title: How to add a fixed bar between a sliver bar and the scrolling content
body: <p>I have a view in my flutter mobile app in which there is:</p>
  
  <ol>
  <li>A top bar that is a <strong>sliver bar</strong> (title + tabs) [red box on image]</li>
  <li>A <strong>scrollable list</strong> of data [green box on image]</li>
  </ol>
  
  <p>When i scroll, the tabs disappear leaving only the page title.</p>
  
  <p>Goal:</p>
  
  <p><img src="https://i.stack.imgur.com/RArId.png" alt="Intended goal"></p>
  
  <p>I want to add a <strong>header</strong> [blue box on image] that does <strong>not disappear when i scroll</strong> and stays between the sliver bar and the scrollable items.</p>
  
  <p>I seem to only be able to put the header inside the title of the sliver bar above the tabs, or in the scrollable list in which case it disappears as i scroll</p>