>>>Have run into occasions where on some computers, comboboxes and dropdown lists don't function quite right. In particular if you use a mouse, you can click on the control, the list does appear, but clicking on an item in the list does not make a selection when you click on a list item (i.e. the list is dismissed w/o making a selection). On the other hand, if you use the keyboard you can indeed make a selection. Does anybody have an idea of what may be causing this issue ?
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>Unfortunately I didn't get much of a chance to "play around" on the systems in question long enough to see if I could notice if the usual "tracking" behavior of the listbox was present (after clicking on listbox, the selection bar "follows" the mouse).

You probably did check, but only omitted mentioning:
rodent population identical physically ?
As today they all have probably have USB tails, differences in drivers possible ?

If you still possess a grandfather mouse without USB, try that -
or a very simplistic one without anything added by the driver for gaming or similar
next step would be to check MoBo to be identical, Windows version and patch level
then deinstall existing USB mouse drivers: identify via device manager
with setting to show disconnected or similar (german version here)
if you have a PE boot install stick, might be interesting if that shows same fluctuation

If you access prg/vcx outside exe, scan disks for older versions

regards
thomas