diff -r 000000000000 -r 6474c204b198 layout/xul/nsStackFrame.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/xul/nsStackFrame.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,63 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//
+// Eric Vaughan
+// Netscape Communications
+//
+// See documentation in associated header file
+//
+
+#include "nsStackFrame.h"
+#include "nsStyleContext.h"
+#include "nsIContent.h"
+#include "nsCOMPtr.h"
+#include "nsHTMLParts.h"
+#include "nsIPresShell.h"
+#include "nsCSSRendering.h"
+#include "nsBoxLayoutState.h"
+#include "nsStackLayout.h"
+#include "nsDisplayList.h"
+
+nsIFrame*
+NS_NewStackFrame(nsIPresShell* aPresShell, nsStyleContext* aContext)
+{
+  return new (aPresShell) nsStackFrame(aPresShell, aContext);
+}
+
+NS_IMPL_FRAMEARENA_HELPERS(nsStackFrame)
+
+nsStackFrame::nsStackFrame(nsIPresShell* aPresShell, nsStyleContext* aContext):
+  nsBoxFrame(aPresShell, aContext)
+{
+  nsCOMPtr<nsBoxLayout> layout;
+  NS_NewStackLayout(aPresShell, layout);
+  SetLayoutManager(layout);
+}
+
+// REVIEW: The old code put everything in the background layer. To be more
+// consistent with the way other frames work, I'm putting everything in the
+// Content() (i.e., foreground) layer (see nsFrame::BuildDisplayListForChild,
+// the case for stacking context but non-positioned, non-floating frames).
+// This could easily be changed back by hacking nsBoxFrame::BuildDisplayListInternal
+// a bit more.
+void
+nsStackFrame::BuildDisplayListForChildren(nsDisplayListBuilder*   aBuilder,
+                                          const nsRect&           aDirtyRect,
+                                          const nsDisplayListSet& aLists)
+{
+  // BuildDisplayListForChild puts stacking contexts into the PositionedDescendants
+  // list. So we need to map that list to aLists.Content(). This is an easy way to
+  // do that.
+  nsDisplayList* content = aLists.Content();
+  nsDisplayListSet kidLists(content, content, content, content, content, content);
+  nsIFrame* kid = mFrames.FirstChild();
+  while (kid) {
+    // Force each child into its own true stacking context.
+    BuildDisplayListForChild(aBuilder, kid, aDirtyRect, kidLists,
+                             DISPLAY_CHILD_FORCE_STACKING_CONTEXT);
+    kid = kid->GetNextSibling();
+  }
+}