Signature used by NestedScrollView for building its header.
innerBoxIsScrolled argument is typically used to control the
SliverAppBar.forceElevated property to ensure that the app bar shows a
shadow, since it would otherwise not necessarily be aware that it had
content ostensibly below it.
typedef NestedScrollViewHeaderSliversBuilder = List<Widget> Function(BuildContext context, bool innerBoxIsScrolled);