See the attached richlet class and example video at http://gyazo.com/aa5ebd7177ad0c6b5ff6eed62e2da75f
Comboboxes are being put into listcells with hflex set to 1.
If render on demand is off (top list) they size properly to fill the full cell width. If ROD is set on (bottom list) they revert to their preferred size when the list is loaded or modified.