Property tree code must account for parent offsets of fixed layers

cc/trees/property_tree_builder.cc

 // Copyright 2014 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/property_tree_builder.h"
 
 #include <map>
 #include <set>
 
 #include "cc/base/math_util.h"
@@ skipping 121 matching lines @@
                        has_significant_transform || has_animated_transform ||
                        is_page_scale_application_layer || flattening_change ||
                        has_surface;
 
   Layer* transform_parent = GetTransformParent(data_from_ancestor, layer);
 
   // May be non-zero if layer is fixed or has a scroll parent.
   gfx::Vector2dF parent_offset;
   if (transform_parent) {
     // TODO(vollick): This is to mimic existing bugs (crbug.com/441447).
-    if (!is_fixed)
+    if (!is_fixed) {
       parent_offset = transform_parent->offset_to_transform_parent();
+    } else if (data_from_ancestor.transform_tree_parent !=
+               data_from_ancestor.transform_fixed_parent) {
+      gfx::Vector2dF fixed_offset = data_from_ancestor.transform_tree_parent
+                                        ->offset_to_transform_parent();
+      gfx::Transform parent_to_parent;
+      data_from_ancestor.transform_tree->ComputeTransform(
+          data_from_ancestor.transform_tree_parent->transform_tree_index(),
+          data_from_ancestor.transform_fixed_parent->transform_tree_index(),
+          &parent_to_parent);
+
+      fixed_offset += parent_to_parent.To2dTranslation();
+      parent_offset += fixed_offset;
+    }
 
     gfx::Transform to_parent;
     Layer* source = data_from_ancestor.transform_tree_parent;
     if (layer->scroll_parent()) {
       source = layer->parent();
       parent_offset += layer->parent()->offset_to_transform_parent();
     }
     data_from_ancestor.transform_tree->ComputeTransform(
         source->transform_tree_index(),
         transform_parent->transform_tree_index(), &to_parent);
@@ skipping 153 matching lines @@
   data_for_recursion.device_transform = &device_transform;
 
   ClipNode root_clip;
   root_clip.data.clip = viewport;
   root_clip.data.transform_id = 0;
   data_for_recursion.clip_tree_parent = clip_tree->Insert(root_clip, 0);
   BuildPropertyTreesInternal(root_layer, data_for_recursion);
 }
 
 }  // namespace cc