title: How to show scrollbar in SliverList?
body: <p>I'm using SliverAppBar and SliverList as main containers in my app. 
  When I try to wrap SliverList in Scrollbar widget, I get an error, and when I wrap whole CustomScrollView in Scrollbar, it overlaps SliverAppBar. 
  So how can I show scrollbar indicator only in my SliverList? 
  Thanks in advance.</p>