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

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 bc9acf5a9e2df5015736e5859b98110e4c496ce4..fa4584c39806a23c968d9ffc226579a782cde6ed 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -259,6 +259,9 @@ struct UpdateTilePrioritiesForLayer {
 };
 
 void LayerTreeImpl::UpdateDrawProperties(UpdateDrawPropertiesReason reason) {
+  if (IsActiveTree() && RootScrollLayer() && RootClipLayer())
+    UpdateRootScrollLayerSizeDelta();
+
   if (settings().solid_color_scrollbars &&
       IsActiveTree() &&
       RootScrollLayer()) {
@@ -666,4 +669,23 @@ bool LayerTreeImpl::HasPinchZoomScrollbars() const {
          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