--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -2458,16 +2458,32 @@ MaxZIndexInList(nsDisplayList* aList, ns
 {
   int32_t maxZIndex = 0;
   for (nsDisplayItem* item = aList->GetBottom(); item; item = item->GetAbove()) {
     maxZIndex = std::max(maxZIndex, item->ZIndex());
   }
   return maxZIndex;
 }
 
+// Finds the max z-index of the items in aList that meet the following conditions
+//   1) have z-index auto or z-index >= 0.
+//   2) aFrame is a proper ancestor of the item's frame.
+// Returns -1 if there is no such item.
+static int32_t
+MaxZIndexInListOfItemsContainedInFrame(nsDisplayList* aList, nsIFrame* aFrame)
+{
+  int32_t maxZIndex = -1;
+  for (nsDisplayItem* item = aList->GetBottom(); item; item = item->GetAbove()) {
+    if (nsLayoutUtils::IsProperAncestorFrame(aFrame, item->Frame())) {
+      maxZIndex = std::max(maxZIndex, item->ZIndex());
+    }
+  }
+  return maxZIndex;
+}
+
 static void
 AppendToTop(nsDisplayListBuilder* aBuilder, const nsDisplayListSet& aLists,
             nsDisplayList* aSource, nsIFrame* aSourceFrame, bool aOwnLayer,
             bool aPositioned)
 {
   if (aSource->IsEmpty())
     return;
 
@@ -2992,28 +3008,35 @@ ScrollFrameHelper::BuildDisplayList(nsDi
       wrapper.WrapListsInPlace(aBuilder, mOuter, scrolledContent);
     }
 
     // In case we are not using displayport or the nsDisplayScrollLayers are
     // flattened during visibility computation, we still need to export the
     // metadata about this scroll box to the compositor process.
     nsDisplayScrollInfoLayer* layerItem = new (aBuilder) nsDisplayScrollInfoLayer(
       aBuilder, mScrolledFrame, mOuter);
+    nsDisplayList* positionedDescendants = scrolledContent.PositionedDescendants();
     if (BuildScrollContainerLayers()) {
       // We process display items from bottom to top, so if we need to flatten after
       // the scroll layer items have been processed we need to be on the top.
-      nsDisplayList* positionedDescendants = scrolledContent.PositionedDescendants();
       if (!positionedDescendants->IsEmpty()) {
         layerItem->SetOverrideZIndex(MaxZIndexInList(positionedDescendants, aBuilder));
         positionedDescendants->AppendNewToTop(layerItem);
       } else {
         aLists.Outlines()->AppendNewToTop(layerItem);
       }
     } else {
-      scrolledContent.BorderBackground()->AppendNewToBottom(layerItem);
+      int32_t zindex =
+        MaxZIndexInListOfItemsContainedInFrame(positionedDescendants, mOuter);
+      if (zindex >= 0) {
+        layerItem->SetOverrideZIndex(zindex);
+        positionedDescendants->AppendNewToTop(layerItem);
+      } else {
+        scrolledContent.Outlines()->AppendNewToTop(layerItem);
+      }
     }
   }
   // Now display overlay scrollbars and the resizer, if we have one.
   AppendScrollPartsTo(aBuilder, aDirtyRect, scrolledContent, usingDisplayport,
                       createLayersForScrollbars, true);
   scrolledContent.MoveTo(aLists);
 }