Maybe this is again one of the "it is too hot to think clearly", but ...
in some cases there is a difference if
a) I click on a button and the OnClick-event is triggered or
b) I set the focus to the same button via tab - tab - tab ... and press "Enter": And nothing happens
Example for
a) OpendclDemo, "List Box": the buttons an the right accept mouse-click and focus-plus-Enter
b) Opendcldemo, Message Box Sample: The button "Show Message Box" accepts only the mouseclick, but no "Enter"
What is the difference?
They are different kinds of controls: the message box sample uses text buttons, the other uses graphic buttons with no picture. I don't recall why they behave differently.