KDM brings up a "login" screen.  I can use the tab key to go from the
"user" field to the "password" field, and then additional tab-key
presses  select in turn the various buttons below.  When one of the
buttons is selected, shouldn't i be able to press "enter" and press the
button?  In other words, this login screen should be completely workable
without the mouse.

I discovered this because I have another problem that logging out of a
user's session sometimes leaves me at the KDM login screen with an
unresponsive mouse cursor (another problem ...)