Show Overlay Scrollbar when GestureScrollUpdate

cc/trees/layer_tree_host_impl.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 <stdint.h>
 
 #include <algorithm>
@@ skipping 2838 matching lines @@
 
 InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
     const gfx::Point& viewport_point,
     const gfx::Vector2dF& scroll_delta,
     base::TimeDelta delayed_by) {
   InputHandler::ScrollStatus scroll_status;
   scroll_status.main_thread_scrolling_reasons =
       MainThreadScrollingReason::kNotScrollingOnMain;
   ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
   ScrollNode* scroll_node = scroll_tree.CurrentlyScrollingNode();
+
+  LayerImpl* currently_scrolling_layer = CurrentlyScrollingLayer();
+
+  if (currently_scrolling_layer) {
+    ScrollbarAnimationController* animation_controller =
+        ScrollbarAnimationControllerForId(currently_scrolling_layer->id());
+
+    if (animation_controller)
+      animation_controller->WillUpdateScroll();
+  }
+
   if (scroll_node) {
     gfx::Vector2dF delta = scroll_delta;
     if (!scroll_node->user_scrollable_horizontal)
       delta.set_x(0);
     if (!scroll_node->user_scrollable_vertical)
       delta.set_y(0);
 
     if (ScrollAnimationUpdateTarget(scroll_node, delta, delayed_by)) {
       scroll_status.thread = SCROLL_ON_IMPL_THREAD;
     } else {
@@ skipping 266 matching lines @@
   scroll_state->set_scroll_chain_and_layer_tree(current_scroll_chain,
                                                 active_tree());
   scroll_state->DistributeToScrollChainDescendant();
 }
 
 InputHandlerScrollResult LayerTreeHostImpl::ScrollBy(
     ScrollState* scroll_state) {
   DCHECK(scroll_state);
 
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBy");
-  if (!CurrentlyScrollingLayer())
+
+  LayerImpl* currently_scrolling_layer = CurrentlyScrollingLayer();
+  if (!currently_scrolling_layer)
     return InputHandlerScrollResult();
 
+  ScrollbarAnimationController* animation_controller =
+      ScrollbarAnimationControllerForId(currently_scrolling_layer->id());
+
+  if (animation_controller)
+    animation_controller->WillUpdateScroll();
+
   float initial_top_controls_offset =
       browser_controls_offset_manager_->ControlsTopOffset();
 
   scroll_state->set_delta_consumed_for_scroll_sequence(
       did_lock_scrolling_layer_);
   scroll_state->set_is_direct_manipulation(!wheel_scrolling_);
   scroll_state->set_current_native_scrolling_node(
       active_tree()->property_trees()->scroll_tree.CurrentlyScrollingNode());
 
   DistributeScrollDelta(scroll_state);
@@ skipping 982 matching lines @@
       worker_context_visibility_ =
           worker_context->CacheController()->ClientBecameVisible();
     } else {
       worker_context->CacheController()->ClientBecameNotVisible(
           std::move(worker_context_visibility_));
     }
   }
 }
 
 }  // namespace cc