Measure a child on the layer it draws on, not twice on two

Fixed point cost 3x in layout: `many` went from 0.179 ms a frame to 0.544,
and `scroll` from 0.011 to 0.030. The counters said why -- eight more "placed
by redrawing" a frame -- and the reason was mine rather than the grid's. A
retained drawing belongs to the layer it was made on, which `4e28f10` started
enforcing, and `Stack` measures the child that sizes it by drawing it on its
own layer and then draws it again on the child layer. So every stacked child
redrew twice a frame, forever.

`Painter::child_layer_at` addresses a child's layer rather than walking to
it, and `Stack` measures on the layer that child ends up on. The second ask
is then a reuse. Its glyphs are written once rather than once under the
background and once over it.

Measured on the same fixture: `scroll` 0.031 ms to 0.020, `many` 0.570 to
0.283, and the scroll phase's counters are back to what they were before
fixed point -- 4 widget draws and 12 draw requests a frame, exactly. What is
left above that baseline is not this.

`ReuseOutcome` could not say "another layer" or "the region-node choice
changed"; both returned without a counter, which is why the first look at
this said nothing. They have counters now.

Checked: fmt, clippy, 105 tests, five shrinker cases at 300 seeds, 100
generated seeds, and the examples byte-identical but for 36 pixels of
`random` at one level -- edges that were being drawn twice.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/layout_diagnostics.rs

@@ -40,6 +40,8 @@ pub(crate) enum Counter {
     ReuseWrongParent,
     ReuseRemapped,
     ReuseOutside,
+    ReuseWrongLayer,
+    ReuseWrongNode,
     PlaceRedraws,
     QueuePops,
     DepthReads,
@@ -73,6 +75,8 @@ impl Counter {
         "reuse: wrong parent",
         "reuse remapped",
         "reuse: outside what it holds for",
+        "reuse: another layer",
+        "reuse: region-node choice changed",
         "placed by redrawing",
         "redraw queue pops",
         "depth reads",

core/src/ui/painter.rs

@@ -42,6 +42,9 @@ pub struct Painter<'a> {
     /// its own when opted in, otherwise the nearest ancestor's.
     pub(super) move_idx: MoveIdx,
     pub layer: usize,
+    /// The layer this widget was entered on, which its children's layers are
+    /// counted from however far `layer` has walked.
+    pub(super) own_layer: usize,
     pub(super) depth: usize,
     pub(super) id: WidgetId,
 }
@@ -420,6 +423,18 @@ impl<'a> Painter<'a> {
         self.layer = self.state.layers.child(self.layer);
     }
 
+    /// The layer this widget's `n`th child draws on, addressed rather than
+    /// walked to. A container that measures one child by drawing it can ask
+    /// on the layer that child will end up on, and then the second ask is a
+    /// reuse rather than a second drawing on another layer.
+    pub fn child_layer_at(&mut self, n: usize) {
+        let mut at = self.state.layers.child(self.own_layer);
+        for _ in 0..n {
+            at = self.state.layers.next(at);
+        }
+        self.layer = at;
+    }
+
     pub fn next_layer(&mut self) {
         self.layer = self.state.layers.next(self.layer);
     }

core/src/ui/render_state.rs

@@ -307,6 +307,7 @@ impl UiRenderState {
             region: local,
             mask: info.mask,
             layer: info.layer,
+            own_layer: info.layer,
             id,
             textures: Vec::new(),
             primitives: Vec::new(),
@@ -349,6 +350,7 @@ impl UiRenderState {
             under,
             move_idx,
             layer,
+            own_layer: _,
             depth: _,
             id,
         } = painter;
@@ -586,6 +588,8 @@ impl UiRenderState {
         }
         let has_region_node = active.move_idx != active.parent_move;
         if has_region_node != info.region_node {
+            #[cfg(feature = "layout-diagnostics")]
+            diag::bump(Counter::ReuseWrongNode);
             return None;
         }
         // Drawn on another layer: the drawing sits in that layer's list and
@@ -595,7 +599,10 @@ impl UiRenderState {
         // on a layer the first answer is not good for.
         if active.layer != info.layer {
             #[cfg(feature = "layout-diagnostics")]
-            diag::reuse(id, ReuseOutcome::WrongLayer);
+            {
+                diag::bump(Counter::ReuseWrongLayer);
+                diag::reuse(id, ReuseOutcome::WrongLayer);
+            }
             return None;
         }
         // Drawn somewhere else in the tree: its box is in coordinates it no

