[web] Fix potential race condition in ClickDebouncer (flutter#173294)

Based on Gemini's comment:
flutter#173072 (comment)

Author: mdebbar

engine/src/flutter/lib/web_ui/lib/src/engine/pointer_binding.dart

@@ -350,6 +350,12 @@ class ClickDebouncer {
   ///
   /// This method is called asynchronously from [_maybeStartDebouncing].
   void _doStartDebouncing(DomEvent event, List<ui.PointerData> data) {
+    // It's possible that debouncing was canceled between the pointerdown event and the execution
+    // of this method.
+    if (!isDebouncing) {
+      return;
+    }
+
     _state = (
       target: event.target!,
       // The 200ms duration was chosen empirically by testing tapping, mouse

engine/src/flutter/lib/web_ui/test/engine/pointer_binding_test.dart

@@ -2651,6 +2651,44 @@ void _testClickDebouncer({required PointerBinding Function() getBinding}) {
     expect(semanticsActions, isEmpty);
   });
 
+  testWithSemantics('Does not start debouncing if reset before scheduled execution', () async {
+    expect(EnginePlatformDispatcher.instance.semanticsEnabled, isTrue);
+    expect(PointerBinding.clickDebouncer.isDebouncing, isFalse);
+    expect(PointerBinding.clickDebouncer.debugState, isNull);
+
+    final DomElement testElement = createDomElement('flt-semantics');
+    testElement.setAttribute('flt-tappable', '');
+    view.dom.semanticsHost.appendChild(testElement);
+
+    // 1. Trigger _maybeStartDebouncing, which sets _isDebouncing = true and schedules _doStartDebouncing.
+    testElement.dispatchEvent(context.primaryDown());
+
+    // At this point, _isDebouncing is true, but _doStartDebouncing (which sets _state and creates the Timer)
+    // has not yet executed because it was scheduled with Timer.run().
+    expect(PointerBinding.clickDebouncer.isDebouncing, isTrue);
+    expect(PointerBinding.clickDebouncer.debugState, isNull); // _state is still null
+
+    // 2. Simulate a scenario where reset() is called before _doStartDebouncing gets a chance to run.
+    // This could happen due to a hot restart or other lifecycle events.
+    PointerBinding.clickDebouncer.reset();
+
+    // After reset(), _isDebouncing should be false and _state should still be null.
+    expect(PointerBinding.clickDebouncer.isDebouncing, isFalse);
+    expect(PointerBinding.clickDebouncer.debugState, isNull);
+
+    // 3. Allow the scheduled _doStartDebouncing to run. With the fix, it should now check
+    // `!isDebouncing` and return early.
+    await nextEventLoop();
+
+    // Verify that _doStartDebouncing did not proceed to set _state or create a Timer.
+    expect(PointerBinding.clickDebouncer.isDebouncing, isFalse);
+    expect(PointerBinding.clickDebouncer.debugState, isNull);
+
+    // Ensure no events were sent to the framework as debouncing was effectively cancelled.
+    expect(pointerPackets, isEmpty);
+    expect(semanticsActions, isEmpty);
+  });
+
   testWithSemantics('Starts debouncing after event loop', () async {
     expect(EnginePlatformDispatcher.instance.semanticsEnabled, isTrue);
     expect(PointerBinding.clickDebouncer.isDebouncing, isFalse);