Support bottom-right anchored fixed-position elements during a pinch gesture

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/animation.h"
 #include "cc/animation/animation_id_provider.h"
 #include "cc/animation/keyframed_animation_curve.h"
@@ skipping 198 matching lines @@
     return;
 
   gfx::Vector2dF max_scroll = gfx::Rect(ScrollableSize()).bottom_right() -
       gfx::RectF(ScrollableViewportSize()).bottom_right();
 
   // The viewport may be larger than the contents in some cases, such as
   // having a vertical scrollbar but no horizontal overflow.
   max_scroll.ClampToMin(gfx::Vector2dF());
 
   root_scroll_layer_->SetMaxScrollOffset(gfx::ToFlooredVector2d(max_scroll));
+
+  gfx::Vector2dF scrollable_viewport_size =
+      gfx::RectF(ScrollableViewportSize()).bottom_right() - gfx::PointF();
+  gfx::Vector2dF original_scrollable_viewport_size =
+      gfx::RectF(layer_tree_host_impl_->VisibleViewportSize()).bottom_right() -
+      gfx::PointF();
+  original_scrollable_viewport_size.Scale(1 / page_scale_factor());
+  root_scroll_layer_->SetFixedContainerSizeDelta(
+      scrollable_viewport_size - original_scrollable_viewport_size);
 }
 
 gfx::Transform LayerTreeImpl::ImplTransform() const {
   gfx::Transform transform;
   transform.Scale(total_page_scale_factor(), total_page_scale_factor());
   return transform;
 }
 
 void LayerTreeImpl::UpdateSolidColorScrollbars() {
   DCHECK(settings().solid_color_scrollbars);
@@ skipping 423 matching lines @@
   StartFadeOutAnimation(PinchZoomScrollbarVertical());
 }
 
 bool LayerTreeImpl::HasPinchZoomScrollbars() const {
   return pinch_zoom_scrollbar_horizontal_layer_id_ != Layer::INVALID_ID &&
          pinch_zoom_scrollbar_vertical_layer_id_ != Layer::INVALID_ID;
 }
 
 
 } // namespace cc