Keep track of fixed positioning in accessibility tree.

ui/accessibility/ax_relative_bounds.h

 // Copyright 2016 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.
 
 #ifndef UI_ACCESSIBILITY_AX_RELATIVE_BOUNDS_H_
 #define UI_ACCESSIBILITY_AX_RELATIVE_BOUNDS_H_
 
 #include <memory>
 
 #include "ui/accessibility/ax_export.h"
 #include "ui/gfx/geometry/rect_f.h"
 
 namespace gfx {
 class Transform;
 };
 
 namespace ui {
 
 // The relative bounding box of an AXNode.
 //
 // This is an efficient, compact, serializable representation of a node's
 // bounding box that requires minimal changes to the tree when layers are
 // moved or scrolled. Computing the absolute bounding box of a node requires
-// walking up the tree and applying node offsets and transforms until reaching
-// the top.
+// walking up the tree and applying node offsets, scroll positions, and
+// transforms until reaching the top.
 //
 // If the offset container id is valid, the bounds are relative
 // to the node with that offset container id.
 //
 // Otherwise, for a node other than the root, the bounds are relative to
 // the root of the tree, and for the root of a tree, the bounds are relative
 // to its immediate containing node.
+//
+// If |is_fixed_positioned| is true, the scroll offsets of the container
+// node should be ignored.
+//
+// The canonical implementation of converting a node's bounding box
+// from local coordinates to tree coordinates is AXTree::GetTreeBounds().
+// Always try to use that rather than reimplementing it.
 struct AX_EXPORT AXRelativeBounds {
   AXRelativeBounds();
   virtual ~AXRelativeBounds();
 
   AXRelativeBounds(const AXRelativeBounds& other);
   AXRelativeBounds& operator=(AXRelativeBounds other);
   bool operator!=(const AXRelativeBounds& other);
   bool operator==(const AXRelativeBounds& other);
 
   std::string ToString() const;
 
   // The id of an ancestor node in the same AXTree that this object's
   // bounding box is relative to, or -1 if there's no offset container.
   int offset_container_id;
 
   // The relative bounding box of this node.
   gfx::RectF bounds;
 
   // An additional transform to apply to position this object and its subtree.
   // NOTE: this member is a std::unique_ptr because it's rare and gfx::Transform
   // takes up a fair amount of space. The assignment operator and copy
   // constructor both make a duplicate of the owned pointer, so it acts more
   // like a member than a pointer.
   std::unique_ptr<gfx::Transform> transform;
+
+  // If true, the container's scroll position should be ignored.
+  bool is_fixed_positioned;
 };
 
 }  // namespace ui
 
 #endif  // UI_ACCESSIBILITY_AX_NODE_DATA_H_