Remove sync call from DetachRootView to fix deadlock, add quirk option to re-enable if needed

Co-authored-by: anupriya13 <54227869+anupriya13@users.noreply.github.com>

Author: Copilot

change/remove-sync-detach-20250905060318.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Remove sync call from DetachRootView to fix deadlock, add quirk option to re-enable if needed",
+  "packageName": "@rnw-dependencies",
+  "email": "noreply@microsoft.com",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative/ReactHost/React.h

@@ -251,6 +251,12 @@ struct ReactOptions {
   //! It is not safe to expose to Custom Function. Add this flag so we can turn it off for Custom Function.
   bool EnableNativePerformanceNow{true};
 
+  //! QUIRK: Enable synchronous wait in DetachRootView for JS thread completion.
+  //! This flag re-enables the legacy runOnQueueSync call that can cause deadlocks
+  //! with Hermes GC. Only enable if removal of sync behavior breaks your application.
+  //! Default: false (sync call is disabled to prevent deadlocks)
+  bool EnableSyncDetachRootView{false};
+
   ReactDevOptions DeveloperSettings = {};
 
   //! This controls the availability of various developer support functionality including

vnext/Microsoft.ReactNative/ReactHost/ReactInstanceWin.cpp

@@ -1514,15 +1514,13 @@ void ReactInstanceWin::DetachRootView(facebook::react::IReactRootView *rootView,
     CallJsFunction("AppRegistry", "unmountApplicationComponentAtRootTag", std::move(params));
   }
 
-  // Schedule an async task on the JS thread to ensure previous operations complete
-  // This avoids the deadlock that can occur with runOnQueueSync when the JS thread
-  // is busy with garbage collection or other operations
-  if (auto jsMessageThread = m_jsMessageThread.Load()) {
-    jsMessageThread->runOnQueue([]() {
-      // This async operation ensures that any previously queued JS operations
-      // (like the unmount call above) have a chance to complete
-    });
+  // QUIRK: Legacy sync behavior can be re-enabled via EnableSyncDetachRootView option
+  // The sync call was removed to prevent deadlocks with Hermes GC operations
+  if (m_options.EnableSyncDetachRootView) {
+    // Legacy behavior: wait for JS thread to finish executing (can cause deadlocks)
+    m_jsMessageThread.Load()->runOnQueueSync([]() {});
   }
+  // Default behavior: no synchronization call to prevent deadlocks
 }
 
 Mso::CntPtr<IReactInstanceInternal> MakeReactInstance(