title: WPF ListBox fixed width
body: <p>I have a <code>ListBox</code> in a <code>PopupControl</code>. Problem is that while scrolling, Popup resizes
  to fit the actual widest element.</p>
  
  <p>How can I avoid this resizing and adjust automatically to the widest element in the whole list?</p>
  
  <p>I tried to put it in a <code>Grid</code> but without success.</p>