anchor.diff:                          itemPositionsNotifier: backItemPositionsNotifier,
                           scrollDirection: widget.scrollDirection,
                           reverse: widget.reverse,
  -                        cacheExtent: _cacheExtent(constraints),
  +                        cacheExtent: cacheExtent,
                           alignment: backAlignment,
                           physics: widget.physics,
                           addSemanticIndexes: widget.addSemanticIndexes,
  
positive.code:   }
  
    @override
    Widget build(BuildContext context) => GestureDetector(
          onPanDown: (_) => cancelScrollCallback?.call(),
          excludeFromSemantics: true,
          child: Stack(
            children: <Widget>[
              if (_showBackList)
                PostMountCallback(
                  key: const ValueKey<String>('Back'),
                  callback: startAnimationCallback,
                  child: FadeTransition(
                    opacity: ReverseAnimation(opacity),
                    child: NotificationListener<ScrollNotification>(
                      onNotification: (_) {
                        return scrollNotificationCallback?.call() ==
                            _ListDisplay.back;
                      },
                      child: LayoutBuilder(
                        builder: (context, constraints) => PositionedList(
                          itemBuilder: widget.itemBuilder,
                          separatorBuilder: widget.separatorBuilder,
                          itemCount: widget.itemCount,
  
negative.code:                         controller: frontScrollController,
                          scrollDirection: widget.scrollDirection,
                          reverse: widget.reverse,
                          cacheExtent: cacheExtent,
                          alignment: frontAlignment,
                          physics: widget.physics,
                          addSemanticIndexes: widget.addSemanticIndexes,
  