--- a/layout/generic/nsIFrame.h
+++ b/layout/generic/nsIFrame.h
@@ -3171,16 +3171,17 @@ public:
   inline bool IsTableCaption() const;
 
   inline bool IsBlockInside() const;
   inline bool IsBlockOutside() const;
   inline bool IsInlineOutside() const;
   inline uint8_t GetDisplay() const;
   inline bool IsFloating() const;
   inline bool IsAbsPosContainingBlock() const;
+  inline bool IsFixedPosContainingBlock() const;
   inline bool IsRelativelyPositioned() const;
   inline bool IsAbsolutelyPositioned() const;
 
   /**
    * Returns the vertical-align value to be used for layout, if it is one
    * of the enumerated values.  If this is an SVG text frame, it returns a value
    * that corresponds to the value of dominant-baseline.  If the
    * vertical-align property has length or percentage value, this returns

--- a/layout/generic/nsIFrameInlines.h
+++ b/layout/generic/nsIFrameInlines.h
@@ -50,16 +50,22 @@ nsIFrame::IsFloating() const
 
 bool
 nsIFrame::IsAbsPosContainingBlock() const
 {
   return StyleDisplay()->IsAbsPosContainingBlock(this);
 }
 
 bool
+nsIFrame::IsFixedPosContainingBlock() const
+{
+  return StyleDisplay()->IsFixedPosContainingBlock(this);
+}
+
+bool
 nsIFrame::IsRelativelyPositioned() const
 {
   return StyleDisplay()->IsRelativelyPositioned(this);
 }
 
 bool
 nsIFrame::IsAbsolutelyPositioned() const
 {