Allow scrolling a page with selection after long-press on whitespace

ui/touch_selection/longpress_drag_selector.h

Index: ui/touch_selection/longpress_drag_selector.h
diff --git a/ui/touch_selection/longpress_drag_selector.h b/ui/touch_selection/longpress_drag_selector.h
index f6e84ecf1cd7463786720c79dd3779f383134cb3..35335eb2861c57fd6d78b6bcbd1d4f49d68980cd 100644
--- a/ui/touch_selection/longpress_drag_selector.h
+++ b/ui/touch_selection/longpress_drag_selector.h
@@ -40,6 +40,13 @@ class UI_TOUCH_SELECTION_EXPORT LongPressDragSelector
   void OnLongPressEvent(base::TimeTicks event_time,
                         const gfx::PointF& position);
 
+  // Called when a scroll is going to happen to cancel longpress-drag gesture.
+  // TODO(mohsen): This is a workaround to avoid starting longpress-drag gesture

[mfomitchev, 2016/01/04 17:07:55]

It seems like the root of the issue is that (without this fix) TSC calls
OnSelectionActivated() on LongPressDragSelector when a scroll happens. It does
so because it interprets the selection _update_ (resulting from a scroll) as a
_new_ selection - because it has response_pending_input_event_ set to
LONG_PRESS.

Can we please describe this in a bit more detail? Also, I suggest we the
description of the issue to TSC::OnScrollBeginEvent() (in the .cc file). We
should also mention that this happens when the long press does not result in a
selection.

[mohsen, 2016/01/04 18:45:59]

Done.

+  // if the longpress is happening on an empty area far from an existing
+  // selection. Get rid of this workaround when we have enough information about
+  // the cause of the selection (see https://crbug.com/571897).
+  void OnScrollBeginEvent();
+
   // Called when the active selection changes.
   void OnSelectionActivated();
   void OnSelectionDeactivated();