I suppose you could use the on change, or key down, but I'm a bit reluctant to doing so, cause it will keep invoking the dropdown per each keystroke, and I don't know if that causes any not desired sideffects.
Try it out, see what happens, and if you have side effects, try using a variable with form level scope, set it to 0 in the on enter/gotfocus event of the combo, then per each keydown/on change, increment by one, but only do a dropdown when it's 0 (before the increment), or something like that;-)
Roy-Vidar