Serhiy, could you take at least a quick look at this troublesome issue?  The issue seems to be this: idlelib.autocomplete_w.py has

# at the top
LISTUPDATE_SEQUENCE = "<B1-ButtonRelease>"

# within show_window method
        self.listupdateid = listbox.bind(LISTUPDATE_SEQUENCE,
                                         self.listselect_event)
# and later
    def listselect_event(self, event):

However, when one clicks on a completion box, the click is only seen by the text window underneath, and the box disappears.  On Mac, there are also nasty symptoms like IDLE freezing.  I that changing the sequence to '<Button-1>' and addint 'return "break"' to the end of listselect_event might solve the problem.  When it did not, I added prints to the top of __init__ and listselect_event.  Running in the console, the first printed, the second did not.  Do you have any idea why not?