--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -3014,16 +3014,17 @@ AppendInternalItemToTop(const nsDisplayL
     aLists.Content()->AppendToTop(aItem);
   }
 }
 
 static const uint32_t APPEND_OWN_LAYER = 0x1;
 static const uint32_t APPEND_POSITIONED = 0x2;
 static const uint32_t APPEND_SCROLLBAR_CONTAINER = 0x4;
 static const uint32_t APPEND_OVERLAY = 0x8;
+static const uint32_t APPEND_TOP = 0x10;
 
 static void
 AppendToTop(nsDisplayListBuilder* aBuilder, const nsDisplayListSet& aLists,
             nsDisplayList* aSource, nsIFrame* aSourceFrame, uint32_t aFlags)
 {
   if (aSource->IsEmpty())
     return;
 
@@ -3051,17 +3052,19 @@ AppendToTop(nsDisplayListBuilder* aBuild
     newItem = MakeDisplayItem<nsDisplayWrapList>(aBuilder, aSourceFrame, aSource, asr, false, 1);
   }
 
   if (aFlags & APPEND_POSITIONED) {
     // We want overlay scrollbars to always be on top of the scrolled content,
     // but we don't want them to unnecessarily cover overlapping elements from
     // outside our scroll frame.
     Maybe<int32_t> zIndex = Nothing();
-    if (aFlags & APPEND_OVERLAY) {
+    if (aFlags & APPEND_TOP) {
+      zIndex = Some(INT32_MAX);
+    } else if (aFlags & APPEND_OVERLAY) {
       zIndex = MaxZIndexInList(aLists.PositionedDescendants(), aBuilder);
     } else if (aSourceFrame->StylePosition()->mZIndex.GetUnit() == eStyleUnit_Integer) {
       zIndex = Some(aSourceFrame->StylePosition()->mZIndex.GetIntValue());
 
     }
     AppendInternalItemToTop(aLists, newItem, zIndex);
   } else {
     aLists.BorderBackground()->AppendToTop(newItem);
@@ -3182,20 +3185,25 @@ ScrollFrameHelper::AppendScrollPartsTo(n
     }
 
     if (createLayer) {
       appendToTopFlags |= APPEND_OWN_LAYER;
     }
     if (aPositioned) {
       appendToTopFlags |= APPEND_POSITIONED;
     }
-    if (overlayScrollbars ||
+
+    if (isOverlayScrollbar ||
         scrollParts[i] == mResizerBox) {
-      appendToTopFlags |= APPEND_OVERLAY;
-      aBuilder->SetDisablePartialUpdates(true);
+      if (isOverlayScrollbar && mIsRoot) {
+        appendToTopFlags |= APPEND_TOP;
+      } else {
+        appendToTopFlags |= APPEND_OVERLAY;
+        aBuilder->SetDisablePartialUpdates(true);
+      }
     }
 
     {
       nsDisplayListBuilder::AutoBuildingDisplayList
         buildingForChild(aBuilder, scrollParts[i],
                          visible + mOuter->GetOffsetTo(scrollParts[i]),
                          dirty + mOuter->GetOffsetTo(scrollParts[i]), true);
       nsDisplayListBuilder::AutoCurrentScrollbarInfoSetter