John,

As an experiment, add an offscreen textbox with the myclass.visible=.f. in the WHEN or GOTFOCUS of the textbox. Then set focus to it when you want the class to be rendered invisible. Does the combo stay visible now? IME that's a reliable way to avoid curious display artifacts that can consume happy days pursuing phantoms!

FWIW, I remember something similar where the .visible setting was in an object inside the class being made invisible (especially if it has focus at the time) and/or code was present along the event sequence to display/update the combo after it is rendered invisible. Thisform.lockscreen=.f. can help if it's to do with code in events, but there's no value in picking through screeds of working event code if you can prevent it every time by setting focus elsewhere...