Enumerate the return value of dispatchEvent so it is clear.

third_party/WebKit/Source/core/events/EventTarget.h

Index: third_party/WebKit/Source/core/events/EventTarget.h
diff --git a/third_party/WebKit/Source/core/events/EventTarget.h b/third_party/WebKit/Source/core/events/EventTarget.h
index 5f56ecb04adcf376f578f59ca61a2f76aede39e3..2d22c16a80e4706f20cf6987a17ec1bc385f16ca 100644
--- a/third_party/WebKit/Source/core/events/EventTarget.h
+++ b/third_party/WebKit/Source/core/events/EventTarget.h
@@ -79,6 +79,23 @@ public:
     OwnPtr<FiringEventIteratorVector> firingEventIterators;
 };
 
+enum class DispatchEventResult {
+    // Event was not canceled by event handler or default event handler.
+    NotCanceled,
+    // Event was canceled by event handler; ie. a script handler calling preventDefault.

[philipj_slow, 2016/02/25 04:21:27]

i.e. is a much more common spelling, in Source/core at least.

[dtapuska, 2016/02/25 14:20:24]

Done.

+    CanceledByEventHandler,
+    // Event was canceled by the default event handler; ie. executing the default action.
+    // This result should be used sparingly as it deviates from the DOM Event Dispatch
+    // model. Default event handlers really shouldn't be invoked inside of dispatch.
+    CanceledByDefaultEventHandler,
+    // Event was canceled but suppressed before dispatched to event handler.
+    // This result should be used sparingly; and likely its usage likely indicates

[philipj_slow, 2016/02/25 04:21:27]

two likely

[dtapuska, 2016/02/25 14:20:24]

Done.

+    // there is potential for a bug. Trusted events may return this code; but untrusted
+    // events likely should always execute the event handler the developer intends to
+    // execute.
+    CanceledBeforeDispatch,
+};
+
 // This is the base class for all DOM event targets. To make your class an
 // EventTarget, follow these steps:
 // - Make your IDL interface inherit from EventTarget.
@@ -138,7 +155,7 @@ public:
     bool removeEventListener(const AtomicString& eventType, PassRefPtrWillBeRawPtr<EventListener>, EventListenerOptions&);
     virtual void removeAllEventListeners();
 
-    bool dispatchEvent(PassRefPtrWillBeRawPtr<Event>);
+    DispatchEventResult dispatchEvent(PassRefPtrWillBeRawPtr<Event>);
 
     // dispatchEventForBindings is intended to only be called from
     // javascript originated calls. This method will validate and may adjust
@@ -156,7 +173,7 @@ public:
     EventListenerVector* getEventListeners(const AtomicString& eventType);
     Vector<AtomicString> eventTypes();
 
-    bool fireEventListeners(Event*);
+    DispatchEventResult fireEventListeners(Event*);
 
     DEFINE_INLINE_VIRTUAL_TRACE() { }
 
@@ -167,7 +184,7 @@ protected:
 
     virtual bool addEventListenerInternal(const AtomicString& eventType, PassRefPtrWillBeRawPtr<EventListener>, const EventListenerOptions&);
     virtual bool removeEventListenerInternal(const AtomicString& eventType, PassRefPtrWillBeRawPtr<EventListener>, const EventListenerOptions&);
-    virtual bool dispatchEventInternal(PassRefPtrWillBeRawPtr<Event>);
+    virtual DispatchEventResult dispatchEventInternal(PassRefPtrWillBeRawPtr<Event>);
 
     // Subclasses should likely not override these themselves; instead, they should subclass EventTargetWithInlineData.
     virtual EventTargetData* eventTargetData() = 0;