--- a/layout/generic/nsSubDocumentFrame.cpp
+++ b/layout/generic/nsSubDocumentFrame.cpp
@@ -571,23 +571,23 @@ nsSubDocumentFrame::BuildDisplayList(nsD
   }
   if (constructResolutionItem) {
     nsDisplayResolution* resolutionItem =
       new (aBuilder) nsDisplayResolution(aBuilder, subdocRootFrame, &childItems,
                                          flags);
     childItems.AppendToTop(resolutionItem);
     needsOwnLayer = false;
   }
-  if (needsOwnLayer) {
-    // We always want top level content documents to be in their own layer.
-    nsDisplaySubDocument* layerItem = new (aBuilder) nsDisplaySubDocument(
-      aBuilder, subdocRootFrame ? subdocRootFrame : this, this,
-      &childItems, flags);
-    childItems.AppendToTop(layerItem);
-  }
+
+  // We always want top level content documents to be in their own layer.
+  nsDisplaySubDocument* layerItem = new (aBuilder) nsDisplaySubDocument(
+    aBuilder, subdocRootFrame ? subdocRootFrame : this, this,
+    &childItems, flags);
+  childItems.AppendToTop(layerItem);
+  layerItem->SetShouldFlattenAway(!needsOwnLayer);
 
   // If we're using containers for root frames, then the earlier call
   // to AddCanvasBackgroundColorItem won't have been able to add an
   // unscrolled color item for overscroll. Try again now that we're
   // outside the scrolled ContainerLayer.
   if (!aBuilder->IsForEventDelivery() &&
       gfxPrefs::LayoutUseContainersForRootFrames() &&
       !nsLayoutUtils::NeedsPrintPreviewBackground(presContext)) {

--- a/layout/painting/nsDisplayList.cpp
+++ b/layout/painting/nsDisplayList.cpp
@@ -6838,16 +6838,17 @@ nsDisplayOwnLayer::UpdateScrollData(mozi
 }
 
 nsDisplaySubDocument::nsDisplaySubDocument(nsDisplayListBuilder* aBuilder,
                                            nsIFrame* aFrame,
                                            nsSubDocumentFrame* aSubDocFrame,
                                            nsDisplayList* aList, uint32_t aFlags)
     : nsDisplayOwnLayer(aBuilder, aFrame, aList, aBuilder->CurrentActiveScrolledRoot(), aFlags)
     , mScrollParentId(aBuilder->GetCurrentScrollParentId())
+    , mShouldFlatten(false)
     , mSubDocFrame(aSubDocFrame)
 {
   MOZ_COUNT_CTOR(nsDisplaySubDocument);
   mForceDispatchToContentRegion =
     aBuilder->IsBuildingLayerEventRegions() &&
     nsLayoutUtils::HasDocumentLevelListenersForApzAwareEvents(aFrame->PresContext()->PresShell());
 
   // The SubDocument display item is conceptually outside the viewport frame,

--- a/layout/painting/nsDisplayList.h
+++ b/layout/painting/nsDisplayList.h
@@ -5091,27 +5091,38 @@ public:
 
   virtual nsSubDocumentFrame* SubDocumentFrame() { return mSubDocFrame; }
 
   virtual bool ComputeVisibility(nsDisplayListBuilder* aBuilder,
                                  nsRegion* aVisibleRegion) override;
 
   virtual bool ShouldBuildLayerEvenIfInvisible(nsDisplayListBuilder* aBuilder) const override;
 
+  virtual bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override
+  {
+    return mShouldFlatten;
+  }
+
+  void SetShouldFlattenAway(bool aShouldFlatten)
+  {
+    mShouldFlatten = aShouldFlatten;
+  }
+
   virtual nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
                                    bool* aSnap) const override;
 
   NS_DISPLAY_DECL_NAME("SubDocument", TYPE_SUBDOCUMENT)
 
   mozilla::UniquePtr<ScrollMetadata> ComputeScrollMetadata(Layer* aLayer,
                                                            const ContainerLayerParameters& aContainerParameters);
 
 protected:
   ViewID mScrollParentId;
   bool mForceDispatchToContentRegion;
+  bool mShouldFlatten;
   nsSubDocumentFrame* mSubDocFrame;
 };
 
 /**
  * A display item for subdocuments to capture the resolution from the presShell
  * and ensure that it gets applied to all the right elements. This item creates
  * a container layer.
  */