InitEvent should not do anything if dispatch flag is set.

third_party/WebKit/Source/core/events/MouseEvent.cpp

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2005, 2006, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 163 matching lines @@
     }
     ASSERT_NOT_REACHED();
     return 0;
 }
 
 void MouseEvent::initMouseEvent(ScriptState* scriptState, const AtomicString& type, bool canBubble, bool cancelable, AbstractView* view,
                                 int detail, int screenX, int screenY, int clientX, int clientY,
                                 bool ctrlKey, bool altKey, bool shiftKey, bool metaKey,
                                 short button, EventTarget* relatedTarget, unsigned short buttons)
 {
-    if (dispatched())
+    if (isBeingDispatched())
         return;
 
     if (scriptState && scriptState->world().isIsolatedWorld())
         UIEventWithKeyState::didCreateEventInIsolatedWorld(ctrlKey, altKey, shiftKey, metaKey);
 
     initModifiers(ctrlKey, altKey, shiftKey, metaKey);
     initMouseEventInternal(type, canBubble, cancelable, view, detail, screenX, screenY, clientX, clientY, modifiers(), button, relatedTarget, nullptr, buttons);
 }
 
 void MouseEvent::initMouseEventInternal(const AtomicString& type, bool canBubble, bool cancelable, AbstractView* view,
@@ skipping 111 matching lines @@
     doubleClickEvent->setTrusted(mouseEvent.isTrusted());
     if (mouseEvent.defaultHandled())
         doubleClickEvent->setDefaultHandled();
     DispatchEventResult doubleClickDispatchResult = EventDispatcher::dispatchEvent(dispatcher.node(), MouseEventDispatchMediator::create(doubleClickEvent));
     if (doubleClickDispatchResult != DispatchEventResult::NotCanceled)
         return doubleClickDispatchResult;
     return dispatchResult;
 }
 
 } // namespace blink