Keep expanded if mouse moves off of scrollbar while dragging

cc/trees/layer_tree_host_impl.h

 // 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.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_IMPL_H_
 #define CC_TREES_LAYER_TREE_HOST_IMPL_H_
 
 #include <stddef.h>
 
 #include <bitset>
@@ skipping 160 matching lines @@
       base::TimeDelta delayed_by = base::TimeDelta()) override;
   void ApplyScroll(ScrollNode* scroll_node, ScrollState* scroll_state);
   InputHandlerScrollResult ScrollBy(ScrollState* scroll_state) override;
   bool ScrollVerticallyByPage(const gfx::Point& viewport_point,
                               ScrollDirection direction) override;
   void RequestUpdateForSynchronousInputHandler() override;
   void SetSynchronousInputHandlerRootScrollOffset(
       const gfx::ScrollOffset& root_offset) override;
   void ScrollEnd(ScrollState* scroll_state) override;
   InputHandler::ScrollStatus FlingScrollBegin() override;
+  void MouseDownAt(const gfx::Point& viewport_point) override;

[bokan, 2016/09/23 00:39:12]

Nit: add a blank line above

+  void MouseUp() override;
   void MouseMoveAt(const gfx::Point& viewport_point) override;
+
   void PinchGestureBegin() override;
   void PinchGestureUpdate(float magnify_delta,
                           const gfx::Point& anchor) override;
   void PinchGestureEnd() override;
   void StartPageScaleAnimation(const gfx::Vector2d& target_offset,
                                bool anchor_point,
                                float page_scale,
                                base::TimeDelta duration);
   void SetNeedsAnimateInput() override;
   bool IsCurrentlyScrollingInnerViewport() const override;
@@ skipping 645 matching lines @@
   bool requires_high_res_to_draw_;
   bool is_likely_to_require_a_draw_;
 
   std::unique_ptr<Viewport> viewport_;
 
   std::unique_ptr<LayerTreeMutator> mutator_;
 
   std::unique_ptr<PendingTreeDurationHistogramTimer>
       pending_tree_duration_timer_;
 
+  int captured_scrollbar_layer_id_;
+
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_IMPL_H_