title: Create combobox with non selectable separator
body: <p>I'd like to custom combobox.
  Indeed, I like to create a combox with two array separate by a line like that.</p>
  
  <p><img src="https://i.stack.imgur.com/3SAsA.jpg" alt="enter image description here"></p>
  
  <p>The line between All and above 40 must no selectable.</p>
  
  <p>Do you know how to do that?</p>
  
  <p>Thanks for helping</p>