title: Flutter CustomScrollView slivers stacking
body: <p>I am trying to create a scrollView using <a href="https://docs.flutter.io/flutter/widgets/CustomScrollView-class.html" rel="noreferrer">CustomScrollView</a>.
  The effect that I need, is very similar to <a href="https://www.youtube.com/watch?v=4R_-YeHnej0" rel="noreferrer">this one</a>.</p>
  
  <p>I need the <a href="https://docs.flutter.io/flutter/widgets/SliverList-class.html" rel="noreferrer">SliverList</a> to be stacked above the <a href="https://docs.flutter.io/flutter/material/SliverAppBar-class.html" rel="noreferrer">SliverAppbar</a>, without the list taking the whole screen and hiding the SliverAppbar.
  The reason I want to do this, is that i need to attach a persistent Positioned widget on top of that list, and it won't appear unless the list is stacked above the SliverAppbar.</p>
  
  <p>Here's my <a href="https://github.com/Macromeda/scrollme/blob/master/lib/main.dart" rel="noreferrer">code</a>.</p>