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 241 matching lines @@
   }
 }
 
 struct UpdateTilePrioritiesForLayer {
   void operator()(LayerImpl *layer) {
     layer->UpdateTilePriorities();
   }
 };
 
 void LayerTreeImpl::UpdateDrawProperties(UpdateDrawPropertiesReason reason) {
+  if (IsActiveTree() && RootScrollLayer() && RootClipLayer())
+    UpdateRootScrollLayerSizeDelta();
+
   if (settings().solid_color_scrollbars &&
       IsActiveTree() &&
       RootScrollLayer()) {
     UpdateSolidColorScrollbars();
 
     // The top controls manager is incompatible with the WebKit-created cliprect
     // because it can bring into view a larger amount of content when it
     // hides. It's safe to deactivate the clip rect if no non-overlay scrollbars
     // are present.
     if (RootClipLayer() && layer_tree_host_impl_->top_controls_manager())
@@ skipping 387 matching lines @@
 
   StartFadeOutAnimation(PinchZoomScrollbarHorizontal());
   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;
 }
 
+void LayerTreeImpl::UpdateRootScrollLayerSizeDelta() {
+  LayerImpl* root_scroll = RootScrollLayer();
+  LayerImpl* root_clip = RootClipLayer();
+  DCHECK(root_scroll);
+  DCHECK(root_clip);
+  DCHECK(IsActiveTree());
+
+  gfx::Vector2dF scrollable_viewport_size =
+      gfx::RectF(ScrollableViewportSize()).bottom_right() - gfx::PointF();
+
+  gfx::Vector2dF original_viewport_size =
+      gfx::RectF(root_clip->bounds()).bottom_right() -
+      gfx::PointF();
+  original_viewport_size.Scale(1 / page_scale_factor());
+
+  root_scroll->SetFixedContainerSizeDelta(
+      scrollable_viewport_size - original_viewport_size);
+}
+
 }  // namespace cc