--- a/layout/generic/nsSubDocumentFrame.cpp
+++ b/layout/generic/nsSubDocumentFrame.cpp
@@ -447,16 +447,41 @@ nsSubDocumentFrame::BuildDisplayList(nsD
   }
 
   if (!needsOwnLayer && aBuilder->IsBuildingLayerEventRegions() &&
       nsLayoutUtils::HasDocumentLevelListenersForApzAwareEvents(presShell))
   {
     needsOwnLayer = true;
   }
 
+  if (aBuilder->IsRetainingDisplayList()) {
+    // The value of needsOwnLayer can change between builds without
+    // an invalidation recorded for this frame (like if the root
+    // scrollframe becomes active). If this happens,
+    // then we need to notify the builder so that merging can
+    // happen correctly.
+    if (!mPreviouslyNeededLayer ||
+        mPreviouslyNeededLayer.value() != needsOwnLayer) {
+      dirty = visible;
+      aBuilder->MarkFrameModifiedDuringBuilding(this);
+    }
+    mPreviouslyNeededLayer = Some(needsOwnLayer);
+
+    // Caret frame changed, rebuild the entire subdoc.
+    // We could just invalidate the old and new frame
+    // areas and save some work here. RetainedDisplayListBuilder
+    // does this, so we could teach it to find and check all
+    // subdocs in advance.
+    if (mPreviousCaret != aBuilder->GetCaretFrame()) {
+      dirty = visible;
+      aBuilder->MarkFrameModifiedDuringBuilding(this);
+    }
+    mPreviousCaret = aBuilder->GetCaretFrame();
+  }
+
   nsDisplayList childItems(aBuilder);
 
   {
     DisplayListClipState::AutoSaveRestore nestedClipState(aBuilder);
     if (needsOwnLayer) {
       // Clear current clip. There's no point in propagating it down, since
       // the layer we will construct will be clipped by the current clip.
       // In fact for nsDisplayZoom propagating it down would be incorrect since

--- a/layout/generic/nsSubDocumentFrame.h
+++ b/layout/generic/nsSubDocumentFrame.h
@@ -162,15 +162,17 @@ protected:
   nsIFrame* ObtainIntrinsicSizeFrame();
 
   nsView* GetViewInternal() const override { return mOuterView; }
   void SetViewInternal(nsView* aView) override { mOuterView = aView; }
 
   RefPtr<nsFrameLoader> mFrameLoader;
   nsView* mOuterView;
   nsView* mInnerView;
+  Maybe<bool> mPreviouslyNeededLayer;
   bool mIsInline;
   bool mPostedReflowCallback;
   bool mDidCreateDoc;
   bool mCallingShow;
+  WeakFrame mPreviousCaret;
 };
 
 #endif /* NSSUBDOCUMENTFRAME_H_ */

--- a/layout/painting/nsDisplayList.cpp
+++ b/layout/painting/nsDisplayList.cpp
@@ -1258,16 +1258,30 @@ void
 nsDisplayListBuilder::EnterPresShell(nsIFrame* aReferenceFrame,
                                      bool aPointerEventsNoneDoc)
 {
   PresShellState* state = mPresShellStates.AppendElement();
   state->mPresShell = aReferenceFrame->PresContext()->PresShell();
   state->mCaretFrame = nullptr;
   state->mFirstFrameMarkedForDisplay = mFramesMarkedForDisplay.Length();
 
+  nsIScrollableFrame* sf = state->mPresShell->GetRootScrollFrameAsScrollable();
+  if (sf) {
+    // We are forcing a rebuild of nsDisplayCanvasBackgroundColor to make sure
+    // that the canvas background color will be set correctly, and that only one
+    // unscrollable item will be created.
+    // This is done to avoid, for example, a case where only scrollbar frames
+    // are invalidated - we would skip creating nsDisplayCanvasBackgroundColor
+    // and possibly end up with an extra nsDisplaySolidColor item.
+    nsCanvasFrame* canvasFrame = do_QueryFrame(sf->GetScrolledFrame());
+    if (canvasFrame) {
+      MarkFrameForDisplayIfVisible(canvasFrame);
+    }
+  }
+
 #ifdef DEBUG
   state->mAutoLayoutPhase.emplace(aReferenceFrame->PresContext(), eLayoutPhase_DisplayListBuilding);
 #endif
 
   state->mPresShell->UpdateCanvasBackground();
 
   bool buildCaret = mBuildCaret;
   if (mIgnoreSuppression || !state->mPresShell->IsPaintingSuppressed()) {