Separate measured-answer dependencies from retained drawing validity

core/src/ui/layout_holds.rs

@@ -15,6 +15,25 @@ impl LayoutHolds {
         placement: None,
     };
 
+    pub fn and(self, other: Self) -> Self {
+        debug_assert!(
+            self.placement.is_none()
+                || other.placement.is_none()
+                || self.placement == other.placement
+        );
+        Self {
+            frame: [
+                self.frame[0].and(other.frame[0]),
+                self.frame[1].and(other.frame[1]),
+            ],
+            extent: [
+                self.extent[0].and(other.extent[0]),
+                self.extent[1].and(other.extent[1]),
+            ],
+            placement: self.placement.or(other.placement),
+        }
+    }
+
     pub fn contains(self, px: PxVec2, placement: UiRegion) -> bool {
         self.placement.is_none_or(|old| old == placement)
             && [Axis::X, Axis::Y].into_iter().all(|axis| {