…Serhat is right. The layering behaviour of Combo*-Controls is poor (they always shine through). Avoid usage of Combo*-Controls (or use RENDERASFIELD option) within screens where you provide for a field valuehelp or drop down menu (MENU control).
And by the way: the FIELD’s valuehelp is loaded on request (click). In respect to performance this is much better than having the options within the net data stream on page load.
BTW: the problem with the “shining through” combo boxes is an annoying and inherent Itnernet Explorer problem. It does not appear with Mozilla. Combos are somehow rendered as own window object internally, and do not react on any z-indices - they alway are on the top.