Answer "have I asked this child?" in one read

A container's draw asked it once per child by searching the list of children
it had added so far, and four other per-child steps searched a list too, so
one draw cost the square of its children: 70% of a 1,600-child redraw was
those searches. A draw takes a DrawId and leaves it on every widget it asks
about; one note per widget is enough because the handle a container holds a
child by cannot be cloned. tests/children_cost.rs is the rig that shows it,
and it is the only one here that varies width: 3.680 ms to 0.811 ms at 1,600
children, and flat per child.

Beside it, the rest of the fourteenth sweep of #19: a mask's rectangle
resolved once per fragment instead of once per instance, which takes the
storage buffers out of the fragment stage and is 8.8x on a screenful of
deeply nested clips; TextBuffer::shape copying its attrs before the check
that would not need them, which allocated once per named-family text per
frame; a should_panic test on a debug assertion that made cargo test
--release fail; Fixed::div, reached only by its own test; Moves::remove
re-uploading an array it cannot have changed; and two comments the branch
itself falsified.

docs/LAYOUT_LOG.md has all eight with their measurements, the five things
looked at and left, and what was verified.

core/src/ui/render_state.rs

@@ -1,9 +1,9 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::{
-    ActiveData, Answer, Axis, Bounds, Declared, DrawLayers, IdLike, LayoutHolds, LayoutLen, Len,
-    MaskIdx, MoveIdx, Moves, Painter, PixelRegion, PlaceDesc, PxVec2, Size, StrongWidget, UiRegion,
-    UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    ActiveData, Answer, Axis, Bounds, Declared, DrawId, DrawLayers, IdLike, LayoutHolds, LayoutLen,
+    Len, MaskIdx, MoveIdx, Moves, Painter, PixelRegion, PlaceDesc, PxVec2, Size, StrongWidget,
+    UiRegion, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
     ui::painter::Ask,
     util::{HashMap, Vec2},
 };
@@ -97,6 +97,10 @@ pub struct UiRenderState {
     pending: std::collections::BinaryHeap<(usize, WidgetId)>,
     pub(super) requests: crate::RequestArena,
     changed: Vec<WidgetId>,
+    /// The last draw id handed out. Each draw takes a fresh one and leaves it
+    /// on every widget it asks about, which is how it knows in one read
+    /// whether it has asked already.
+    last_draw: DrawId,
     request_readers: HashMap<WidgetId, crate::util::HashSet<WidgetId>>,
     pub moves: Moves,
 }
@@ -113,6 +117,7 @@ impl UiRenderState {
             pending: Default::default(),
             requests: Default::default(),
             changed: Vec::new(),
+            last_draw: DrawId::NONE,
             request_readers: Default::default(),
             moves: Default::default(),
             resized: false,
@@ -322,6 +327,14 @@ impl UiRenderState {
         old: Option<ActiveData>,
         rsc: &mut dyn UiRsc,
     ) -> Answer {
+        let draw = self.next_draw();
+        // Whoever asked about this widget wrote this, and a draw of the widget
+        // itself is not that: the record is rebuilt below, so it is carried
+        // across rather than reset.
+        let asked_by = old
+            .as_ref()
+            .or_else(|| self.active.get(&id))
+            .map_or(DrawId::NONE, |active| active.asked_by);
         let rel_base = info.rel_base;
         let (move_idx, region, retired_move) = match info.region_node {
             // A node entry is only a translation. Its local box keeps the
@@ -383,6 +396,7 @@ impl UiRenderState {
             answer_under: LayoutHolds::ANY,
             depth: info.depth,
             move_idx,
+            draw,
             rsc,
         };
 
@@ -419,6 +433,7 @@ impl UiRenderState {
             layer,
             own_layer: _,
             depth: _,
+            draw: _,
             id,
         } = painter;
 
@@ -507,7 +522,7 @@ impl UiRenderState {
             region.to_px(window),
         );
         for c in &old_children {
-            if !children.contains(c) {
+            if !self.asked_in(*c, draw) {
                 self.undraw_rec(*c, rsc);
             }
         }
@@ -531,7 +546,7 @@ impl UiRenderState {
         // and a change there has to reach it. Asking answered whatever mark
         // it had: a hint is read live, and a drawing is not kept past one.
         for &dep in &size_deps {
-            if !children.contains(&dep) {
+            if !self.asked_in(dep, draw) {
                 self.asked(
                     dep,
                     DrawInfo {
@@ -574,6 +589,7 @@ impl UiRenderState {
             region,
             // Whoever asked writes the answer.
             answer: None,
+            asked_by,
             re_asked: info.re_asked,
             size,
             holds,
@@ -603,6 +619,21 @@ impl UiRenderState {
         }
     }
 
+    fn next_draw(&mut self) -> DrawId {
+        self.last_draw = self.last_draw.next();
+        self.last_draw
+    }
+
+    /// Whether `draw` has asked about this widget, which is what it left on
+    /// the widget's own record when it did. One widget is asked about by one
+    /// container, since the handle a container holds a child by cannot be
+    /// cloned, so one note per widget is enough to answer this.
+    pub(super) fn asked_in(&self, id: WidgetId, draw: DrawId) -> bool {
+        self.active
+            .get(&id)
+            .is_some_and(|active| active.asked_by == draw)
+    }
+
     /// Keeps a region node's entry across redraws because descendants retain
     /// its index.
     fn move_slot(&mut self, id: WidgetId, parent: MoveIdx, region: UiRegion) -> MoveIdx {
@@ -956,6 +987,7 @@ impl UiRenderState {
                 asked: PlaceDesc::WHOLE,
                 region: UiRegion::FULL,
                 answer: None,
+                asked_by: DrawId::NONE,
                 re_asked: false,
                 size,
                 holds: LayoutHolds::ANY,