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

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

Index: third_party/WebKit/Source/core/input/EventHandler.cpp
diff --git a/third_party/WebKit/Source/core/input/EventHandler.cpp b/third_party/WebKit/Source/core/input/EventHandler.cpp
index d547f455326b37a4db2347ff4e82de9b0b233a70..4d92954a4469511f448ebac08030ac38c5e99130 100644
--- a/third_party/WebKit/Source/core/input/EventHandler.cpp
+++ b/third_party/WebKit/Source/core/input/EventHandler.cpp
@@ -1797,6 +1797,7 @@ WebInputEventResult EventHandler::handleWheelEvent(const PlatformWheelEvent& eve
         // 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);
@@ -1804,13 +1805,20 @@ WebInputEventResult EventHandler::handleWheelEvent(const PlatformWheelEvent& eve
                     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);
+            }
         }
     }