Articulate the cancel behavior in WebMouseWheelEvent.

content/renderer/input/main_thread_event_queue.cc

Index: content/renderer/input/main_thread_event_queue.cc
diff --git a/content/renderer/input/main_thread_event_queue.cc b/content/renderer/input/main_thread_event_queue.cc
index 10629f5593e3e5a7028b56e02a181aa9e221a275..2a0acffe9574d8aa4cc891654cf1a1baa2e0273d 100644
--- a/content/renderer/input/main_thread_event_queue.cc
+++ b/content/renderer/input/main_thread_event_queue.cc
@@ -30,21 +30,33 @@ bool MainThreadEventQueue::HandleEvent(
                    : DISPATCH_TYPE_BLOCKING_NOTIFY_MAIN;
 
   if (event->type == blink::WebInputEvent::MouseWheel) {
+    PendingMouseWheelEvent modified_dispatch_type_event =
+        PendingMouseWheelEvent(
+            *static_cast<const blink::WebMouseWheelEvent*>(event), latency,
+            dispatch_type);
+
+    // Adjust the |dispatchType| on the event since it was
+    // adjusted by the compositor.

[tdresser, 2016/04/20 13:20:10]

Maybe "since the compositor determined all event listeners are passive."?

[dtapuska, 2016/04/20 14:03:23]

Done.

+    if (non_blocking) {
+      modified_dispatch_type_event.event.dispatchType =
+          blink::WebInputEvent::ListenersNonBlockingPassive;
+    }
+
     if (wheel_events_.state() == WebInputEventQueueState::ITEM_PENDING) {
-      wheel_events_.Queue(PendingMouseWheelEvent(
-          *static_cast<const blink::WebMouseWheelEvent*>(event), latency,
-          dispatch_type));
+      wheel_events_.Queue(modified_dispatch_type_event);
     } else {
       if (non_blocking) {
         wheel_events_.set_state(WebInputEventQueueState::ITEM_PENDING);
-        client_->SendEventToMainThread(routing_id_, event, latency,
-                                       dispatch_type);
+        client_->SendEventToMainThread(routing_id_,
+                                       &modified_dispatch_type_event.event,
+                                       latency, dispatch_type);
       } else {
         // If there is nothing in the event queue and the event is
         // blocking pass the |original_dispatch_type| to avoid
         // having the main thread call us back as an optimization.
-        client_->SendEventToMainThread(routing_id_, event, latency,
-                                       original_dispatch_type);
+        client_->SendEventToMainThread(routing_id_,
+                                       &modified_dispatch_type_event.event,
+                                       latency, original_dispatch_type);
       }
     }
   } else if (blink::WebInputEvent::isTouchEventType(event->type)) {