diff --git a/mozilla/layout/generic/nsFrame.cpp b/mozilla/layout/generic/nsFrame.cpp
index 72abc9c3b8c..61818eebef6 100644
--- a/mozilla/layout/generic/nsFrame.cpp
+++ b/mozilla/layout/generic/nsFrame.cpp
@@ -1368,7 +1368,7 @@ nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder*   aBuilder,
 
   PRBool isComposited = disp->mOpacity != 1.0f;
   PRBool isPositioned = disp->IsPositioned();
-  if (isComposited || isPositioned) {
+  if (isComposited || isPositioned || (aFlags & DISPLAY_CHILD_FORCE_STACKING_CONTEXT)) {
     // If you change this, also change IsPseudoStackingContextFromStyle()
     pseudoStackingContext = PR_TRUE;
   }
@@ -1405,7 +1405,7 @@ nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder*   aBuilder,
   nsDisplayList extraPositionedDescendants;
   const nsStylePosition* pos = aChild->GetStylePosition();
   if ((isPositioned && pos->mZIndex.GetUnit() == eStyleUnit_Integer) ||
-      isComposited) {
+      isComposited || (aFlags & DISPLAY_CHILD_FORCE_STACKING_CONTEXT)) {
     // True stacking context
     rv = aChild->BuildDisplayListForStackingContext(aBuilder, dirty, &list);
     if (NS_SUCCEEDED(rv)) {
@@ -1453,7 +1453,8 @@ nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder*   aBuilder,
   }
   NS_ENSURE_SUCCESS(rv, rv);
     
-  if (isPositioned || isComposited) {
+  if (isPositioned || isComposited ||
+      (aFlags & DISPLAY_CHILD_FORCE_STACKING_CONTEXT)) {
     // Genuine stacking contexts, and positioned pseudo-stacking-contexts,
     // go in this level.
     rv = aLists.PositionedDescendants()->AppendNewToTop(new (aBuilder)
diff --git a/mozilla/layout/generic/nsIFrame.h b/mozilla/layout/generic/nsIFrame.h
index f5cd8995655..a90b253b031 100644
--- a/mozilla/layout/generic/nsIFrame.h
+++ b/mozilla/layout/generic/nsIFrame.h
@@ -762,16 +762,17 @@ public:
 
   enum {
     DISPLAY_CHILD_FORCE_PSEUDO_STACKING_CONTEXT = 0x01,
-    DISPLAY_CHILD_INLINE = 0x02
-  };                 
+    DISPLAY_CHILD_FORCE_STACKING_CONTEXT = 0x02,
+    DISPLAY_CHILD_INLINE = 0x04
+  };
   /**
    * Adjusts aDirtyRect for the child's offset, checks that the dirty rect
    * actually intersects the child (or its descendants), calls BuildDisplayList
    * on the child if necessary, and puts things in the right lists if the child
    * is positioned.
    *
-   * @param aFlags combination of DISPLAY_CHILD_FORCE_PSEUDO_STACKING_CONTEXT
-   * and DISPLAY_CHILD_INLINE
+   * @param aFlags combination of DISPLAY_CHILD_FORCE_PSEUDO_STACKING_CONTEXT,
+   *    DISPLAY_CHILD_FORCE_STACKING_CONTEXT and DISPLAY_CHILD_INLINE
    */
   nsresult BuildDisplayListForChild(nsDisplayListBuilder*   aBuilder,
                                     nsIFrame*               aChild,
diff --git a/mozilla/layout/xul/base/src/nsStackFrame.cpp b/mozilla/layout/xul/base/src/nsStackFrame.cpp
index 637c6af3143..313c2460445 100644
--- a/mozilla/layout/xul/base/src/nsStackFrame.cpp
+++ b/mozilla/layout/xul/base/src/nsStackFrame.cpp
@@ -57,6 +57,7 @@
 #include "nsIViewManager.h"
 #include "nsBoxLayoutState.h"
 #include "nsStackLayout.h"
+#include "nsDisplayList.h"
 
 nsIFrame*
 NS_NewStackFrame (nsIPresShell* aPresShell, nsStyleContext* aContext, nsIBoxLayout* aLayoutManager)
@@ -88,12 +89,17 @@ 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 stacking context.
+    // Force each child into its own true stacking context.
     nsresult rv =
-      BuildDisplayListForChild(aBuilder, kid, aDirtyRect, aLists,
-                               DISPLAY_CHILD_FORCE_PSEUDO_STACKING_CONTEXT);
+      BuildDisplayListForChild(aBuilder, kid, aDirtyRect, kidLists,
+                               DISPLAY_CHILD_FORCE_STACKING_CONTEXT);
     NS_ENSURE_SUCCESS(rv, rv);
     kid = kid->GetNextSibling();
   }