--- a/storage/browser/file_system/sandbox_file_system_backend_delegate.cc	2026-05-25 00:56:09.636682003 +0000
+++ b/storage/browser/file_system/sandbox_file_system_backend_delegate.cc	2026-06-06 00:26:01.813353579 +0000
@@ -441,8 +441,22 @@
 #if DCHECK_IS_ON()
   DCHECK(!is_filesystem_opened_ || io_thread_checker_.CalledOnValidThread());
 #endif
-  change_observers_[type] =
+  // ---------------------------------------------------------------------------
+  // Race-window amplification for the data race against GetChangeObservers().
+  // The original code performs the unsynchronised read-modify-write below
+  // exactly once.  Because the IO-thread reader hits this map for only a few
+  // hundred nanoseconds per OPFS operation, we repeat the same unsynchronised
+  // assignment in a tight loop so the IO thread has a realistic chance of
+  // copying the ChangeObserverList while it is being reassigned.  The data
+  // race is unchanged; only its duration is extended.
+  // ---------------------------------------------------------------------------
+  ChangeObserverList added =
       change_observers_[type].AddObserver(std::move(observer), task_runner);
+  for (int i = 0; i < 5000000; ++i) {
+    change_observers_[type] = added;
+    change_observers_[type] = ChangeObserverList();
+  }
+  change_observers_[type] = added;
 }
 
 void SandboxFileSystemBackendDelegate::AddFileAccessObserver(