Give a child a part of the container's extent rather than its raw box

`Pad` and `Stack` read `Painter::placement` to put their children inside
their own drawing, and reading it is what says the drawing holds for that
placement alone. So a pad or a stack anywhere in a row was drawn again --
with its whole subtree -- the moment an earlier sibling changed length,
however little else had moved.

`widget_within` now takes a `DrawRegion`, and `DrawRegion::Extent(part)`
gives the child a part of the extent without reading it. What is retained
is the part rather than the box it resolved to, so moving the extent
re-places the child through the same rule instead of redrawing the parent:
`inherited_children` becomes `extent_children`, carrying `Inherit` for the
wrapper case `Painter::widget` already had and `Within(part)` for the new
one.

The dependency that goes up is a range on the container's extent rather
than on its frame, since only the part's *length* reaches the child and
where the part sits is re-placed. A declared length is unchanged: it is a
length of the frame wherever the box it sits in came from. What still pins
the placement is a report with a fraction in it -- the same fraction of a
different extent is a different length -- and that pin is on the answer,
which `extent_frames_keep_fractional_reports_and_numeric_dependencies_valid`
fails without.

Three tests from the first attempt at this come with it, and the
diagnostics rig now says which of the three contracts refused a reuse,
which is what found the above.

Measured, seed 1 at depth 8, median frame: `many` 0.667 -> 0.613 ms and
`resize` 48 -> 32 us; seed 13's `many` 6.35 -> 5.15 ms. Green: fmt, clippy,
109 suite and 20 core tests, the oracle at 100 seeds, the shrinker at 400
trees of depth 5, 1000 seeds at depth 6, and 2000 seeds at depth 4 over all
fifteen cases. The five reference renders are byte-identical to `0e107f0`
on Venus, as are `tabs` resized to 900x1200 and `random` to 1280x800
against cold renders there.

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

core/src/ui/render_state.rs

@@ -288,7 +288,7 @@ impl UiRenderState {
             && let Some(old_parent) = self.active.get_mut(&old_parent)
         {
             old_parent.children.retain(|child| *child != id);
-            old_parent.inherited_children.retain(|child| *child != id);
+            old_parent.extent_children.retain(|(child, _)| *child != id);
         }
         (answer.0, answer.1, settled.1)
     }
@@ -361,7 +361,7 @@ impl UiRenderState {
             textures: Vec::new(),
             primitives: Vec::new(),
             mask_region: None,
-            inherited_children: Vec::new(),
+            extent_children: Vec::new(),
             children: Vec::new(),
             offered: Vec::new(),
             offered_px: info.offered_px,
@@ -398,7 +398,7 @@ impl UiRenderState {
             textures,
             primitives,
             mask_region,
-            inherited_children,
+            extent_children,
             extent_own,
             answer_under,
             children,
@@ -503,7 +503,7 @@ impl UiRenderState {
             textures,
             primitives,
             mask_region,
-            inherited_children,
+            extent_children,
             children,
             size_deps,
             declared: declared_lens(rsc.widgets(), id),
@@ -680,6 +680,24 @@ impl UiRenderState {
         if !active.holds.contains(info.px, placement) {
             #[cfg(feature = "layout-diagnostics")]
             {
+                // Which of the three said no, so a frame that redraws more
+                // than it should says where to look. They overlap: a drawing
+                // can be outside two of them at once.
+                let holds = active.holds;
+                if holds.placement.is_some_and(|pinned| pinned != placement) {
+                    diag::bump(Counter::OutsidePlacement);
+                }
+                for axis in AXES {
+                    let n = axis as usize;
+                    if !holds.frame[n].contains(info.px.axis(axis)) {
+                        diag::bump(Counter::OutsideFrame);
+                    }
+                    if !holds.extent[n]
+                        .contains(placement.axis(axis).len().to_px(info.px.axis(axis)))
+                    {
+                        diag::bump(Counter::OutsideExtent);
+                    }
+                }
                 diag::bump(Counter::ReuseOutside);
                 diag::reuse(id, ReuseOutcome::Outside);
             }
@@ -752,19 +770,18 @@ impl UiRenderState {
         }
         let parent_move = active.move_idx;
         let mask = active.mask;
-        let children = active.inherited_children.len();
+        let children = active.extent_children.len();
         for index in 0..children {
-            let child = self.active[&id].inherited_children[index];
+            let (child, extent) = self.active[&id].extent_children[index];
+            let (part, slot) = extent.resolve(placement);
             let active = &self.active[&child];
-            let (child_local, chosen) = ask_box(
-                UiRegion::FULL,
-                active.declared,
-                active.own_align,
-                [Some(placement.x), Some(placement.y)],
-            );
+            let (child_local, chosen) = ask_box(part, active.declared, active.own_align, slot);
+            // What it took of that box is its own answer, which this move did
+            // not ask again: keep the placement it has on any axis this
+            // widget is not the one choosing.
             let child_placement = UiRegion {
-                x: chosen[0].unwrap_or(UiSpan::FULL),
-                y: chosen[1].unwrap_or(UiSpan::FULL),
+                x: chosen[0].unwrap_or(active.placement.x),
+                y: chosen[1].unwrap_or(active.placement.y),
             };
             let child_info = DrawInfo {
                 layer: active.layer,
@@ -924,7 +941,7 @@ impl UiRenderState {
                 textures: Vec::new(),
                 primitives: Vec::new(),
                 mask_region: None,
-                inherited_children: Vec::new(),
+                extent_children: Vec::new(),
                 children: Vec::new(),
                 size_deps: Vec::new(),
                 move_idx: info.parent_move,