anchor.diff:        scrollTo(0.0);
       } else if (selectedIndex == config.items.length - 1) {
         _itemIndices = <int>[selectedIndex - 1, selectedIndex];
  -      scrollTo(config.itemExtent);
  +      scrollTo(1.0);
       } else {
         _itemIndices = <int>[selectedIndex - 1, selectedIndex, selectedIndex + 1];
  -      scrollTo(config.itemExtent);
  +      scrollTo(1.0);
       }
     }
   
  
positive.code: 
    void _handleProgressChange() {
      if (_scrollDirection == AnimationDirection.forward)
        scrollTo(config.itemExtent * _performance.progress);
      else
        scrollTo(config.itemExtent * (1.0 - _performance.progress));
    }
  
    int get itemCount => _itemIndices.length;
  
negative.code:   bool get isScrollable => false;
  }
  
  class _TabBarViewState<T> extends PageableListState<T, TabBarView<T>> {
  
    final _NotScrollable _notScrollable = new _NotScrollable();
    ScrollBehavior createScrollBehavior() => _notScrollable;
    ExtentScrollBehavior get scrollBehavior => _notScrollable;
  
    List<int> _itemIndices = [0, 1];
    AnimationDirection _scrollDirection = AnimationDirection.forward;
  