Handle touchpad flings with passive event listeners on compositor thread.

ui/events/blink/input_handler_proxy_client.h

 // 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.
 
 #ifndef UI_EVENTS_BLINK_INPUT_HANDLER_PROXY_CLIENT_H_
 #define UI_EVENTS_BLINK_INPUT_HANDLER_PROXY_CLIENT_H_
 
+#include "ui/events/blink/scoped_web_input_event.h"
+
 namespace blink {
 class WebGestureCurve;
 struct WebActiveWheelFlingParameters;
 struct WebFloatPoint;
 struct WebSize;
 }
 
 namespace ui {
 
 // All callbacks invoked from the compositor thread.
 class InputHandlerProxyClient {
  public:
   // Called just before the InputHandlerProxy shuts down.
   virtual void WillShutdown() = 0;
 
   // Transfers an active wheel fling animation initiated by a previously
   // handled input event out to the client.
   virtual void TransferActiveWheelFlingAnimation(
       const blink::WebActiveWheelFlingParameters& params) = 0;
 
+  // Gesture flings from touchpads are processed via a gesture curve
+  // animation on the compositor thread, and these events generate MouseWheel
+  // events which need to propagate to the main thread. If there are blocking
+  // event listeners the entire fling animation is moved to the main thread
+  // via |TransferActiveWheelFlingAnimation|.

[tdresser, 2016/09/14 18:58:08]

This comment doesn't obviously connect with the method its commenting on. Can
you make the connection clearer?

[dtapuska, 2016/09/15 20:43:04]

Done.

+  virtual void DispatchNonBlockingEventToMainThread(
+      ui::ScopedWebInputEvent event) = 0;
+
   // Creates a new fling animation curve instance for device |device_source|
   // with |velocity| and already scrolled |cumulative_scroll| pixels.
   virtual blink::WebGestureCurve* CreateFlingAnimationCurve(
       blink::WebGestureDevice device_source,
       const blink::WebFloatPoint& velocity,
       const blink::WebSize& cumulative_scroll) = 0;
 
   // |HandleInputEvent/WithLatencyInfo| will respond to overscroll by calling
   // the passed in callback.
   // Otherwise |DidOverscroll| will be fired.
   virtual void DidOverscroll(
       const gfx::Vector2dF& accumulated_overscroll,
       const gfx::Vector2dF& latest_overscroll_delta,
       const gfx::Vector2dF& current_fling_velocity,
       const gfx::PointF& causal_event_viewport_point) = 0;
 
   virtual void DidStartFlinging() = 0;
 
   virtual void DidStopFlinging() = 0;
 
   virtual void DidAnimateForInput() = 0;
 
  protected:
   virtual ~InputHandlerProxyClient() {}
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_BLINK_INPUT_HANDLER_PROXY_CLIENT_H_