Async Wheel Event with only the first one cancellable behind a flag.

ui/events/blink/input_handler_proxy.cc

 // Copyright 2013 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 "ui/events/blink/input_handler_proxy.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 648 matching lines @@
 }
 
 InputHandlerProxy::EventDisposition InputHandlerProxy::HandleMouseWheel(
     const WebMouseWheelEvent& wheel_event) {
   // Only call |CancelCurrentFling()| if a fling was active, as it will
   // otherwise disrupt an in-progress touch scroll.
   if (!wheel_event.has_precise_scrolling_deltas && fling_curve_)
     CancelCurrentFling();
 
   InputHandlerProxy::EventDisposition result = DROP_EVENT;
+
+  if (wheel_event.dispatch_type == WebInputEvent::kEventNonBlocking) {
+    // The first wheel event in the sequence should be cancellable.
+    DCHECK(wheel_event.phase != WebMouseWheelEvent::kPhaseBegan);
+
+    DCHECK(mouse_wheel_result_ != kEventDispositionUndefined);
+    result = static_cast<EventDisposition>(mouse_wheel_result_);
+
+    if (wheel_event.phase == WebMouseWheelEvent::kPhaseEnded ||
+        wheel_event.phase == WebMouseWheelEvent::kPhaseCancelled ||
+        wheel_event.momentum_phase == WebMouseWheelEvent::kPhaseEnded ||
+        wheel_event.momentum_phase == WebMouseWheelEvent::kPhaseCancelled) {
+      mouse_wheel_result_ = kEventDispositionUndefined;
+    }
+
+    return result;

[dtapuska, 2017/06/20 20:53:14]

So should we really be sending back undefined? ie; in the release version. What
happens?

The touch path will force a hit test if it is undefined. Do we want to send this
to main?

[sahel, 2017/06/21 18:09:59]

Done. I changed it to follow the blocking path if the result is
undefined. I can return DID_NOT_HANDLE directly if you prefer it. 

+  }
+
   cc::EventListenerProperties properties =
       input_handler_->GetEventListenerProperties(
           cc::EventListenerClass::kMouseWheel);
   switch (properties) {
     case cc::EventListenerProperties::kPassive:
       result = DID_HANDLE_NON_BLOCKING;
       break;
     case cc::EventListenerProperties::kBlockingAndPassive:
     case cc::EventListenerProperties::kBlocking:
       result = DID_NOT_HANDLE;
@@ skipping 982 matching lines @@
                  scroll_elasticity_controller_->GetWeakPtr(), gesture_event,
                  scroll_result));
 }
 
 void InputHandlerProxy::SetTickClockForTesting(
     std::unique_ptr<base::TickClock> tick_clock) {
   tick_clock_ = std::move(tick_clock);
 }
 
 }  // namespace ui