Wheel scroll latching enabled behind flag.

content/browser/renderer_host/input/mouse_wheel_event_queue.h

 // 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.
 
 #ifndef CONTENT_BROWSER_RENDERER_HOST_INPUT_MOUSE_WHEEL_EVENT_QUEUE_H_
 #define CONTENT_BROWSER_RENDERER_HOST_INPUT_MOUSE_WHEEL_EVENT_QUEUE_H_
 
 #include <deque>
 
 #include "base/time/time.h"
 #include "content/browser/renderer_host/event_with_latency_info.h"
 #include "content/common/content_export.h"
 #include "content/common/input/input_event_ack_state.h"
 #include "third_party/WebKit/public/web/WebInputEvent.h"
 
 namespace content {
 
 // The duration in which a ScrollEnd will be sent after the last
 // ScrollUpdate was sent for wheel based gesture scrolls.
 // Set the default wheel transaction to 0ms until
 // crbug.com/526463 is fully implemented.
-const int64_t kDefaultWheelScrollTransactionMs = 0;  // 100;
+const int64_t kDefaultWheelScrollTransactionMs = 0;
+const int64_t kDefaultWheelScrollLatchingTransactionMs = 100;

[tdresser, 2016/07/22 15:43:15]

What's the difference between these constants? Why do we need a transaction time
for Mac?

[sahel, 2016/07/25 15:54:53]

This is for handling the cases that phase info is not available. e.g. Wheel
scroll events On Mac.
Maybe I should change the title of the patch, so that it represents both
latching cases.

[tdresser, 2016/07/25 16:46:55]

We don't need a timeout when phase information is available.

I think we just need one number, the transaction timeout for wheel. If we aren't
latching for wheel, then we'll use 0ms, otherwise, we'll use the value of the
flag.

 
 class QueuedWebMouseWheelEvent;
 
 // Interface with which MouseWheelEventQueue can forward mouse wheel events,
 // and dispatch mouse wheel event responses.
 class CONTENT_EXPORT MouseWheelEventQueueClient {
  public:
   virtual ~MouseWheelEventQueueClient() {}
 
   virtual void SendMouseWheelEventImmediately(
       const MouseWheelEventWithLatencyInfo& event) = 0;
   virtual void ForwardGestureEventWithLatencyInfo(
       const blink::WebGestureEvent& event,
       const ui::LatencyInfo& latency_info) = 0;
   virtual void OnMouseWheelEventAck(const MouseWheelEventWithLatencyInfo& event,
                                     InputEventAckState ack_result) = 0;
 };
 
 // A queue for throttling and coalescing mouse wheel events.
 class CONTENT_EXPORT MouseWheelEventQueue {
  public:
   // The |client| must outlive the MouseWheelEventQueue. |send_gestures|
   // indicates whether mouse wheel events should generate
   // Scroll[Begin|Update|End] on unhandled acknowledge events.
   // |scroll_transaction_ms| is the duration in which the
   // ScrollEnd should be sent after a ScrollUpdate.
   MouseWheelEventQueue(MouseWheelEventQueueClient* client,
-                       int64_t scroll_transaction_ms);
+                       bool touchpad_scroll_latching);
 
   ~MouseWheelEventQueue();
 
   // Adds an event to the queue. The event may be coalesced with previously
   // queued events (e.g. consecutive mouse-wheel events can be coalesced into a
   // single mouse-wheel event). The event may also be immediately forwarded to
   // the renderer (e.g. when there are no other queued mouse-wheel event).
   void QueueEvent(const MouseWheelEventWithLatencyInfo& event);
 
   // Notifies the queue that a mouse wheel event has been processed by the
@@ skipping 29 matching lines @@
   WheelEventQueue wheel_queue_;
   std::unique_ptr<QueuedWebMouseWheelEvent> event_sent_for_gesture_ack_;
 
   // True if a non-synthetic GSB needs to be sent before a GSU is sent.
   bool needs_scroll_begin_;
 
   // True if a non-synthetic GSE needs to be sent because a non-synthetic
   // GSB has been sent in the past.
   bool needs_scroll_end_;
 
+  // True if the touchpad scroll latching is enabled.
+  bool touchpad_scroll_latching_;
+
   int64_t scroll_transaction_ms_;
   blink::WebGestureDevice scrolling_device_;
 
   DISALLOW_COPY_AND_ASSIGN(MouseWheelEventQueue);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_INPUT_MOUSE_WHEEL_EVENT_QUEUE_H_