[macOS] Do not block raster thread when shutting down

shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm

@@ -6,6 +6,7 @@
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine_Internal.h"
 
 #include <functional>
+#include <thread>
 
 #include "flutter/fml/synchronization/waitable_event.h"
 #include "flutter/lib/ui/window/platform_message.h"
@@ -668,6 +669,19 @@ - (nonnull NSView*)createWithViewIdentifier:(int64_t)viewId arguments:(nullable
   }
 }
 
+TEST(EngineTest, ThreadSynchronizerNotBlockingRasterThreadAfterShutdown) {
+  FlutterThreadSynchronizer* threadSynchronizer = [[FlutterThreadSynchronizer alloc] init];
+  [threadSynchronizer shutdown];
+
+  std::thread rasterThread([&threadSynchronizer] {
+    [threadSynchronizer performCommit:CGSizeMake(100, 100)
+                               notify:^{
+                               }];
+  });
+
+  rasterThread.join();
+}
+
 }  // namespace flutter::testing
 
 // NOLINTEND(clang-analyzer-core.StackAddressEscape)

shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.mm

@@ -79,6 +79,11 @@ - (void)performCommit:(CGSize)size notify:(nonnull dispatch_block_t)notify {
   fml::AutoResetWaitableEvent event;
   {
     std::unique_lock<std::mutex> lock(_mutex);
+    if (_shuttingDown) {
+      // Engine is shutting down, main thread may be blocked by the engine
+      // waiting for raster thread to finish.
+      return;
+    }
     fml::AutoResetWaitableEvent& e = event;
     _scheduledBlocks.push_back(^{
       notify();