diff --git a/layout/base/nsDisplayList.cpp b/layout/base/nsDisplayList.cpp
index 7c338e216d6f..ac1eb6d99a82 100644
--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -61,6 +61,12 @@ using namespace mozilla::layers;
 using namespace mozilla::dom;
 typedef FrameMetrics::ViewID ViewID;
 
+static inline nsIFrame*
+GetTransformRootFrame(nsIFrame* aFrame)
+{
+  return nsLayoutUtils::GetTransformRootFrame(aFrame);
+}
+
 static void AddTransformFunctions(nsCSSValueList* aList,
                                   nsStyleContext* aContext,
                                   nsPresContext* aPresContext,
@@ -2878,15 +2884,6 @@ nsDisplayBoxShadowInner::ComputeVisibility(nsDisplayListBuilder* aBuilder,
   return true;
 }
 
-nsIFrame *GetTransformRootFrame(nsIFrame* aFrame)
-{
-  nsIFrame *parent = nsLayoutUtils::GetCrossDocParentFrame(aFrame);
-  while (parent && parent->Preserves3DChildren()) {
-    parent = nsLayoutUtils::GetCrossDocParentFrame(parent);
-  }
-  return parent;
-}
-
 nsDisplayWrapList::nsDisplayWrapList(nsDisplayListBuilder* aBuilder,
                                      nsIFrame* aFrame, nsDisplayList* aList)
   : nsDisplayItem(aBuilder, aFrame) {
diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 64e381d266ab..8d830ce00719 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -4803,6 +4803,16 @@ nsLayoutUtils::GetReferenceFrame(nsIFrame* aFrame)
   }
 }
 
+/* static */ nsIFrame*
+nsLayoutUtils::GetTransformRootFrame(nsIFrame* aFrame)
+{
+  nsIFrame *parent = nsLayoutUtils::GetCrossDocParentFrame(aFrame);
+  while (parent && parent->Preserves3DChildren()) {
+    parent = nsLayoutUtils::GetCrossDocParentFrame(parent);
+  }
+  return parent;
+}
+
 /* static */ uint32_t
 nsLayoutUtils::GetTextRunFlagsForStyle(nsStyleContext* aStyleContext,
                                        const nsStyleFont* aStyleFont,
diff --git a/layout/base/nsLayoutUtils.h b/layout/base/nsLayoutUtils.h
index 3b81498d2b4d..a195d72c9b48 100644
--- a/layout/base/nsLayoutUtils.h
+++ b/layout/base/nsLayoutUtils.h
@@ -1520,7 +1520,8 @@ public:
    * Get the reference frame that would be used when constructing a
    * display item for this frame.  (Note, however, that
    * nsDisplayTransform use the reference frame appropriate for their
-   * parent, rather than using their own frame as a reference frame.)
+   * GetTransformRootFrame(), rather than using their own frame as a
+   * reference frame.)
    *
    * This duplicates some of the logic of GetDisplayRootFrame above and
    * of nsDisplayListBuilder::FindReferenceFrameFor.
@@ -1530,6 +1531,16 @@ public:
    */
   static nsIFrame* GetReferenceFrame(nsIFrame* aFrame);
 
+  /**
+   * Get the parent of this frame, except if that parent is part of a
+   * preserve-3d hierarchy, get the parent of the root of the
+   * preserve-3d hierarchy.
+   *
+   * (This is used as the starting point for reference frame computation
+   * for nsDisplayTransform display items.)
+   */
+  static nsIFrame* GetTransformRootFrame(nsIFrame* aFrame);
+
   /**
    * Get textrun construction flags determined by a given style; in particular
    * some combination of: