Wheel scroll latching enabled behind flag.

content/browser/renderer_host/input/mouse_wheel_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/browser/renderer_host/input/mouse_wheel_event_queue.h"
 
 #include "base/metrics/histogram_macros.h"
 #include "base/stl_util.h"
 #include "base/trace_event/trace_event.h"
 #include "content/common/input/web_input_event_traits.h"
@@ skipping 16 matching lines @@
 
   ~QueuedWebMouseWheelEvent() {
     TRACE_EVENT_ASYNC_END0("input", "MouseWheelEventQueue::QueueEvent", this);
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(QueuedWebMouseWheelEvent);
 };
 
 MouseWheelEventQueue::MouseWheelEventQueue(MouseWheelEventQueueClient* client,
-                                           int64_t scroll_transaction_ms)
+                                           bool touchpad_scroll_latching)
     : client_(client),
       needs_scroll_begin_(true),
       needs_scroll_end_(false),
-      scroll_transaction_ms_(scroll_transaction_ms),
+      touchpad_scroll_latching_(touchpad_scroll_latching),

[nasko, 2016/07/25 16:00:18]

It doesn't seem this member variable is used anywhere in this class. Can't we
just set the scroll_transaction_ms_ based on the constructor parameter and
remove the member?

[sahel, 2016/07/25 16:05:11]

You are right, for now it is not used. But I am working on a dependent patch
that uses this flag to latch the touchpad scrolls. (we discussed with tdresser@
and he recommended to split the previous cl into two.) I didn't want to revert
it to use the number in constructor and then change it back to this boolean
parameter.

       scrolling_device_(blink::WebGestureDeviceUninitialized) {
   DCHECK(client);
+  scroll_transaction_ms_ = touchpad_scroll_latching_

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

The timeout is only for wheel, not touchpad, right?
So this bool is named incorrectly?

[sahel, 2016/07/25 17:58:26]

Done.

+                               ? kDefaultWheelScrollLatchingTransactionMs
+                               : kDefaultWheelScrollTransactionMs;
 }
 
 MouseWheelEventQueue::~MouseWheelEventQueue() {
   if (!wheel_queue_.empty())
     STLDeleteElements(&wheel_queue_);
 }
 
 void MouseWheelEventQueue::QueueEvent(
     const MouseWheelEventWithLatencyInfo& event) {
   TRACE_EVENT0("input", "MouseWheelEventQueue::QueueEvent");
@@ skipping 230 matching lines @@
   scroll_begin.data.scrollBegin.targetViewport = false;
   scroll_begin.data.scrollBegin.deltaHintUnits =
       gesture_update.data.scrollUpdate.deltaUnits;
 
   needs_scroll_begin_ = false;
   needs_scroll_end_ = true;
   client_->ForwardGestureEventWithLatencyInfo(scroll_begin, ui::LatencyInfo());
 }
 
 }  // namespace content