Touchpad scroll latching enabled for Mac behind flag.

cc/input/scroll_state_data.h

 // Copyright 2015 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 CC_INPUT_SCROLL_STATE_DATA_H_
 #define CC_INPUT_SCROLL_STATE_DATA_H_
 
 #include <stdint.h>
 
 #include <list>
@@ skipping 14 matching lines @@
   // Pointer (i.e. cursor/touch point) position in viewport coordinates (DIP).
   int position_x;
   int position_y;
   // Scroll velocity in DIP/seconds.
   double velocity_x;
   double velocity_y;
 
   bool is_beginning;
   bool is_in_inertial_phase;
   bool is_ending;
+  // True if the next event may be the start of a fling (is_beginning &&
+  // in_inertial_phase). May only be true if is_ending == true.
+  bool may_precede_fling;

[tdresser, 2016/11/22 21:44:40]

Shouldn't this be |precedes_fling| now? Don't we only dispatch it when we're
positive it precedes a fling?

[sahel, 2016/11/24 15:28:55]

In case of a touchpad fling, the scrollBegin/scrollEnd are called in
InputHandlerProxy::ScrollByMouseWheel.  The scrollEnd state has
may_precede_fling = true. Because the next scrollBegin might be in inertial
phase. Other than this case, we can accurately tell if a fling is going to
happen after a scrollEnd or not. 

[tdresser, 2016/11/28 15:25:17]

I'm not quite following this.

You're saying the only case we don't know may_precede_fling for sure is in
ScrollByMouseWheel? Isn't ScrollByMouseWheel only used for inertial scroll
updates, so we're never going to be followed by a fling start?

 
   bool should_propagate;
   bool from_user_input;
 
   // Whether the scroll sequence has had any delta consumed, in the
   // current frame, or any child frames.
   bool delta_consumed_for_scroll_sequence;
   // True if the user interacts directly with the display, e.g., via
   // touch.
   bool is_direct_manipulation;
@@ skipping 23 matching lines @@
   // The last scroll node to respond to a scroll, or null if none exists.
   ScrollNode* current_native_scrolling_node_;
   // The id of the last native element to respond to a scroll, or 0 if none
   // exists.
   ElementId current_native_scrolling_element_;
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SCROLL_STATE_DATA_H_