1 files changed, 21 insertions, 1 deletions
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index 4fdc60a2e..d8f876291 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -719,7 +719,18 @@ public:
    *         it was propagated from.
    */
   nsIContent* UpdateViewportScrollbarStylesOverride();
-  const ScrollbarStyles& GetViewportScrollbarStylesOverride()
+
+  /**
+   * Returns the cached result from the last call to
+   * UpdateViewportScrollbarStylesOverride() -- i.e. return the node
+   * whose scrollbar styles we have propagated to the viewport (or nullptr if
+   * there is no such node).
+   */
+  nsIContent* GetViewportScrollbarStylesOverrideNode() const {
+    return mViewportScrollbarOverrideNode;
+  }
+
+  const ScrollbarStyles& GetViewportScrollbarStylesOverride() const
   {
     return mViewportStyleScrollbar;
   }
@@ -1310,7 +1321,16 @@ protected:
 
   nscolor               mBodyTextColor;
 
+  // This is a non-owning pointer. May be null. If non-null, it's guaranteed
+  // to be pointing to a node that's still alive, because we'll reset it in
+  // UpdateViewportScrollbarStylesOverride() as part of the cleanup code
+  // when this node is removed from the document. (For <body> and the root node,
+  // this call happens in nsCSSFrameConstructor::ContentRemoved(). For
+  // fullscreen elements, it happens in the fullscreen-specific cleanup
+  // invoked by Element::UnbindFromTree().)
+  nsIContent* MOZ_NON_OWNING_REF mViewportScrollbarOverrideNode;
   ScrollbarStyles       mViewportStyleScrollbar;
+
   uint8_t               mFocusRingWidth;
 
   bool mExistThrottledUpdates;