Fix crash when reloading an instance with an active ReactNativeIsland

change/react-native-windows-2e987ef7-fe10-47ca-9390-fb04d7839be3.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Fix crash when reloading an instance with an active ReactNativeIsland",
+  "packageName": "react-native-windows",
+  "email": "30809111+acoates-ms@users.noreply.github.com",
+  "dependentChangeType": "patch"
+}

packages/playground/Samples/simple.tsx

@@ -6,17 +6,15 @@
 import React from 'react';
 import {AppRegistry, View} from 'react-native';
 
-export default class Bootstrap extends React.Component {
-  render() {
-    return (
-      <View
-        accessible={true}
-        accessibilityValue={{now: 10, min: 0, max: 20}}
-        style={{borderRadius: 30, width: 60, height: 60, margin: 10}}>
-        <View style={{backgroundColor: 'magenta', width: 60, height: 60}} />
-      </View>
-    );
-  }
-}
+const Bootstrap = () => {
+  return (
+    <View
+      accessible={true}
+      accessibilityValue={{now: 10, min: 0, max: 20}}
+      style={{borderRadius: 30, width: 60, height: 60, margin: 10}}>
+      <View style={{backgroundColor: 'magenta', width: 60, height: 60}} />
+    </View>
+  );
+};
 
 AppRegistry.registerComponent('Bootstrap', () => Bootstrap);

vnext/Microsoft.ReactNative/Fabric/Composition/ReactNativeIsland.cpp

@@ -520,13 +520,6 @@ void ReactNativeIsland::UpdateRootViewInternal() noexcept {
   }
 }
 
-struct AutoMRE {
-  ~AutoMRE() {
-    mre.Set();
-  }
-  Mso::ManualResetEvent mre;
-};
-
 void ReactNativeIsland::UninitRootView() noexcept {
   if (!m_isInitialized) {
     return;
@@ -539,17 +532,6 @@ void ReactNativeIsland::UninitRootView() noexcept {
     auto uiManager = ::Microsoft::ReactNative::FabricUIManager::FromProperties(
         winrt::Microsoft::ReactNative::ReactPropertyBag(m_context.Properties()));
     uiManager->stopSurface(static_cast<facebook::react::SurfaceId>(RootTag()));
-
-    // This is needed to ensure that the unmount JS logic is completed before the the instance is shutdown during
-    // instance destruction. Aligns with similar code in ReactInstanceWin::DetachRootView for paper Future: Instead
-    // this method should return a Promise, which should be resolved when the JS logic is complete. The task will auto
-    // set the event on destruction to ensure that the event is set if the JS Queue has already been shutdown
-    Mso::ManualResetEvent mre;
-    m_context.JSDispatcher().Post([autoMRE = std::make_unique<AutoMRE>(AutoMRE{mre})]() {});
-    mre.Wait();
-
-    // Paper version gives the JS thread time to finish executing - Is this needed?
-    // m_jsMessageThread.Load()->runOnQueueSync([]() {});
   }
 
   m_rootTag = -1;