--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -3053,17 +3053,19 @@ AppendToTop(nsDisplayListBuilder* aBuild
       scrollbarData.mTargetViewId = aBuilder->GetCurrentScrollbarTarget();
       // The flags here should be exactly one scrollbar direction
       MOZ_ASSERT(flags != nsDisplayOwnLayerFlags::eNone);
       flags |= nsDisplayOwnLayerFlags::eScrollbarContainer;
     }
 
     newItem = MakeDisplayItem<nsDisplayOwnLayer>(aBuilder, aSourceFrame, aSource, asr, flags, scrollbarData);
   } else {
-    newItem = MakeDisplayItem<nsDisplayWrapList>(aBuilder, aSourceFrame, aSource, asr);
+    // Build the wrap list with an index of 1, since the scrollbar frame itself might have already
+    // built an nsDisplayWrapList.
+    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) {

--- a/layout/painting/nsDisplayList.cpp
+++ b/layout/painting/nsDisplayList.cpp
@@ -6076,20 +6076,22 @@ nsDisplayWrapList::nsDisplayWrapList(nsD
                                      nsIFrame* aFrame, nsDisplayList* aList)
   : nsDisplayWrapList(aBuilder, aFrame, aList,
                       aBuilder->CurrentActiveScrolledRoot())
 {}
 
 nsDisplayWrapList::nsDisplayWrapList(nsDisplayListBuilder* aBuilder,
                                      nsIFrame* aFrame, nsDisplayList* aList,
                                      const ActiveScrolledRoot* aActiveScrolledRoot,
-                                     bool aClearClipChain)
+                                     bool aClearClipChain,
+                                     uint32_t aIndex)
   : nsDisplayItem(aBuilder, aFrame, aActiveScrolledRoot)
   , mFrameActiveScrolledRoot(aBuilder->CurrentActiveScrolledRoot())
   , mOverrideZIndex(0)
+  , mIndex(aIndex)
   , mHasZIndexOverride(false)
   , mClearingClipChain(aClearClipChain)
 {
   MOZ_COUNT_CTOR(nsDisplayWrapList);
 
   mBaseVisibleRect = mVisibleRect;
 
   mListPtr = &mList;
@@ -6122,16 +6124,17 @@ nsDisplayWrapList::nsDisplayWrapList(nsD
 
   SetVisibleRect(visible, true);
 }
 
 nsDisplayWrapList::nsDisplayWrapList(nsDisplayListBuilder* aBuilder,
                                      nsIFrame* aFrame, nsDisplayItem* aItem)
   : nsDisplayItem(aBuilder, aFrame)
   , mOverrideZIndex(0)
+  , mIndex(0)
   , mHasZIndexOverride(false)
 {
   MOZ_COUNT_CTOR(nsDisplayWrapList);
 
   mBaseVisibleRect = mVisibleRect;
 
   mListPtr = &mList;
   mListPtr->AppendToTop(aItem);

--- a/layout/painting/nsDisplayList.h
+++ b/layout/painting/nsDisplayList.h
@@ -4997,23 +4997,25 @@ public:
   /**
    * Takes all the items from aList and puts them in our list.
    */
   nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                     nsDisplayList* aList);
   nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                     nsDisplayList* aList,
                     const ActiveScrolledRoot* aActiveScrolledRoot,
-                    bool aClearClipChain = false);
+                    bool aClearClipChain = false,
+                    uint32_t aIndex = 0);
   nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                     nsDisplayItem* aItem);
   nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
     : nsDisplayItem(aBuilder, aFrame)
     , mFrameActiveScrolledRoot(aBuilder->CurrentActiveScrolledRoot())
     , mOverrideZIndex(0)
+    , mIndex(0)
     , mHasZIndexOverride(false)
   {
     MOZ_COUNT_CTOR(nsDisplayWrapList);
     mBaseVisibleRect = mVisibleRect;
     mListPtr = &mList;
   }
 
   /**
@@ -5024,16 +5026,17 @@ public:
   nsDisplayWrapList(nsDisplayListBuilder* aBuilder, const nsDisplayWrapList& aOther)
     : nsDisplayItem(aBuilder, aOther)
     , mListPtr(&mList)
     , mFrameActiveScrolledRoot(aOther.mFrameActiveScrolledRoot)
     , mMergedFrames(aOther.mMergedFrames)
     , mBounds(aOther.mBounds)
     , mBaseVisibleRect(aOther.mBaseVisibleRect)
     , mOverrideZIndex(aOther.mOverrideZIndex)
+    , mIndex(aOther.mIndex)
     , mHasZIndexOverride(aOther.mHasZIndexOverride)
     , mClearingClipChain(aOther.mClearingClipChain)
   {
     MOZ_COUNT_CTOR(nsDisplayWrapList);
   }
 
   virtual ~nsDisplayWrapList();
 
@@ -5093,16 +5096,21 @@ public:
                            bool* aSnap) const override;
   virtual nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
                                    bool* aSnap) const override;
   virtual mozilla::Maybe<nscolor> IsUniform(nsDisplayListBuilder* aBuilder) const override;
   virtual void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
   virtual bool ComputeVisibility(nsDisplayListBuilder* aBuilder,
                                  nsRegion* aVisibleRegion) override;
 
+  virtual uint32_t GetPerFrameKey() const override
+  {
+    return (mIndex << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
+  }
+
   virtual bool CanMerge(const nsDisplayItem* aItem) const override
   {
     return false;
   }
 
   virtual void Merge(const nsDisplayItem* aItem) override
   {
     MOZ_ASSERT(CanMerge(aItem));
@@ -5207,16 +5215,17 @@ protected:
   // The frames from items that have been merged into this item, excluding
   // this item's own frame.
   nsTArray<nsIFrame*> mMergedFrames;
   nsRect mBounds;
   // Visible rect contributed by this display item itself.
   // Our mVisibleRect may include the visible areas of children.
   nsRect mBaseVisibleRect;
   int32_t mOverrideZIndex;
+  uint32_t mIndex;
   bool mHasZIndexOverride;
   bool mClearingClipChain = false;
 };
 
 /**
  * We call WrapDisplayList on the in-flow lists: BorderBackground(),
  * BlockBorderBackgrounds() and Content().
  * We call WrapDisplayItem on each item of Outlines(), PositionedDescendants(),