anchor.diff:        double alignment,
         @required Duration duration,
         Curve curve = Curves.linear}) async {
  -    final lastTarget = _showFrontList ? frontTarget : bottomTarget;
  +    final lastTarget = _showFrontList ? frontTarget : backTarget;
       final direction = index > lastTarget ? 1 : -1;
       final startingListDisplay = listDisplay;
       final startingScrollController =
  
positive.code:   final frontOpacity =
        ProxyAnimation(const AlwaysStoppedAnimation<double>(1.0));
  
    int bottomTarget = 0;
    double bottomAlignment = 0;
    int frontTarget;
    double backAlignment;
    Function cancelScrollCallback;
    Function endScrollCallback;
    _ListDisplay listDisplay = _ListDisplay.front;
  
negative.code:   void initState() {
      super.initState();
      frontTarget = widget.initialScrollIndex;
      frontAlignment = widget.initialAlignment;
      widget.itemScrollController?._attach(this);
      frontItemPositionNotifier.itemPositions.addListener(_updatePositions);
      backItemPositionNotifier.itemPositions.addListener(_updatePositions);
  