anchor.diff:    final frontOpacity =
         ProxyAnimation(const AlwaysStoppedAnimation<double>(1.0));
   
  -  int bottomTarget = 0;
  -  double bottomAlignment = 0;
  +  int backTarget = 0;
  +  double backAlignment = 0;
     int frontTarget;
  -  double backAlignment;
  +  double frontAlignment;
     Function cancelScrollCallback;
     Function endScrollCallback;
     _ListDisplay listDisplay = _ListDisplay.front;
  
positive.code:       double alignment,
        @required Duration duration,
        Curve curve = Curves.linear}) async {
      final lastTarget = _showFrontList ? frontTarget : bottomTarget;
      final direction = index > lastTarget ? 1 : -1;
      final startingListDisplay = listDisplay;
      final startingScrollController =
  
negative.code:       };
        setState(() {
          if (_showFrontList) {
            backTarget = index;
          } else {
            frontTarget = index;
          }
  