title: Change the ComboBox Popup control to be right-aligned with the its parent
body: <p>I'm trying to get a <code>ComboBox</code> to display its <code>Popup</code> right aligned rather than left-aligned as is the default.
  I mean I want the right edge of the <code>Popup</code> to be aligned with the right edge of its parent.</p>
  
  <p>I tried overriding the <code>ComboBox</code> class but that didn't quite work as the <code>Popup</code> HorizontalOffset seems to be reset somehow.
  I thought that would be a fairly easy change but I can't find any resource talking about this issur.</p>