From 30335d9588c1ae5bb1cbd973b0d378ac1c445539 Mon Sep 17 00:00:00 2001
From: biplavbarua <innocentsavage.bb@gmail.com>
Date: Sat, 3 Jan 2026 01:03:09 +0530
Subject: [PATCH] Fix: Guard against re-entry in Scheduler (Issue #17355)

Adds a check for isPerformingWork inside performWorkUntilDeadline. This prevents recursive execution of the work loop when the browser environment (e.g., Firefox) fires message events while the JavaScript main thread is paused by a breakpoint or alert.
---
 packages/scheduler/src/forks/Scheduler.js | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/packages/scheduler/src/forks/Scheduler.js b/packages/scheduler/src/forks/Scheduler.js
index 88239b71067..f25ae19dde0 100644
--- a/packages/scheduler/src/forks/Scheduler.js
+++ b/packages/scheduler/src/forks/Scheduler.js
@@ -500,6 +500,14 @@ const performWorkUntilDeadline = () => {
     // remain true, and we'll continue the work loop.
     let hasMoreWork = true;
     try {
+      if (isPerformingWork) {
+        // A scheduled task triggered a host event (e.g. valid re-entry).
+        // If we process it now, we'll lose key context. Schedulers that
+        // support re-entry should verify that the queue is empty before
+        // processing.
+        // For now, we'll just yield.
+        return;
+      }
       hasMoreWork = flushWork(currentTime);
     } finally {
       if (hasMoreWork) {