GSB uses delta_hints to calculate scrolling chain.

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 151 matching lines @@
 
   // InputHandler implementation
   void BindToClient(InputHandlerClient* client,
                     bool wheel_scroll_latching_enabled) override;
   InputHandler::ScrollStatus ScrollBegin(
       ScrollState* scroll_state,
       InputHandler::ScrollInputType type) override;
   InputHandler::ScrollStatus RootScrollBegin(
       ScrollState* scroll_state,
       InputHandler::ScrollInputType type) override;
-  ScrollStatus ScrollAnimatedBegin(const gfx::Point& viewport_point) override;
+  ScrollStatus ScrollAnimatedBegin(ScrollState* scroll_state) override;
   InputHandler::ScrollStatus ScrollAnimated(
       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;
@@ skipping 443 matching lines @@
 
   // Removes empty or orphan RenderPasses from the frame.
   static void RemoveRenderPasses(FrameData* frame);
 
   LayerTreeHostImplClient* client_;
   TaskRunnerProvider* task_runner_provider_;
 
   BeginFrameTracker current_begin_frame_tracker_;
 
  private:
+  // Transforms viewport start point and scroll delta to local start point and
+  // local delta, respectively.

[bokan, 2017/06/23 18:56:44]

Please also document what the return value means.

[sahel, 2017/06/23 20:00:59]

Done.

+  bool CalculateLocalScrollDeltaAndStartPoint(
+      const ScrollNode& scroll_node,
+      const gfx::PointF& viewport_point,
+      const gfx::Vector2dF& viewport_delta,
+      const ScrollTree& scroll_tree,
+      gfx::Vector2dF* out_local_scroll_delta,
+      gfx::PointF* out_local_start_point = nullptr);
   gfx::Vector2dF ScrollNodeWithViewportSpaceDelta(
       ScrollNode* scroll_node,
       const gfx::PointF& viewport_point,
       const gfx::Vector2dF& viewport_delta,
       ScrollTree* scroll_tree);
 
   void CleanUpTileManagerAndUIResources();
   void CreateTileManagerResources();
   void ReleaseTreeResources();
   void ReleaseTileResources();
@@ skipping 11 matching lines @@
   void UpdateTreeResourcesForGpuRasterizationIfNeeded();
 
   Viewport* viewport() const { return viewport_.get(); }
 
   InputHandler::ScrollStatus ScrollBeginImpl(
       ScrollState* scroll_state,
       ScrollNode* scrolling_node,
       InputHandler::ScrollInputType type);
   bool IsInitialScrollHitTestReliable(LayerImpl* layer, const gfx::PointF&);
   void DistributeScrollDelta(ScrollState* scroll_state);
+  bool CanConsumeDelta(ScrollNode* scroll_node,
+                       const ScrollState& scroll_state);
 
   bool AnimatePageScale(base::TimeTicks monotonic_time);
   bool AnimateScrollbars(base::TimeTicks monotonic_time);
   bool AnimateBrowserControls(base::TimeTicks monotonic_time);
 
   void UpdateTileManagerMemoryPolicy(const ManagedMemoryPolicy& policy);
 
   // This function should only be called from PrepareToDraw, as DidDrawAllLayers
   // must be called if this helper function is called.  Returns DRAW_SUCCESS if
   // the frame should be drawn.
@@ skipping 202 matching lines @@
   bool touchpad_and_wheel_scroll_latching_enabled_;
 
   ImplThreadPhase impl_thread_phase_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_IMPL_H_