Remove sync call from DetachRootView to fix deadlock, add public QuirkSettings API for backward compatibility

change/1756980197-fix-deadlock.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Fix deadlock in ReactInstanceWin::DetachRootView by replacing sync call with async",
+  "packageName": "react-native-windows",
+  "email": "copilot@github.com",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative/QuirkSettings.cpp

@@ -77,6 +77,12 @@ winrt::Microsoft::ReactNative::ReactPropertyId<bool> UseRuntimeSchedulerProperty
   return propId;
 }
 
+winrt::Microsoft::ReactNative::ReactPropertyId<bool> UseRunOnQueueSyncProperty() noexcept {
+  winrt::Microsoft::ReactNative::ReactPropertyId<bool> propId{L"ReactNative.QuirkSettings", L"UseRunOnQueueSync"};
+
+  return propId;
+}
+
 #pragma region IDL interface
 
 /*static*/ void QuirkSettings::SetMatchAndroidAndIOSStretchBehavior(
@@ -121,6 +127,12 @@ winrt::Microsoft::ReactNative::ReactPropertyId<bool> UseRuntimeSchedulerProperty
   ReactPropertyBag(settings.Properties()).Set(UseRuntimeSchedulerProperty(), value);
 }
 
+/*static*/ void QuirkSettings::SetUseRunOnQueueSync(
+    winrt::Microsoft::ReactNative::ReactInstanceSettings settings,
+    bool value) noexcept {
+  ReactPropertyBag(settings.Properties()).Set(UseRunOnQueueSyncProperty(), value);
+}
+
 #pragma endregion IDL interface
 
 /*static*/ bool QuirkSettings::GetMatchAndroidAndIOSStretchBehavior(ReactPropertyBag properties) noexcept {
@@ -153,4 +165,8 @@ winrt::Microsoft::ReactNative::ReactPropertyId<bool> UseRuntimeSchedulerProperty
   return properties.Get(UseRuntimeSchedulerProperty()).value_or(true);
 }
 
+/*static*/ bool QuirkSettings::GetUseRunOnQueueSync(ReactPropertyBag properties) noexcept {
+  return properties.Get(UseRunOnQueueSyncProperty()).value_or(false);
+}
+
 } // namespace winrt::Microsoft::ReactNative::implementation

vnext/Microsoft.ReactNative/QuirkSettings.h

@@ -30,6 +30,7 @@ struct QuirkSettings : QuirkSettingsT<QuirkSettings> {
       winrt::Microsoft::ReactNative::ReactPropertyBag properties) noexcept;
   static bool GetSuppressWindowFocusOnViewFocus(winrt::Microsoft::ReactNative::ReactPropertyBag properties) noexcept;
   static bool GetUseRuntimeScheduler(winrt::Microsoft::ReactNative::ReactPropertyBag properties) noexcept;
+  static bool GetUseRunOnQueueSync(winrt::Microsoft::ReactNative::ReactPropertyBag properties) noexcept;
 
   static bool GetEnableFabric(winrt::Microsoft::ReactNative::ReactPropertyBag properties) noexcept;
 
@@ -65,6 +66,8 @@ struct QuirkSettings : QuirkSettingsT<QuirkSettings> {
       winrt::Microsoft::ReactNative::ReactInstanceSettings settings,
       bool value) noexcept;
 
+  static void SetUseRunOnQueueSync(winrt::Microsoft::ReactNative::ReactInstanceSettings settings, bool value) noexcept;
+
 #pragma endregion Public API - part of IDL interface
 };
 

vnext/Microsoft.ReactNative/QuirkSettings.idl

@@ -64,6 +64,11 @@ namespace Microsoft.ReactNative
         "By default `react-native-windows` will use the new RuntimeScheduler."
         "Setting this to false will revert the behavior to previous scheduling logic.")
     static void SetUseRuntimeScheduler(ReactInstanceSettings settings, Boolean value);
+
+    DOC_STRING(
+      "By default `react-native-windows` will not call sync JS call: runOnQueueSync."
+      "Setting this to true will revert the behavior to previous logic and might result in hang.")
+    static void SetUseRunOnQueueSync(ReactInstanceSettings settings, Boolean value);
   }
 
 } // namespace Microsoft.ReactNative

vnext/Microsoft.ReactNative/ReactHost/ReactInstanceWin.cpp

@@ -1514,8 +1514,15 @@ void ReactInstanceWin::DetachRootView(facebook::react::IReactRootView *rootView,
     CallJsFunction("AppRegistry", "unmountApplicationComponentAtRootTag", std::move(params));
   }
 
-  // Give the JS thread time to finish executing
-  m_jsMessageThread.Load()->runOnQueueSync([]() {});
+  // QUIRK: Legacy sync behavior can be re-enabled via EnableSyncDetachRootView option
+  // The sync call was removed to prevent deadlocks with Hermes GC operations
+  bool useSyncCall = winrt::Microsoft::ReactNative::implementation::QuirkSettings::GetUseRunOnQueueSync(
+      winrt::Microsoft::ReactNative::ReactPropertyBag(m_reactContext->Properties()));
+  if (useSyncCall) {
+    // 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(