title: How to keep at least one panel in an accordion expanded?
body: <p>Say I have an accordion which has 2 expansion panels, by default, there are three states of this accordion:</p>
  
  <p>State 1: Panel 1 closed, Panel 2 closed</p>
  
  <p>State 2: Panel 1 expanded, Panel 2 closed</p>
  
  <p>State 3: Panel 1 closed, Panel 2 expanded</p>
  
  <p>I want to forbid user from closing all panels. They can open panel 1 close panel 2, or open panel 2 close panel 1, but not close them both.</p>