well: their own boxes have not changed, so the mark is the only
-        // thing stopping the draw reusing its way past this widget.
-        if let Some(top) = self.mark_readers(id, rsc) {
+        // A widget can only answer whether its size changed by drawing in the
+        // box its parent chose. If that box changed in pixels, its retained
+        // placement is stale and the highest size reader must choose the new
+        // box first. Otherwise the widget can draw locally, and its readers
+        // only matter if the returned size actually changed.
+        let box_changed = self
+            .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) {
             self.redraw(top, rsc);
-            // Cleared by that draw if it reached here; if it did not, this is
-            // no longer drawn and asking again would not end.
             rsc.widgets_mut().needs_redraw.remove(&id);
             return;
         }
@@ -470,7 +477,8 @@ impl UiRenderState {
             return;
         };
 
-        self.draw_inner(
+        let was = active.size;
+        let size = self.draw_inner(
             active.layer,
             id,
             active.region,
@@ -478,9 +486,16 @@ impl UiRenderState {
             active.parent_move,
             active.move_idx != active.parent_move,
             active.mask,
-            Some(active.children),
+            Some(active),
             rsc,
         );
+
+        if size != was
+            && let Some(top) = self.mark_readers(id, rsc)
+        {
+            self.redraw(top, rsc);
+            rsc.widgets_mut().needs_redraw.remove(&id);
+        }
     }
 
     /// The furthest ancestor that read this widget's size, directly or through

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);

tests/retained.rs

@@ -79,10 +79,9 @@ fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
     h.rsc[first].size = Size::from((150, 200));
     h.frame();
 
-    // Twice: once for the span to measure it, once for its real box. A child
-    // that can hint its length is spared the first, and a smaller number here
-    // means someone has made that cheaper rather than broken it.
-    assert_eq!(draws.get(), settled + 2);
+    // The preceding fixed child makes the remaining box this child's real
+    // box, so measuring it also draws it in its final place.
+    assert_eq!(draws.get(), settled + 1);
     assert_corners!(h, second, (150, 0), (400, 200));
 }
 
@@ -116,6 +115,50 @@ fn a_span_relays_out_when_a_child_it_measured_changes() {
     assert_corners!(h, second, (250, 0), (400, 200));
 }
 
+#[test]
+fn a_repaint_that_keeps_its_size_does_not_relay_out() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws) = counted(&mut h, Size::from((100, 200)), OnResize::Translate);
+    let (second, _) = counted(&mut h, Size::REST, OnResize::Translate);
+    h.set_root((first, second).span(Dir::RIGHT));
+    let settled = draws.get();
+
+    // Taking mutable access is the ordinary content-change signal. This
+    // widget returns the same size, so the parent has nothing to lay out.
+    let _ = h.rsc.widgets_mut().get_dyn_mut(first.id());
+    h.frame();
+
+    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));