diff --git a/dom/base/nsDocument.h b/dom/base/nsDocument.h
index bb05a4fb9b..16b93d245d 100644
--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -904,11 +904,11 @@ public:
   void CleanupFullscreenState();
 
   // Pushes aElement onto the top layer
-  bool TopLayerPush(Element* aElement);
+  bool TopLayerPush(Element* aElement) override;
 
   // Removes the topmost element which have aPredicate return true from the top
   // layer. The removed element, if any, is returned.
-  Element* TopLayerPop(FunctionRef<bool(Element*)> aPredicateFunc);
+  Element* TopLayerPop(FunctionRef<bool(Element*)> aPredicateFunc) override;
 
   // Pops the fullscreen element from the top layer and clears its
   // fullscreen flag.
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index 81b5b51c8e..2a18c2259e 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -100,6 +100,8 @@ class nsIGlobalObject;
 class nsStyleSet;
 struct nsCSSSelectorList;
 
+using mozilla::FunctionRef;
+
 namespace mozilla {
 class CSSStyleSheet;
 class ErrorResult;
@@ -1339,6 +1341,13 @@ public:
    * in this document.
    */
   virtual void SetFullscreenRoot(nsIDocument* aRoot) = 0;
+  
+  /**
+   * Push elements to and pop elements from the top layer.
+   * Currently in use for fullscreen and modal version of <dialog>.
+   */
+  virtual bool TopLayerPush(Element* aElement) = 0;
+  virtual Element* TopLayerPop(FunctionRef<bool(Element*)> aPredicateFunc) = 0;
 
   /**
    * Synchronously cleans up the fullscreen state on the given document.