diff -r 000000000000 -r 6474c204b198 layout/base/ActiveLayerTracker.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/base/ActiveLayerTracker.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,86 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef ACTIVELAYERTRACKER_H_
+#define ACTIVELAYERTRACKER_H_
+
+#include "nsCSSProperty.h"
+
+class nsIFrame;
+
+namespace mozilla {
+
+/**
+ * This class receives various notifications about style changes and content
+ * changes that affect layerization decisions, and implements the heuristics
+ * that drive those decisions. It manages per-frame state to support those
+ * heuristics.
+ */
+class ActiveLayerTracker {
+public:
+  static void Shutdown();
+
+  /*
+   * We track style changes to selected styles:
+   *   eCSSProperty_transform
+   *   eCSSProperty_opacity
+   *   eCSSProperty_left, eCSSProperty_top, eCSSProperty_right, eCSSProperty_bottom
+   * and use that information to guess whether style changes are animated.
+   */
+
+  /**
+   * Notify aFrame's style property as having changed due to a restyle,
+   * and therefore possibly wanting an active layer to render that style.
+   * Any such marking will time out after a short period.
+   * @param aProperty the property that has changed
+   */
+  static void NotifyRestyle(nsIFrame* aFrame, nsCSSProperty aProperty);
+  /**
+   * Notify aFrame's left/top/right/bottom properties as having (maybe)
+   * changed due to a restyle, and therefore possibly wanting an active layer
+   * to render that style. Any such marking will time out after a short period.
+   */
+  static void NotifyOffsetRestyle(nsIFrame* aFrame);
+  /**
+   * Mark aFrame as being known to have an animation of aProperty.
+   * Any such marking will time out after a short period.
+   */
+  static void NotifyAnimated(nsIFrame* aFrame, nsCSSProperty aProperty);
+  /**
+   * Notify that a property in the inline style rule of aFrame's element
+   * has been modified.
+   * This notification is incomplete --- not all modifications to inline
+   * style will trigger this.
+   */
+  static void NotifyInlineStyleRuleModified(nsIFrame* aFrame, nsCSSProperty aProperty);
+  /**
+   * Return true if aFrame's aProperty style should be considered as being animated
+   * for constructing active layers.
+   */
+  static bool IsStyleAnimated(nsIFrame* aFrame, nsCSSProperty aProperty);
+  /**
+   * Return true if any of aFrame's offset property styles should be considered
+   * as being animated for constructing active layers.
+   */
+  static bool IsOffsetOrMarginStyleAnimated(nsIFrame* aFrame);
+
+  /*
+   * We track modifications to the content of certain frames (i.e. canvas frames)
+   * and use that to make layering decisions.
+   */
+
+  /**
+   * Mark aFrame's content as being active. This marking will time out after
+   * a short period.
+   */
+  static void NotifyContentChange(nsIFrame* aFrame);
+  /**
+   * Return true if this frame's content is still marked as active.
+   */
+  static bool IsContentActive(nsIFrame* aFrame);
+};
+
+}
+
+#endif /* ACTIVELAYERTRACKER_H_ */