Events should have an initialized flag.

third_party/WebKit/Source/core/events/Event.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 71 matching lines @@
     : m_type(eventType)
     , m_canBubble(canBubbleArg)
     , m_cancelable(cancelableArg)
     , m_scoped(scoped)
     , m_relatedTargetScoped(relatedTargetScoped)
     , m_propagationStopped(false)
     , m_immediatePropagationStopped(false)
     , m_defaultPrevented(false)
     , m_defaultHandled(false)
     , m_cancelBubble(false)
+    , m_initialized(true)
     , m_isTrusted(false)
     , m_handlingPassive(false)
     , m_eventPhase(0)
     , m_currentTarget(nullptr)
     , m_createTime(convertSecondsToDOMTimeStamp(currentTime()))
     , m_platformTimeStamp(platformTimeStamp)
 {
 }
 
 Event::Event(const AtomicString& eventType, const EventInit& initializer)
     : Event(eventType, initializer.bubbles(), initializer.cancelable(), initializer.scoped(), initializer.relatedTargetScoped(), monotonicallyIncreasingTime())
 {
 }
 
 Event::~Event()
 {
 }
 
 void Event::initEvent(const AtomicString& eventTypeArg, bool canBubbleArg, bool cancelableArg)
 {
     initEvent(eventTypeArg, canBubbleArg, cancelableArg, nullptr);
 }
 
 void Event::initEvent(const AtomicString& eventTypeArg, bool canBubbleArg, bool cancelableArg, EventTarget* relatedTarget)
 {
     if (dispatched())
         return;
 
+    m_initialized = true;
     m_propagationStopped = false;
     m_immediatePropagationStopped = false;
     m_defaultPrevented = false;
     m_isTrusted = false;
 
     m_type = eventTypeArg;
     m_canBubble = canBubbleArg;
     m_cancelable = cancelableArg;
 
     m_relatedTargetScoped = relatedTarget ? true : false;
@@ skipping 227 matching lines @@
 
 DEFINE_TRACE(Event)
 {
     visitor->trace(m_currentTarget);
     visitor->trace(m_target);
     visitor->trace(m_underlyingEvent);
     visitor->trace(m_eventPath);
 }
 
 } // namespace blink