anchor.diff:  }
   
   class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
  +  Animation<double> _scaffoldAnimation;
  +
  +  void _handleTick() {
  +    setState(() {
  +      // The animation's state is our build state, and it changed already.
  +    });
  +  }
  +
  +  @override
  +  void deactivate() {
  +    _scaffoldAnimation?.removeListener(_handleTick);
  +    _scaffoldAnimation = null;
  +  }
  +
     @override
     Widget build(BuildContext context) {
       assert(debugCheckHasScaffold(context));
       final double statusBarHeight = MediaQuery.of(context).padding.top;
  -    final Animation<double> animation = Scaffold.of(context).appBarAnimation;
  -    final double appBarHeight = Scaffold.of(context).appBarHeight + statusBarHeight;
  +    final ScaffoldState scaffold = Scaffold.of(context);
  +    _scaffoldAnimation ??= scaffold.appBarAnimation..addListener(_handleTick);
  +    final double appBarHeight = scaffold.appBarHeight + statusBarHeight;
       final double toolBarHeight = kToolBarHeight + statusBarHeight;
       final List<Widget> children = <Widget>[];
   
  
positive.code:       final double fadeStart = (appBarHeight - toolBarHeight) / appBarHeight;
        final double fadeEnd = (appBarHeight - toolBarHeight / 2.0) / appBarHeight;
        final CurvedAnimation opacityCurve = new CurvedAnimation(
          parent: animation,
          curve: new Interval(fadeStart, fadeEnd)
        );
        TextStyle titleStyle = Theme.of(context).primaryTextTheme.title;
  
negative.code:       final double fadeStart = (appBarHeight - toolBarHeight * 2.0) / appBarHeight;
        final double fadeEnd = (appBarHeight - toolBarHeight) / appBarHeight;
        final CurvedAnimation opacityCurve = new CurvedAnimation(
          parent: _scaffoldAnimation,
          curve: new Interval(math.max(0.0, fadeStart), math.min(fadeEnd, 1.0))
        );
        final double parallax = new Tween<double>(begin: 0.0, end: appBarHeight / 4.0).evaluate(_scaffoldAnimation);
        children.add(new Positioned(
          top: -parallax,
          left: 0.0,
  