Account for fixed positioning once when computing parent offset

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 113 matching lines @@
           Animation::TRANSFORM);
 
   const bool has_surface = !!layer->render_surface();
 
   bool requires_node = is_root || is_scrollable || has_significant_transform ||
                        has_animated_transform || has_surface ||
                        is_page_scale_application_layer;
 
   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 (layer->scroll_parent()) {
+      gfx::Transform to_parent;
+      Layer* source = layer->parent();
+      parent_offset += source->offset_to_transform_parent();
+      data_from_ancestor.transform_tree->ComputeTransform(
+          source->transform_tree_index(),
+          transform_parent->transform_tree_index(), &to_parent);
+      parent_offset += to_parent.To2dTranslation();
+    } else 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);
-
-    parent_offset += to_parent.To2dTranslation();
   }
 
   if (layer->IsContainerForFixedPositionLayers() || is_root)
     data_for_children->transform_fixed_parent = layer;
   data_for_children->transform_tree_parent = layer;
 
   if (!requires_node) {
     data_for_children->should_flatten |= layer->should_flatten_transform();
     gfx::Vector2dF local_offset = layer->position().OffsetFromOrigin() +
                                   layer->transform().To2dTranslation();
@@ skipping 152 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