Use relative bounding boxes throughout Chrome accessibility

ui/accessibility/ax_relative_bounds.h

Index: ui/accessibility/ax_relative_bounds.h
diff --git a/ui/accessibility/ax_relative_bounds.h b/ui/accessibility/ax_relative_bounds.h
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+// 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.
+//
+// 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.
+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;
+};
+
+}  // namespace ui
+
+#endif  // UI_ACCESSIBILITY_AX_NODE_DATA_H_