Disable overlay scrollbars in Blink when hidden by the compositor.

cc/layers/layer.cc

 // Copyright 2010 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/layers/layer.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 79 matching lines @@
       clip_tree_index_(ClipTree::kInvalidNodeId),
       scroll_tree_index_(ScrollTree::kInvalidNodeId),
       property_tree_sequence_number_(-1),
       should_flatten_transform_from_property_tree_(false),
       draws_content_(false),
       use_local_transform_for_backface_visibility_(false),
       should_check_backface_visibility_(false),
       force_render_surface_for_testing_(false),
       subtree_property_changed_(false),
       may_contain_video_(false),
+      scrollbars_hidden_(false),

[aelias_OOO_until_Jul13, 2016/10/26 19:01:03]

This state is redundant with the one in ScrollableArea, please delete it.  This
would mean getting rid of the early-return in
Layer::SetScrollbarsHiddenFromImplSide().  There's already early-return coverage
in ScrollableArea::didChangeScrollbarsHidden() and you just need to add
analogous early-return to FrameView::didChangeScrollbarsHidden().

[bokan, 2016/10/27 20:56:46]

Done.
FrameView::didChangeScrollbarsHidden and
PaintLayerScrollableArea::didChangeScrollbarsHidden are only called from
ScrollableArea::setScrollbarsHidden (that's what you meant above) if there's a
change so they don't need an early return.

       safe_opaque_background_color_(0),
       draw_blend_mode_(SkXfermode::kSrcOver_Mode),
       num_unclipped_descendants_(0) {}
 
 Layer::~Layer() {
   // Our parent should be holding a reference to us so there should be no
   // way for us to be destroyed while we still have a parent.
   DCHECK(!parent());
   // Similarly we shouldn't have a layer tree host since it also keeps a
   // reference to us.
@@ skipping 708 matching lines @@
     transform_node->needs_local_transform_update = true;
     property_trees->transform_tree.set_needs_update(true);
     needs_rebuild = false;
   }
 
   if (needs_rebuild)
     property_trees->needs_rebuild = true;
 
   if (!inputs_.did_scroll_callback.is_null())
     inputs_.did_scroll_callback.Run();
+

[aelias_OOO_until_Jul13, 2016/10/26 19:01:03]

nit: unnecessary newline

[bokan, 2016/10/27 20:56:46]

Done.

   // The callback could potentially change the layer structure:
   // "this" may have been destroyed during the process.
 }
 
 void Layer::SetScrollClipLayerId(int clip_layer_id) {
   DCHECK(IsPropertyChangeAllowed());
   if (inputs_.scroll_clip_layer_id == clip_layer_id)
     return;
   inputs_.scroll_clip_layer_id = clip_layer_id;
   SetNeedsCommit();
@@ skipping 546 matching lines @@
   base->set_safe_opaque_background_color(safe_opaque_background_color_);
   // TODO(nyquist): Figure out what to do with debug info. See crbug.com/570372.
   base->set_transform_free_index(transform_tree_index_);
   base->set_effect_tree_index(effect_tree_index_);
   base->set_clip_tree_index(clip_tree_index_);
   base->set_scroll_tree_index(scroll_tree_index_);
   Vector2dFToProto(offset_to_transform_parent_,
                    base->mutable_offset_to_transform_parent());
   base->set_draws_content(draws_content_);
   base->set_may_contain_video(may_contain_video_);
+  // TODO(bokan): Should we serialize the scrollbars_enabled flag?
   base->set_hide_layer_and_subtree(inputs_.hide_layer_and_subtree);
   base->set_subtree_property_changed(subtree_property_changed_);
 
   // TODO(nyquist): Add support for serializing FilterOperations for
   // |filters_| and |background_filters_|. See crbug.com/541321.
 
   base->set_masks_to_bounds(inputs_.masks_to_bounds);
   base->set_main_thread_scrolling_reasons(
       inputs_.main_thread_scrolling_reasons);
   RegionToProto(inputs_.non_fast_scrollable_region,
@@ skipping 43 matching lines @@
 
   transform_tree_index_ = base.transform_free_index();
   effect_tree_index_ = base.effect_tree_index();
   clip_tree_index_ = base.clip_tree_index();
   scroll_tree_index_ = base.scroll_tree_index();
   offset_to_transform_parent_ =
       ProtoToVector2dF(base.offset_to_transform_parent());
   inputs_.double_sided = base.double_sided();
   draws_content_ = base.draws_content();
   may_contain_video_ = base.may_contain_video();
+  // TODO(bokan): scrollbars_hidden_
   inputs_.hide_layer_and_subtree = base.hide_layer_and_subtree();
   subtree_property_changed_ = base.subtree_property_changed();
   inputs_.masks_to_bounds = base.masks_to_bounds();
   inputs_.main_thread_scrolling_reasons = base.main_thread_scrolling_reasons();
   inputs_.non_fast_scrollable_region =
       RegionFromProto(base.non_fast_scrollable_region());
   inputs_.touch_event_handler_region =
       RegionFromProto(base.touch_event_handler_region());
   inputs_.contents_opaque = base.contents_opaque();
   inputs_.opacity = base.opacity();
@@ skipping 133 matching lines @@
   SetNeedsPushProperties();
 }
 
 void Layer::SetMayContainVideo(bool yes) {
   if (may_contain_video_ == yes)
     return;
   may_contain_video_ = yes;
   SetNeedsPushProperties();
 }
 
+void Layer::SetScrollbarsHiddenFromImplSide(bool hidden) {
+  if (scrollbars_hidden_ == hidden)
+    return;
+
+  scrollbars_hidden_ = hidden;
+  if (inputs_.client)
+    inputs_.client->didChangeScrollbarsHidden(hidden);
+}
+
 bool Layer::FilterIsAnimating() const {
   return GetAnimationHost()->IsAnimatingFilterProperty(
       element_id(), GetElementTypeForAnimation());
 }
 
 bool Layer::TransformIsAnimating() const {
   return GetAnimationHost()->IsAnimatingTransformProperty(
       element_id(), GetElementTypeForAnimation());
 }
 
@@ skipping 265 matching lines @@
 
 LayerTree* Layer::GetLayerTree() const {
   return layer_tree_;
 }
 
 void Layer::SetLayerIdForTesting(int id) {
   inputs_.layer_id = id;
 }
 
 }  // namespace cc