anchor.diff:                          itemBuilder: widget.itemBuilder,
                           separatorBuilder: widget.separatorBuilder,
                           itemCount: widget.itemCount,
  -                        positionedIndex: backTarget,
  -                        controller: backScrollController,
  -                        itemPositionsNotifier: backItemPositionsNotifier,
  +                        positionedIndex: back.target,
  +                        controller: back.scrollController,
  +                        itemPositionsNotifier: back.itemPositionsNotifier,
                           scrollDirection: widget.scrollDirection,
                           reverse: widget.reverse,
                           cacheExtent: cacheExtent,
  -                        alignment: backAlignment,
  +                        alignment: back.alignment,
                           physics: widget.physics,
                           addSemanticIndexes: widget.addSemanticIndexes,
                           semanticChildCount: widget.semanticChildCount,
  
positive.code: 
  class _ScrollablePositionedListState extends State<ScrollablePositionedList>
      with TickerProviderStateMixin {
    final frontItemPositionsNotifier = ItemPositionsNotifier();
    final backItemPositionsNotifier = ItemPositionsNotifier();
    final frontScrollController = ScrollController(keepScrollOffset: false);
    final backScrollController = ScrollController(keepScrollOffset: false);
    final opacity = ProxyAnimation(const AlwaysStoppedAnimation<double>(1.0));
  
    int backTarget = 0;
    double backAlignment = 0;
    int frontTarget;
    double frontAlignment;
    void Function() cancelScrollCallback;
    void Function() endScrollCallback;
    _ListDisplay Function() scrollNotificationCallback;
  
negative.code:       final scrollAmount = _screenScrollCount *
            startingScrollController.position.viewportDimension;
        final endingScrollController =
            _showFrontList ? back.scrollController : front.scrollController;
        final startCompleter = Completer<void>();
        final endCompleter = Completer<void>();
        startAnimationCallback = () {
  