Reland of Show Overlay Scrollbar when GestureScrollUpdate

cc/trees/layer_tree_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_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 225 matching lines @@
 
   gfx::ScrollOffset current_offset = scroll_layer->CurrentScrollOffset();
   if (IsViewportLayerId(scroll_layer_id)) {
     current_offset += InnerViewportScrollLayer()->CurrentScrollOffset();
     if (OuterViewportContainerLayer())
       clip_size.SetToMin(OuterViewportContainerLayer()->BoundsForScrolling());
     clip_size.Scale(1 / current_page_scale_factor());
   }
 
   bool scrollbar_needs_animation = false;
+  bool clip_layer_size_did_change = false;
   bool scroll_layer_size_did_change = false;
   bool y_offset_did_change = false;
   for (ScrollbarLayerImplBase* scrollbar : ScrollbarsFor(scroll_layer_id)) {
     if (scrollbar->orientation() == HORIZONTAL) {
       scrollbar_needs_animation |= scrollbar->SetCurrentPos(current_offset.x());
-      scrollbar_needs_animation |=
+      clip_layer_size_did_change |=
           scrollbar->SetClipLayerLength(clip_size.width());
-      scrollbar_needs_animation |= scroll_layer_size_did_change |=
+      scroll_layer_size_did_change |=
           scrollbar->SetScrollLayerLength(scroll_size.width());
     } else {
       scrollbar_needs_animation |= y_offset_did_change |=
           scrollbar->SetCurrentPos(current_offset.y());
-      scrollbar_needs_animation |=
+      clip_layer_size_did_change |=
           scrollbar->SetClipLayerLength(clip_size.height());
-      scrollbar_needs_animation |= scroll_layer_size_did_change |=
+      scroll_layer_size_did_change |=
           scrollbar->SetScrollLayerLength(scroll_size.height());
     }
     scrollbar_needs_animation |=
         scrollbar->SetVerticalAdjust(clip_layer->bounds_delta().y());
   }
 
+  scrollbar_needs_animation |=
+      (clip_layer_size_did_change || scroll_layer_size_did_change);
+
   if (y_offset_did_change && IsViewportLayerId(scroll_layer_id))
     TRACE_COUNTER_ID1("cc", "scroll_offset_y", scroll_layer->id(),
                       current_offset.y());
 
   if (scrollbar_needs_animation) {
     ScrollbarAnimationController* controller =
         layer_tree_host_impl_->ScrollbarAnimationControllerForId(
             scroll_layer_id);
-    if (controller)
-      controller->DidScrollUpdate(scroll_layer_size_did_change);
+    if (!controller)
+      return;
+
+    // TODO(chaopeng) clip_layer_size_did_change should call DidResize after
+    // crbug.com/701810 got fixed.
+    if (scroll_layer_size_did_change) {
+      controller->DidResize();
+    } else {
+      controller->DidScrollUpdate();
+    }
   }
 }
 
 RenderSurfaceImpl* LayerTreeImpl::RootRenderSurface() const {
   return layer_list_.empty() ? nullptr : layer_list_[0]->GetRenderSurface();
 }
 
 bool LayerTreeImpl::LayerListIsEmpty() const {
   return layer_list_.empty();
 }
@@ skipping 1864 matching lines @@
 
 void LayerTreeImpl::ResetAllChangeTracking() {
   layers_that_should_push_properties_.clear();
   // Iterate over all layers, including masks.
   for (auto& layer : *layers_)
     layer->ResetChangeTracking();
   property_trees_.ResetAllChangeTracking();
 }
 
 }  // namespace cc