Fix maintainVisibleContentPosition prepend skipped when batched with a cells update

packages/virtualized-lists/Lists/VirtualizedList.js

@@ -713,18 +713,26 @@ class VirtualizedList extends StateSafePureComponent<
     // first and last could be stale (e.g. if a new, shorter items props is passed in), so we make
     // sure we're rendering a reasonable range here.
     const itemCount = newProps.getItemCount(newProps.data);
-    if (itemCount === prevState.renderMask.numCells()) {
-      return prevState;
-    }
-
-    let maintainVisibleContentPositionAdjustment: ?number = null;
     const prevFirstVisibleItemKey = prevState.firstVisibleItemKey;
     const minIndexForVisible =
       newProps.maintainVisibleContentPosition?.minIndexForVisible ?? 0;
     const newFirstVisibleItemKey =
-      newProps.getItemCount(newProps.data) > minIndexForVisible
+      itemCount > minIndexForVisible
         ? VirtualizedList._getItemKey(newProps, minIndexForVisible)
         : null;
+    // React applies queued state updaters before getDerivedStateFromProps, so
+    // `_updateCellsToRender` may have already sized `renderMask` for the new
+    // data. An unchanged count then doesn't rule out a prepend, so with
+    // maintainVisibleContentPosition the key must be unchanged as well.
+    if (
+      itemCount === prevState.renderMask.numCells() &&
+      (newProps.maintainVisibleContentPosition == null ||
+        newFirstVisibleItemKey === prevFirstVisibleItemKey)
+    ) {
+      return prevState;
+    }
+
+    let maintainVisibleContentPositionAdjustment: ?number = null;
     if (
       newProps.maintainVisibleContentPosition != null &&
       prevFirstVisibleItemKey != null &&

packages/virtualized-lists/Lists/__tests__/VirtualizedList-test.js

@@ -2640,6 +2640,59 @@ it('handles maintainVisibleContentPosition', async () => {
   expect(component).toMatchSnapshot();
 });
 
+// Trigger: Items prepended in the same render as a queued cells update, e.g. when a scroll event takes
+// the high-priority path of `_scheduleCellsToRenderUpdate` and the app updates its data from that event.
+// Expected: The prepend is handled as if it rendered on its own.
+it('handles a maintainVisibleContentPosition prepend batched with a cells update', async () => {
+  const items = generateItems(20);
+  const ITEM_HEIGHT = 10;
+
+  let component;
+  await act(() => {
+    component = create(
+      <VirtualizedList
+        initialNumToRender={1}
+        windowSize={1}
+        maintainVisibleContentPosition={{minIndexForVisible: 0}}
+        {...baseItemProps(items)}
+        {...fixedHeightItemLayoutProps(ITEM_HEIGHT)}
+      />,
+    );
+  });
+
+  await act(() => {
+    simulateLayout(component, {
+      viewport: {width: 10, height: 50},
+      content: {width: 10, height: items.length * ITEM_HEIGHT},
+    });
+
+    performAllBatches();
+  });
+
+  const instance = component.getInstance();
+  expect(instance.state.cellsAroundViewport).toEqual({first: 0, last: 4});
+
+  const newItems = [...generateItems(10, items.length), ...items];
+  await act(() => {
+    instance._updateCellsToRender();
+    component.update(
+      <VirtualizedList
+        initialNumToRender={1}
+        windowSize={1}
+        maintainVisibleContentPosition={{minIndexForVisible: 0}}
+        {...baseItemProps(newItems)}
+        {...fixedHeightItemLayoutProps(ITEM_HEIGHT)}
+      />,
+    );
+  });
+
+  // The render window moves with the previously rendered cells, and the
+  // adjustment is pending until native reports the new offset.
+  expect(instance.state.cellsAroundViewport).toEqual({first: 10, last: 14});
+  expect(instance.state.pendingScrollUpdateCount).toBe(1);
+  expect(instance.state.firstVisibleItemKey).toBe(20);
+});
+
 // Trigger: Item at anchor position removed from data array.
 // Expected: Anchor shifts to next visible item. MVCP captures new anchor's frame, computes delta, adjusts scroll.
 it('handles maintainVisibleContentPosition when anchor moves before minIndexForVisible', async () => {