How to know when user selects from combo list?

["Edward A. Falk" <falk efalk org>]

Hi all; apologies if this is in the FAQ somewhere; I couldn't
find it.

How can I detect when the user has selected something from the
list associated with a combo?  I can get callbacks while the user
is sliding through the options, but I don't want that.  I want to know
when the user is done with the list and has finalized their choice.

	-ed falk