Hand a redrawn widget the mask it inherited, not its own

`ActiveData::mask` is the mask a widget's drawing is clipped to, which is
either one it set itself or the one it inherited. `redraw` passed it back as
the *inherited* mask, so a `Masked` widget settled on its own was handed its
own mask and `set_mask` asserted -- a panic on any local redraw of one, for
as long as there has been a local-redraw path. The two are separate facts, so
`parent_mask` keeps the second.

That also states the question `remap_subtree` was asking. It compared a
widget's mask with the one threaded down from its parent to find out whether
the widget owned it; the comparison is now between the two fields on the
widget, which is the same question asked where the answer lives, and the
parameter goes.

Checked: fmt, clippy, 87 suite tests including the new one, which panics
without this; 18 core unit tests; the release oracle at 100 seeds; the
fifteen shrinker cases at 400 seeds of depth 5; and `tabs` renders
byte-identical at 1920x1200.

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

core/src/ui/render_state.rs

@@ -437,6 +437,7 @@ impl UiRenderState {
             move_idx,
             parent_move: info.parent_move,
             mask,
+            parent_mask: info.mask,
             layer: info.layer,
         };
         rsc.on_draw(&active);
@@ -623,18 +624,14 @@ impl UiRenderState {
             return None;
         }
         let moved = active.region != region;
-        let (answer, old_region, slot, mask) = (
-            (active.size, active.holds),
-            active.region,
-            active.move_idx,
-            info.mask,
-        );
+        let (answer, old_region, slot) =
+            ((active.size, active.holds), active.region, active.move_idx);
         if moved {
             if has_region_node {
                 self.moves.set(slot, region);
             } else {
                 let remap = RegionRemap::new(old_region, region)?;
-                self.remap_subtree(id, &remap, info.parent_move, mask, rsc);
+                self.remap_subtree(id, &remap, info.parent_move, rsc);
             }
         }
         let active = self.active.get_mut(&id).unwrap();
@@ -674,7 +671,6 @@ impl UiRenderState {
         id: WidgetId,
         remap: &RegionRemap,
         parent_move: MoveIdx,
-        inherited_mask: MaskIdx,
         rsc: &mut dyn UiRsc,
     ) {
         let active = self.active.get_mut(&id).unwrap();
@@ -690,16 +686,18 @@ impl UiRenderState {
             *region = remap.apply(*region);
         }
         active.region = remap.apply(active.region);
-        let mask = active.mask;
+        let own_mask = (active.mask != active.parent_mask).then_some(active.mask);
         let children = active.children.len();
-        if mask != inherited_mask && mask != MaskIdx::NONE {
-            let mask = rsc.ui_mut().masks.get_mut(mask);
+        // A mask the widget set itself moves with it; one it inherited
+        // belongs to the widget that set it, and moves there or not at all.
+        if let Some(idx) = own_mask {
+            let mask = rsc.ui_mut().masks.get_mut(idx);
             debug_assert_eq!(mask.move_idx, parent_move);
             mask.region = remap.apply(mask.region);
         }
         for index in 0..children {
             let child = self.active[&id].children[index];
-            self.remap_subtree(child, remap, parent_move, mask, rsc);
+            self.remap_subtree(child, remap, parent_move, rsc);
         }
     }
 
@@ -793,6 +791,7 @@ impl UiRenderState {
                 own_align: rsc.widgets().alignment(id),
                 parent_move: info.parent_move,
                 mask: info.mask,
+                parent_mask: info.mask,
                 layer: info.layer,
             },
         );
@@ -963,7 +962,7 @@ impl UiRenderState {
         let Some(parent) = active.parent else {
             let region = Self::root_region(id, rsc.widgets());
             let info = DrawInfo {
-                mask: active.mask,
+                mask: active.parent_mask,
                 ..self.root_info(region)
             };
             #[cfg(feature = "layout-diagnostics")]
@@ -990,7 +989,7 @@ impl UiRenderState {
             depth: active.depth,
             parent_move: active.parent_move,
             region_node: rsc.widgets().is_region_node(id),
-            mask: active.mask,
+            mask: active.parent_mask,
             given_len: active.given_len,
             offer_len: active.offer_len,
             px: given_px,