--- a/layout/painting/nsDisplayList.h
+++ b/layout/painting/nsDisplayList.h
@@ -3202,16 +3202,102 @@ private:
   // is empty (i.e. everything that was visible is covered by some
   // opaque content in this list).
   bool mIsOpaque;
   // This is set to true by FrameLayerBuilder if any display item in this
   // list needs to force the surface containing this list to be transparent.
   bool mForceTransparentSurface;
 };
 
+class FlattenedDisplayItemIterator
+{
+public:
+  FlattenedDisplayItemIterator(nsDisplayListBuilder* aBuilder,
+                               nsDisplayList* aList,
+                               const bool aResolveFlattening = true)
+    : mBuilder(aBuilder)
+    , mNext(aList->GetBottom())
+  {
+    if (aResolveFlattening) {
+      // This is done conditionally in case subclass overrides
+      // ShouldFlattenNextItem().
+      ResolveFlattening();
+    }
+  }
+
+  virtual ~FlattenedDisplayItemIterator()
+  {
+    MOZ_ASSERT(!HasNext());
+  }
+
+  nsDisplayItem* GetNext()
+  {
+    nsDisplayItem* next = mNext;
+
+    // Advance mNext to the following item
+    if (next) {
+      mNext = mNext->GetAbove();
+      ResolveFlattening();
+    }
+    return next;
+  }
+
+  bool HasNext() const
+  {
+    return mNext || !mStack.IsEmpty();
+  }
+
+  nsDisplayItem* PeekNext()
+  {
+    return mNext;
+  }
+
+protected:
+  bool AtEndOfNestedList() const
+  {
+    return !mNext && mStack.Length() > 0;
+  }
+
+  virtual bool ShouldFlattenNextItem() const
+  {
+    return mNext && mNext->ShouldFlattenAway(mBuilder);
+  }
+
+  void ResolveFlattening()
+  {
+    // Handle the case where we reach the end of a nested list, or the current
+    // item should start a new nested list. Repeat this until we find an actual
+    // item, or the very end of the outer list.
+    while (AtEndOfNestedList() || ShouldFlattenNextItem()) {
+      if (AtEndOfNestedList()) {
+        // Pop the last item off the stack.
+        mNext = mStack.LastElement();
+        EndNested(mNext);
+        mStack.RemoveElementAt(mStack.Length() - 1);
+        // We stored the item that was flattened, so advance to the next.
+        mNext = mNext->GetAbove();
+      } else {
+        // This item wants to be flattened. Store the current item on the stack,
+        // and use the first item in the child list instead.
+        mStack.AppendElement(mNext);
+        StartNested(mNext);
+        nsDisplayList* childItems = mNext->GetSameCoordinateSystemChildren();
+        mNext = childItems->GetBottom();
+      }
+    }
+  }
+
+  virtual void EndNested(nsDisplayItem* aItem) {}
+  virtual void StartNested(nsDisplayItem* aItem) {}
+
+  nsDisplayListBuilder* mBuilder;
+  nsDisplayItem* mNext;
+  AutoTArray<nsDisplayItem*, 10> mStack;
+};
+
 /**
  * This is passed as a parameter to nsIFrame::BuildDisplayList. That method
  * will put any generated items onto the appropriate list given here. It's
  * basically just a collection with one list for each separate stacking layer.
  * The lists themselves are external to this object and thus can be shared
  * with others. Some of the list pointers may even refer to the same list.
  */
 class nsDisplayListSet {
@@ -6773,82 +6859,16 @@ public:
   // (i.e. left and right respectively in horizontal writing modes,
   // regardless of bidi directionality; top and bottom in vertical modes).
   nscoord mVisIStartEdge;
   nscoord mVisIEndEdge;
   // Cached result of mFrame->IsSelected().  Only initialized when needed.
   mutable mozilla::Maybe<bool> mIsFrameSelected;
 };
 
-class FlattenedDisplayItemIterator
-{
-public:
-  FlattenedDisplayItemIterator(nsDisplayListBuilder* aBuilder,
-                               nsDisplayList* aList)
-    : mBuilder(aBuilder)
-    , mNext(aList->GetBottom())
-  {
-    ResolveFlattening();
-  }
-
-  nsDisplayItem* GetNext()
-  {
-    nsDisplayItem* next = mNext;
-
-    // Advance mNext to the following item
-    if (next) {
-      mNext = mNext->GetAbove();
-      ResolveFlattening();
-    }
-    return next;
-  }
-
-  nsDisplayItem* PeekNext()
-  {
-    return mNext;
-  }
-
-private:
-  bool AtEndOfNestedList()
-  {
-    return !mNext && mStack.Length() > 0;
-  }
-
-  bool ShouldFlattenNextItem()
-  {
-    return mNext && mNext->ShouldFlattenAway(mBuilder);
-  }
-
-  void ResolveFlattening()
-  {
-    // Handle the case where we reach the end of a nested list, or the current
-    // item should start a new nested list. Repeat this until we find an actual
-    // item, or the very end of the outer list.
-    while (AtEndOfNestedList() || ShouldFlattenNextItem()) {
-      if (AtEndOfNestedList()) {
-        // Pop the last item off the stack.
-        mNext = mStack.PopLastElement();
-        // We stored the item that was flattened, so advance to the next.
-        mNext = mNext->GetAbove();
-      } else {
-        // This item wants to be flattened. Store the current item on the stack,
-        // and use the first item in the child list instead.
-        mStack.AppendElement(mNext);
-        nsDisplayList* childItems = mNext->GetSameCoordinateSystemChildren();
-        mNext = childItems->GetBottom();
-      }
-    }
-  }
-
-
-  nsDisplayListBuilder* mBuilder;
-  nsDisplayItem* mNext;
-  AutoTArray<nsDisplayItem*, 10> mStack;
-};
-
 /**
  * A display item that for webrender to handle SVG
  */
 class nsDisplaySVGWrapper : public nsDisplayWrapList {
 public:
   NS_DISPLAY_DECL_NAME("SVGWrapper", TYPE_SVG_WRAPPER)
 
   nsDisplaySVGWrapper(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,