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

cc/layer_tree_impl.cc

Index: cc/layer_tree_impl.cc
diff --git a/cc/layer_tree_impl.cc b/cc/layer_tree_impl.cc
index f16ec0035e93d2d3e7b66be833c00fdac28fa69e..14793a4ad8f460194ed6e7c9ba108a660a0b8aa4 100644
--- a/cc/layer_tree_impl.cc
+++ b/cc/layer_tree_impl.cc
@@ -170,8 +170,7 @@ void LayerTreeImpl::SetPageScaleDelta(float delta)
   set_needs_update_draw_properties();
 }
 
-gfx::SizeF LayerTreeImpl::ScrollableViewportSize() const {
-  gfx::SizeF view_bounds;
+gfx::SizeF LayerTreeImpl::UnscaledScrollableViewportSize() const {
   // The clip layer should be used for scrolling bounds if available since it
   // adjusts for non-overlay scrollbars. Otherwise, fall back to our knowledge
   // of the physical viewport size.
@@ -179,11 +178,15 @@ gfx::SizeF LayerTreeImpl::ScrollableViewportSize() const {
   if (root_scroll_layer_)
     clip_layer = root_scroll_layer_->parent();
   if (clip_layer && clip_layer->masksToBounds()) {
-    view_bounds = clip_layer->bounds();
+    return clip_layer->bounds();
   } else {
-    view_bounds = gfx::ScaleSize(device_viewport_size(),
+    return gfx::ScaleSize(device_viewport_size(),
         1 / device_scale_factor());
   }
+}
+
+gfx::SizeF LayerTreeImpl::ScrollableViewportSize() const {
+  gfx::SizeF view_bounds = UnscaledScrollableViewportSize();
   view_bounds.Scale(1 / total_page_scale_factor());
 
   return view_bounds;
@@ -201,6 +204,15 @@ void LayerTreeImpl::UpdateMaxScrollOffset() {
   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(UnscaledScrollableViewportSize()).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 {