Prevent default actions for JS-generated mouse events other than click

Source/core/events/MouseEvent.h

 /*
  * 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, 2004, 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 60 matching lines @@
     EventTarget* relatedTarget() const { return m_relatedTarget.get(); }
     void setRelatedTarget(PassRefPtrWillBeRawPtr<EventTarget> relatedTarget) { m_relatedTarget = relatedTarget; }
 
     Node* toElement() const;
     Node* fromElement() const;
 
     DataTransfer* dataTransfer() const { return isDragEvent() ? m_dataTransfer.get() : 0; }
 
     bool fromTouch() const { return m_syntheticEventType == PlatformMouseEvent::FromTouch; }
 
+    bool isSynthetic() const { return m_syntheticEventType == PlatformMouseEvent::Synthetic; }

[Rick Byers, 2015/03/05 16:34:18]

Again this name is confusing. IIRC we recently replaced an 'isSynthetic'
accessor with the m_syntheticEventType system to reduce confusion over different
types of 'synthetic' events.  But now I see MouseEventDispatchMediator has it's
own notion of 'synthetic' that you're mirroring here.  To avoid confusion I
think we should unify the two mechanisms together.  What I'd suggest is this:

 - Remove MouseEventDispatchMediator::m_mouseEventType
 - Update the only place that sets this (Node::dispatchEvent) to instead call
MouseEvent::setFromScript() to reset the m_syntheticEventType to 'FromScript'.
 - For consistency, update the MouseEvent constructor / JS creation code to also
set the synthetic event type to 'FromScript'
 - The comment above the definition of 'FromScript' should make clear that the
event was either created or re-dispatched by JavaScript (we don't want script to
be able to catch a real MouseEvent and dispatch it later as if it was still
real).
 - Now you can just test 'isFromScript' in
MouseEventDispatchMediator::dispatchEvent and in your default handling logic.

+    void markAsSynthetic() { m_syntheticEventType = PlatformMouseEvent::Synthetic; }
+
     virtual const AtomicString& interfaceName() const override;
 
     virtual bool isMouseEvent() const override;
     virtual bool isDragEvent() const override final;
     virtual int which() const override final;
 
     DECLARE_VIRTUAL_TRACE();
 
 protected:
     MouseEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtrWillBeRawPtr<AbstractView>,
@@ skipping 39 matching lines @@
     virtual bool dispatchEvent(EventDispatcher&) const override;
     bool isSyntheticMouseEvent() const { return m_mouseEventType == SyntheticMouseEvent; }
     MouseEventType m_mouseEventType;
 };
 
 DEFINE_EVENT_TYPE_CASTS(MouseEvent);
 
 } // namespace blink
 
 #endif // MouseEvent_h