fix(ui): prevent virtual items mounting offscreen

packages/ui/src/components/virtual-item.tsx

@@ -334,7 +334,31 @@ export default function VirtualItem(props: VirtualItemProps) {
       // Ignore measurement failures; IntersectionObserver will correct us.
     }
 
-    intersectionCleanup = subscribeToSharedObserver(wrapperRef, targetRoot, margin, (entry) => {
+    const wrapperEl = wrapperRef
+    intersectionCleanup = subscribeToSharedObserver(wrapperEl, targetRoot, margin, (entry) => {
+      // IntersectionObserver can produce transient false-positives during pane
+      // activation/layout transitions (e.g. `isIntersecting: true` for items far
+      // outside the scroll root). For element roots, prefer explicit rect math.
+      if (targetRoot && !(targetRoot instanceof Document)) {
+        // When rootBounds is null we cannot trust the entry; treat as hidden.
+        if (entry.rootBounds === null) {
+          queueVisibility(false)
+          return
+        }
+        try {
+          const rootRect = (targetRoot as Element).getBoundingClientRect()
+          const visible = shouldRenderByRects({
+            wrapperRect: wrapperEl.getBoundingClientRect(),
+            rootRect: { top: rootRect.top, bottom: rootRect.bottom },
+            margin,
+          })
+          queueVisibility(visible)
+          return
+        } catch {
+          // Fall through to the entry-based heuristic.
+        }
+      }
+
       const nextVisible = shouldRenderEntry(entry)
       queueVisibility(nextVisible)
     })