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 158 matching lines @@
       const gfx::Point& viewport_point,
       const gfx::Vector2dF& scroll_delta,
       base::TimeDelta delayed_by = base::TimeDelta()) override;
   void ApplyScroll(ScrollNode* scroll_node, ScrollState* scroll_state);
   InputHandlerScrollResult ScrollBy(ScrollState* scroll_state) override;
   void RequestUpdateForSynchronousInputHandler() override;
   void SetSynchronousInputHandlerRootScrollOffset(
       const gfx::ScrollOffset& root_offset) override;
   void ScrollEnd(ScrollState* scroll_state) override;
   InputHandler::ScrollStatus FlingScrollBegin() override;
+
+  void MouseDown() override;
+  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 559 matching lines @@
 
   // In impl-side painting mode, inert tree with layers that can be recycled
   // by the next sync from the main thread.
   std::unique_ptr<LayerTreeImpl> recycle_tree_;
 
   InputHandlerClient* input_handler_client_;
   bool did_lock_scrolling_layer_;
   bool wheel_scrolling_;
   bool scroll_affects_scroll_handler_;
   int scroll_layer_id_when_mouse_over_scrollbar_;
+  int captured_scrollbar_layer_id_;
+
   std::vector<std::unique_ptr<SwapPromise>>
       swap_promises_for_main_thread_scroll_update_;
 
   // An object to implement the ScrollElasticityHelper interface and
   // hold all state related to elasticity. May be NULL if never requested.
   std::unique_ptr<ScrollElasticityHelper> scroll_elasticity_helper_;
 
   bool tile_priorities_dirty_;
 
   const LayerTreeSettings settings_;
@@ skipping 76 matching lines @@
 
   std::unique_ptr<PendingTreeDurationHistogramTimer>
       pending_tree_duration_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_IMPL_H_