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:                   builder: (context, constraints) => PositionedList(
                      itemBuilder: widget.itemBuilder,
                      itemCount: widget.itemCount,
                      positionedIndex: bottomTarget,
                      controller: backScrollController,
                      itemPositionNotifier: backItemPositionNotifier,
                      scrollDirection: widget.scrollDirection,
                      reverse: widget.reverse,
                      cacheExtent: constraints.maxHeight * _screenScrollCount,
                      alignment: bottomAlignment,
                      physics: widget.physics,
                      addSemanticIndexes: widget.addSemanticIndexes,
                      semanticChildCount: widget.semanticChildCount,
  
negative.code:       };
        setState(() {
          if (_showFrontList) {
            backTarget = index;
          } else {
            frontTarget = index;
          }
  