src/widget/position/stack.rs

@@ -16,16 +16,19 @@ impl Widget for Stack {
         // Whichever child sizes the stack decides the box every child gets.
         // The stack reports that size, so a child given a longer box would
         // draw outside what the stack says it occupies.
-        let size = match sizing.and_then(|i| self.children.get(i)) {
-            Some(child) => painter.widget(child).size(),
+        let size = match sizing.and_then(|i| self.children.get(i).map(|c| (i, c))) {
+            // On the layer that child ends up on, so the ask below is a reuse
+            // rather than a second drawing of it somewhere else: a retained
+            // drawing belongs to the layer it was made on.
+            Some((i, child)) => {
+                painter.child_layer_at(i);
+                painter.widget(child).size()
+            }
             None => Size::LEFTOVER,
         };
         let region = painter.box_of(size);
         for (i, child) in self.children.iter().enumerate() {
-            match i {
-                0 => painter.child_layer(),
-                _ => painter.next_layer(),
-            }
+            painter.child_layer_at(i);
             painter.widget_aligned(child, region, RegionAlign::NEAR);
         }
         size

tests/cases/retained.rs

@@ -560,13 +560,43 @@ fn a_declared_length_child_is_not_redrawn_when_the_box_around_it_grows() {
     assert_corners!(h, fixed, (200, 0), (280, 200));
 }
 
-/// `Stack` measures the child that sizes it by drawing it, then draws it
-/// again above the background it stacks over. The second ask is for the same
-/// box, so nothing geometric says the answer has gone stale, and reusing it
-/// leaves the drawing under the background. The same tree got away with it
-/// while the two asks differed by a rounding.
+/// A retained drawing belongs to the layer it was made on: asked for again
+/// on another one it has to be drawn there, since nothing about its geometry
+/// says it is in a list that paints at a different moment.
 #[test]
 fn a_widget_asked_again_on_another_layer_is_drawn_there() {
+    /// Draws its child on its own layer, then again one layer in -- which is
+    /// what a container measuring a child by drawing it used to do.
+    struct Twice(StrongWidget);
+
+    impl Widget for Twice {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            let size = painter.widget(&self.0).size();
+            painter.child_layer();
+            painter.widget(&self.0);
+            size
+        }
+    }
+
+    let mut h = Harness::new((400, 200));
+    let (front, draws) = counted(&mut h, Size::from((100, 50)), false);
+    let outer = Twice(front.add_strong(&mut h.rsc)).add(&mut h.rsc);
+    h.set_root(outer);
+    h.frame();
+
+    assert_ne!(
+        h.render.active[&front.id()].layer,
+        h.render.active[&outer.id()].layer,
+        "the first drawing was kept, on the layer it was measured on"
+    );
+    assert_eq!(draws.get(), 2, "the second ask could not reuse the first");
+}
+
+/// Which is why `Stack` measures the child that sizes it on the layer that
+/// child draws on: one drawing, above the background it stacks over, rather
+/// than one on each layer and the wrong one kept.
+#[test]
+fn a_stacks_sizing_child_is_drawn_once_where_it_belongs() {
     let mut h = Harness::new((400, 200));
     let background = rect(Color::RED).add(&mut h.rsc);
     let (front, draws) = counted(&mut h, Size::from((100, 50)), false);
@@ -582,13 +612,7 @@ fn a_widget_asked_again_on_another_layer_is_drawn_there() {
     h.frame();
 
     let layer = |id| h.render.active[&id].layer;
-    assert_ne!(
-        layer(front.id()),
-        layer(stack.id()),
-        "the measured drawing was left on the stack's own layer"
-    );
+    assert_ne!(layer(front.id()), layer(stack.id()));
     assert_ne!(layer(front.id()), layer(background.id()));
-    // Measured once for the size and once where it goes, which is what the
-    // stack costs and not something this test is asserting a number for.
-    assert_eq!(draws.get(), 2);
+    assert_eq!(draws.get(), 1);
 }