GSB uses delta_hints to calculate scrolling chain.

cc/trees/layer_tree_host_impl_unittest.cc

 // 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.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <cmath>
@@ skipping 6242 matching lines @@
     SetScrollOffsetDelta(child_ptr, gfx::Vector2dF());
     gfx::Vector2d gesture_scroll_delta(10, 0);
     EXPECT_EQ(InputHandler::SCROLL_ON_IMPL_THREAD,
               host_impl_
                   ->ScrollBegin(BeginState(gfx::Point(1, 1)).get(),
                                 InputHandler::TOUCHSCREEN)
                   .thread);
     host_impl_->ScrollBy(UpdateState(gfx::Point(), gesture_scroll_delta).get());
     host_impl_->ScrollEnd(EndState().get());
 
-    // The child layer shouldn't have scrolled.
+    // The child layer should have scrolled down in its local coordinates an
+    // amount proportional to the angle between it and the input scroll delta.
     gfx::Vector2d expected_scroll_delta(
         0, -gesture_scroll_delta.x() *
                std::sin(MathUtil::Deg2Rad(child_layer_angle)));
     std::unique_ptr<ScrollAndScaleSet> scroll_info =
         host_impl_->ProcessScrollDeltas();
     EXPECT_TRUE(ScrollInfoContains(*scroll_info.get(), child_scroll_id,
                                    expected_scroll_delta));
 
     // The root scroll layer shouldn't have scrolled.
     ExpectNone(*scroll_info.get(), scroll_layer->element_id());
@@ skipping 4662 matching lines @@
                    scrolling_layer->CurrentScrollOffset());
   EXPECT_EQ(nullptr, host_impl_->CurrentlyScrollingNode());
   host_impl_->DidFinishImplFrame();
 }
 
 TEST_F(LayerTreeHostImplTest, SecondScrollAnimatedBeginNotIgnored) {
   const gfx::Size content_size(1000, 1000);
   const gfx::Size viewport_size(50, 100);
   CreateBasicVirtualViewportLayers(viewport_size, content_size);
 
-  EXPECT_EQ(InputHandler::SCROLL_ON_IMPL_THREAD,
-            host_impl_->ScrollAnimatedBegin(gfx::Point()).thread);
+  EXPECT_EQ(
+      InputHandler::SCROLL_ON_IMPL_THREAD,
+      host_impl_->ScrollAnimatedBegin(BeginState(gfx::Point()).get()).thread);
 
   // The second ScrollAnimatedBegin should not get ignored.
-  EXPECT_EQ(InputHandler::SCROLL_ON_IMPL_THREAD,
-            host_impl_->ScrollAnimatedBegin(gfx::Point()).thread);
+  EXPECT_EQ(
+      InputHandler::SCROLL_ON_IMPL_THREAD,
+      host_impl_->ScrollAnimatedBegin(BeginState(gfx::Point()).get()).thread);
 }
 
 // Verfify that a smooth scroll animation doesn't jump when UpdateTarget gets
 // called before the animation is started.
 TEST_F(LayerTreeHostImplTest, AnimatedScrollUpdateTargetBeforeStarting) {
   const gfx::Size content_size(1000, 1000);
   const gfx::Size viewport_size(50, 100);
   CreateBasicVirtualViewportLayers(viewport_size, content_size);
 
   DrawFrame();
@@ skipping 1700 matching lines @@
   // layer should be prioritized over the hidden layer.
   hidden_layer->set_contributes_to_drawn_render_surface(false);
   hidden_layer->set_raster_even_if_not_drawn(true);
   queue = host_impl_->BuildRasterQueue(TreePriority::SMOOTHNESS_TAKES_PRIORITY,
                                        RasterTilePriorityQueue::Type::ALL);
   EXPECT_EQ(queue->Top().tile()->layer_id(), 3);
 }
 
 }  // namespace
 }  // namespace cc