Articulate the cancel behavior in WebMouseWheelEvent.

content/renderer/input/main_thread_event_queue.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/input/main_thread_event_queue.h"
 
 namespace content {
 
 MainThreadEventQueue::MainThreadEventQueue(int routing_id,
                                            MainThreadEventQueueClient* client)
@@ skipping 12 matching lines @@
          ack_result == INPUT_EVENT_ACK_STATE_NOT_CONSUMED);
 
   bool non_blocking = original_dispatch_type == DISPATCH_TYPE_NON_BLOCKING ||
                       ack_result == INPUT_EVENT_ACK_STATE_SET_NON_BLOCKING;
 
   InputEventDispatchType dispatch_type =
       non_blocking ? DISPATCH_TYPE_NON_BLOCKING_NOTIFY_MAIN
                    : 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)) {
     PendingTouchEvent modified_dispatch_type_event =
         PendingTouchEvent(*static_cast<const blink::WebTouchEvent*>(event),
                           latency, dispatch_type);
 
     // Adjust the |dispatchType| on the event since it was
     // adjusted by the compositor.
     if (non_blocking) {
@@ skipping 43 matching lines @@
                                      event->type);
     } else {
       touch_events_.set_state(WebInputEventQueueState::ITEM_NOT_PENDING);
     }
   } else {
     NOTREACHED() << "Invalid passive event type";
   }
 }
 
 }  // namespace content