Touchpad scroll latching enabled for Mac behind flag.

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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 "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 2669 matching lines @@
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     ScrollState* scroll_state,
     InputHandler::ScrollInputType type) {
   ScrollStatus scroll_status;
   scroll_status.main_thread_scrolling_reasons =
       MainThreadScrollingReason::kNotScrollingOnMain;
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBegin");
 
+  // On Mac a scroll begin with |inertial_phase| = true happens to handle a
+  // fling.
+  if (scroll_state->is_in_inertial_phase())
+    return FlingScrollBegin();
+
   ClearCurrentlyScrollingLayer();
 
   gfx::Point viewport_point(scroll_state->position_x(),
                             scroll_state->position_y());
 
   gfx::PointF device_viewport_point = gfx::ScalePoint(
       gfx::PointF(viewport_point), active_tree_->device_scale_factor());
   LayerImpl* layer_impl =
       active_tree_->FindLayerThatIsHitByPoint(device_viewport_point);
 
@@ skipping 527 matching lines @@
   scroll_affects_scroll_handler_ = false;
   accumulated_root_overscroll_ = gfx::Vector2dF();
 }
 
 void LayerTreeHostImpl::ScrollEnd(ScrollState* scroll_state) {
   DCHECK(scroll_state);
   DCHECK(scroll_state->delta_x() == 0 && scroll_state->delta_y() == 0);
 
   DistributeScrollDelta(scroll_state);
   top_controls_manager_->ScrollEnd();
-  ClearCurrentlyScrollingLayer();
+
+  if (scroll_state->is_in_inertial_phase()) {

[dtapuska, 2016/08/18 00:30:45]

I'm a bit concerned this isn't correct for devices that don't have inertial
phase information or when the feature is off. Shouldnt it be clearing the
scrolling layer in those cases?

[sahel, 2016/08/18 15:41:56]

It's ok if we don't do the latching for whatever reason and don't clear it here,
because in scrollBegin we clear it.

+    // Only clear the currently scrolling layer if we know the scroll is done.
+    // A non-inertial scroll end could be followed by an inertial scroll.
+    ClearCurrentlyScrollingLayer();
+  }
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::FlingScrollBegin() {
   InputHandler::ScrollStatus scroll_status;
   scroll_status.main_thread_scrolling_reasons =
       MainThreadScrollingReason::kNotScrollingOnMain;
   if (!CurrentlyScrollingLayer()) {
     scroll_status.thread = SCROLL_IGNORED;
     scroll_status.main_thread_scrolling_reasons =
         MainThreadScrollingReason::kNoScrollingLayer;
@@ skipping 818 matching lines @@
   return task_runner_provider_->HasImplThread();
 }
 
 bool LayerTreeHostImpl::CommitToActiveTree() const {
   // In single threaded mode we skip the pending tree and commit directly to the
   // active tree.
   return !task_runner_provider_->HasImplThread();
 }
 
 }  // namespace cc