From 409f114d685ea76984d5d1d115ef1be7adeba339 Mon Sep 17 00:00:00 2001
From: Rohan Kulkarni <rohankulkarni@meta.com>
Date: Thu, 13 Aug 2026 10:47:32 -0700
Subject: [PATCH] Fix maintainVisibleContentPosition with rapid data updates
 (#53542)

Fixes #53542 where rapid prepends with coalesced native scroll events leave pendingScrollUpdateCount stuck, freezing render window.

Root cause: pendingScrollUpdateCount increments by 1 per prepend (0->1->2) but EventQueue coalesces N scroll events into 1 via dispatchUniqueEvent (ScrollViewEventEmitter.cpp:13 + EventQueue.cpp:29-50 replaces in place), so single scroll drains 2->1 leaving blocked. Also leak when JS predicate fires but native declines (tag recycled, delta<=0.5, clamped).

Fix: Clamp pending to 1 (not accumulate) and drain to 0 on any scroll, so single coalesced event unblocks. Prevents frozen window, suppressed onViewableItemsChanged and onEndReached.

[GENERAL] [FIXED] - Fix maintainVisibleContentPosition with rapid data updates (#53542)
---
 .../Lists/VirtualizedList.js                  |  6 +-
 .../Lists/__tests__/VirtualizedList-test.js   | 77 +++++++++++++++++++
 2 files changed, 79 insertions(+), 4 deletions(-)

diff --git a/packages/virtualized-lists/Lists/VirtualizedList.js b/packages/virtualized-lists/Lists/VirtualizedList.js
index a8593403e4cd..8545ba4c6ff8 100644
--- a/packages/virtualized-lists/Lists/VirtualizedList.js
+++ b/packages/virtualized-lists/Lists/VirtualizedList.js
@@ -777,7 +777,7 @@ class VirtualizedList extends StateSafePureComponent<
       firstVisibleItemKey: newFirstVisibleItemKey,
       pendingScrollUpdateCount:
         maintainVisibleContentPositionAdjustment != null
-          ? prevState.pendingScrollUpdateCount + 1
+          ? 1
           : prevState.pendingScrollUpdateCount,
     };
   }
@@ -1760,9 +1760,7 @@ class VirtualizedList extends StateSafePureComponent<
       zoomScale,
     };
     if (this.state.pendingScrollUpdateCount > 0) {
-      this.setState<'pendingScrollUpdateCount'>(state => ({
-        pendingScrollUpdateCount: state.pendingScrollUpdateCount - 1,
-      }));
+      this.setState<'pendingScrollUpdateCount'>({pendingScrollUpdateCount: 0});
     }
     this._updateViewableItems(this.props, this.state.cellsAroundViewport);
     if (!this.props) {
diff --git a/packages/virtualized-lists/Lists/__tests__/VirtualizedList-test.js b/packages/virtualized-lists/Lists/__tests__/VirtualizedList-test.js
index 6547145e9201..661be1b1f83c 100644
--- a/packages/virtualized-lists/Lists/__tests__/VirtualizedList-test.js
+++ b/packages/virtualized-lists/Lists/__tests__/VirtualizedList-test.js
@@ -2877,6 +2877,83 @@ it('maintainVisibleContentPosition with inverted VirtualizedList handles prepend
   expect(anchorAfterPrepend).toBeLessThanOrEqual(anchorBeforePrepend + 10);
 });
 
+it('handles rapid prepends with coalesced scroll event (regression for #53542)', 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},
+    });
+    simulateScroll(component, {x: 0, y: 50});
+    performAllBatches();
+  });
+
+  const afterFirstPrepend = [...generateItems(5, items.length), ...items];
+  const afterSecondPrepend = [
+    ...generateItems(3, afterFirstPrepend.length),
+    ...afterFirstPrepend,
+  ];
+
+  // Two rapid prepends WITHOUT intermediate scroll (coalesced native event)
+  await act(() => {
+    component.update(
+      <VirtualizedList
+        initialNumToRender={1}
+        windowSize={1}
+        maintainVisibleContentPosition={{minIndexForVisible: 0}}
+        {...baseItemProps(afterFirstPrepend)}
+        {...fixedHeightItemLayoutProps(ITEM_HEIGHT)}
+      />,
+    );
+  });
+
+  await act(() => {
+    component.update(
+      <VirtualizedList
+        initialNumToRender={1}
+        windowSize={1}
+        maintainVisibleContentPosition={{minIndexForVisible: 0}}
+        {...baseItemProps(afterSecondPrepend)}
+        {...fixedHeightItemLayoutProps(ITEM_HEIGHT)}
+      />,
+    );
+  });
+
+  // Only ONE coalesced scroll event for both prepends (delta 8*ITEM_HEIGHT)
+  // This simulates EventQueue coalescing: dispatchUniqueEvent replaces previous scroll
+  // Previously: pending 0→1→2, scroll 2→1 (still blocked). Now: 0→1→1→0 (unblocked)
+  await act(() => {
+    simulateContentLayout(component, {
+      width: 10,
+      height: afterSecondPrepend.length * ITEM_HEIGHT,
+    });
+    simulateScroll(component, {x: 0, y: 50 + 8 * ITEM_HEIGHT});
+    performAllBatches();
+  });
+
+  // Pending should be 0, not 1 – this fails on main before fix
+  expect(component.getInstance().state.pendingScrollUpdateCount).toBe(0);
+  expect(component.getInstance().state.firstVisibleItemKey).not.toBeNull();
+  expect(
+    component.getInstance().state.cellsAroundViewport.first,
+  ).toBeGreaterThanOrEqual(0);
+});
+
 function generateItems(count, startKey = 0) {
   return Array(count)
     .fill()