Judge a kept contract against the box it was asked in

A local redraw keeps the narrower guarantee its parent holds when the new
drawing covers it, so widening and narrowing back do not churn the parent.
It checked that guarantee against `placement`, where the answer put the
drawing, rather than `region`, the box the drawing was made in and the box
both contracts are about. The two differ on every axis a widget reported
less than it was offered, so any such widget escalated to its parent every
time its contract widened -- which is the churn the retention exists to
avoid. `resize` and `try_reuse` both already ask about `region`.

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

core/src/ui/render_state.rs

@@ -1102,8 +1102,11 @@ impl UiRenderState {
         {
             active.answer = was_answer;
         }
+        // Against the box it was asked in, which is what both contracts are
+        // about. Where the answer put the drawing is shorter than that
+        // wherever the widget reported less than it was offered.
         if active.holds.covers(was_holds)
-            && was_holds.contains(self.output_size, active.rel_base, active.placement)
+            && was_holds.contains(self.output_size, active.rel_base, active.region)
         {
             active.holds = was_holds;
         }