title: Flutter: Persistent item in CustomScrollView
body: <p>Currently I'm creating an application using Flutter which involves several lists which can be ordered and filtered. When an ordering or filter is active, persistent bar appears under the app bar which indicates what filters are active. This is achieved by stacking int on top of a <code>ListView</code> using a <code>StackView</code>, and moving the <code>ListView</code> down a little using it's padding. An example of this can be seen in the image below:</p>
  
  <p><a href="https://i.stack.imgur.com/csMwN.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/csMwN.jpg" alt="Working app bar"></a></p>
  
  <p>Now I'm trying to create a second view using a <code>CustomScrollView</code> and a <code>SliverAppBar</code>, and I want this same "filter bar" behaviour here. I've tried using a <code>SliverList</code> with a single item, and a <code>SliverList</code> below that with the filtered items, but then both lists scroll as if they were one long list. I've also tried adding a bar through the <code>bottom</code> property of the <code>SliverAppBar</code>, but this results in a bar which is placed over the title of the app bar, and as far as I know, there is no way to move this title up. This result can be seen in the image below (the red bar is a placeholder for the filter bar in the first image).</p>
  
  <p><a href="https://i.stack.imgur.com/m4YI4.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/m4YI4.jpg" alt="enter image description here"></a></p>
  
  <p>Is there any way to achieve this "persistent" bar effect in a <code>CustomScrollView</code> with a <code>SliverAppBar</code>?</p>