From ab7bcd3eb31f74e2e488117d832d73e124eb8d56 Mon Sep 17 00:00:00 2001
From: linsk1998 <linsk@live.com>
Date: Fri, 5 Jun 2026 13:41:01 +0800
Subject: [PATCH] fix: use == null instead of === undefined for window.event
 check in resolveUpdatePriority

window.event returns null in IE10, not just undefined in IE11. Using === undefined misses the null case, causing getEventPriority(null.type) to throw. Changed to == null to cover both null and undefined.
---
 .../react-dom-bindings/src/client/ReactDOMUpdatePriority.js    | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/packages/react-dom-bindings/src/client/ReactDOMUpdatePriority.js b/packages/react-dom-bindings/src/client/ReactDOMUpdatePriority.js
index 4f422a06732e..310b1398e218 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMUpdatePriority.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMUpdatePriority.js
@@ -38,7 +38,8 @@ export function resolveUpdatePriority(): EventPriority {
     return updatePriority;
   }
   const currentEvent = window.event;
-  if (currentEvent === undefined) {
+  // IE11 is undefined, IE10 is null
+  if (currentEvent == null) {
     return DefaultEventPriority;
   }
   return getEventPriority(currentEvent.type);