Make scrollable viewport size no longer depend on clip layer.

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 "base/debug/trace_event.h"
 #include "cc/animation/keyframed_animation_curve.h"
 #include "cc/animation/scrollbar_animation_controller.h"
 #include "cc/debug/traced_value.h"
@@ skipping 118 matching lines @@
 
   if (hud_layer())
     target_tree->set_hud_layer(static_cast<HeadsUpDisplayLayerImpl*>(
         LayerTreeHostCommon::FindLayerInSubtree(
             target_tree->root_layer(), hud_layer()->id())));
   else
     target_tree->set_hud_layer(NULL);
 }
 
 LayerImpl* LayerTreeImpl::RootScrollLayer() const {
-  DCHECK(IsActiveTree());
   return root_scroll_layer_;
 }
 
 LayerImpl* LayerTreeImpl::RootClipLayer() const {
   return root_scroll_layer_ ? root_scroll_layer_->parent() : NULL;
 }
 
 LayerImpl* LayerTreeImpl::CurrentlyScrollingLayer() const {
   DCHECK(IsActiveTree());
   return currently_scrolling_layer_;
@@ skipping 52 matching lines @@
   UpdateMaxScrollOffset();
   set_needs_update_draw_properties();
 }
 
 gfx::SizeF LayerTreeImpl::ScrollableViewportSize() const {
   return gfx::ScaleSize(layer_tree_host_impl_->VisibleViewportSize(),
                         1.0f / total_page_scale_factor());
 }
 
 void LayerTreeImpl::UpdateMaxScrollOffset() {
-  if (!root_scroll_layer_ || !root_scroll_layer_->children().size())
+  LayerImpl* root_scroll = RootScrollLayer();
+  if (!root_scroll || !root_scroll->children().size())
     return;
 
   gfx::Vector2dF max_scroll = gfx::Rect(ScrollableSize()).bottom_right() -
       gfx::RectF(ScrollableViewportSize()).bottom_right();
 
+  // The scrollable viewport size is based on device viewport instead of Blink's
+  // container layer, so we need to adjust for non-overlay scrollbars.
+  ScrollbarLayerImpl* horiz = root_scroll->horizontal_scrollbar_layer();
+  ScrollbarLayerImpl* vertical = root_scroll->vertical_scrollbar_layer();
+  if (horiz && !horiz->is_overlay_scrollbar())
+    max_scroll.set_y(max_scroll.y() + horiz->thumb_thickness());

[enne (OOO), 2013/07/12 23:48:05]

I still don't really understand this.  You can scroll more horizontally if you
have a horizontal scrollbar?

Are you sure that Blink doesn't already do the right thing here?

[enne (OOO), 2013/07/12 23:50:30]

Eh, sorry, misreading code late on a Friday.

I guess this all makes sense.

+  if (vertical && !vertical->is_overlay_scrollbar())
+    max_scroll.set_x(max_scroll.x() + vertical->thumb_thickness());
+
   // The viewport may be larger than the contents in some cases, such as
   // having a vertical scrollbar but no horizontal overflow.
   max_scroll.SetToMax(gfx::Vector2dF());
 
   root_scroll_layer_->SetMaxScrollOffset(gfx::ToFlooredVector2d(max_scroll));
 }
 
 void LayerTreeImpl::UpdateSolidColorScrollbars() {
   DCHECK(settings().solid_color_scrollbars);
 
@@ skipping 358 matching lines @@
 const std::vector<LayerImpl*> LayerTreeImpl::LayersWithCopyOutputRequest()
     const {
   // Only the active tree needs to know about layers with copy requests, as
   // they are aborted if not serviced during draw.
   DCHECK(IsActiveTree());
 
   return layers_with_copy_output_request_;
 }
 
 }  // namespace cc