anchor.diff:        };
         setState(() {
           if (_showFrontList) {
  -          bottomTarget = index;
  +          backTarget = index;
           } else {
             frontTarget = index;
           }
  
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:                         scrollDirection: widget.scrollDirection,
                          reverse: widget.reverse,
                          cacheExtent: constraints.maxHeight * _screenScrollCount,
                          alignment: frontAlignment,
                          physics: widget.physics,
                          addSemanticIndexes: widget.addSemanticIndexes,
                          semanticChildCount: widget.semanticChildCount,
  