Make scrollable viewport size no longer depend on clip layer.

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 25cd58a6bf26c695d6bae75db99f1ee2fe0a7afc..e5aef8a3034275739d50627a4a20be43114ed1d2 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -136,7 +136,6 @@ void LayerTreeImpl::PushPropertiesTo(LayerTreeImpl* target_tree) {
 }
 
 LayerImpl* LayerTreeImpl::RootScrollLayer() const {
-  DCHECK(IsActiveTree());
   return root_scroll_layer_;
 }
 
@@ -209,12 +208,22 @@ gfx::SizeF LayerTreeImpl::ScrollableViewportSize() const {
 }
 
 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());