Prevent propagation of wheel events to dispatch event in parent iframe.

third_party/WebKit/Source/core/input/EventHandler.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 1779 matching lines @@
 
     if (m_previousWheelScrolledNode)
         m_previousWheelScrolledNode = nullptr;
 
     bool isOverWidget = result.isOverWidget();
 
     if (node) {
         // Figure out which view to send the event to.
         LayoutObject* target = node->layoutObject();
 
+        bool sendDOMEvent = true;
         if (isOverWidget && target && target->isLayoutPart()) {
             if (Widget* widget = toLayoutPart(target)->widget()) {
                 WebInputEventResult result = passWheelEventToWidget(event, *widget);
                 if (result != WebInputEventResult::NotHandled) {
                     setFrameWasScrolledByUser();
                     return result;
                 }
+                // TODO(dtapuska): Remove this once wheel gesture scroll has
+                // been enabled everywhere; as we can just return early.

[Rick Byers, 2016/02/25 16:34:16]

nit: reference the crbug link for that incase anyone reading this wants more
information.

+                // Don't propagate the DOM event into the parent iframe
+                // but do dispatch the scroll event.
+                sendDOMEvent = false;
             }
         }
 
-        RefPtrWillBeRawPtr<Event> domEvent = WheelEvent::create(event, node->document().domWindow());
-        if (!node->dispatchEvent(domEvent)) {
-            setFrameWasScrolledByUser();
-            return eventToEventResult(domEvent, false);
+        if (sendDOMEvent) {
+            RefPtrWillBeRawPtr<Event> domEvent = WheelEvent::create(event, node->document().domWindow());
+            if (!node->dispatchEvent(domEvent)) {
+                setFrameWasScrolledByUser();
+                return eventToEventResult(domEvent, false);
+            }
         }
     }
 
     // We do another check on the frame view because the event handler can run
     // JS which results in the frame getting destroyed.
     view = m_frame->view();
     if (!view)
         return WebInputEventResult::NotHandled;
 
     // Wheel events which do not scroll are used to trigger zooming.
@@ skipping 2152 matching lines @@
 PlatformEvent::Modifiers EventHandler::accessKeyModifiers()
 {
 #if OS(MACOSX)
     return static_cast<PlatformEvent::Modifiers>(PlatformEvent::CtrlKey | PlatformEvent::AltKey);
 #else
     return PlatformEvent::AltKey;
 #endif
 }
 
 } // namespace blink