I'm testing this on Lazarus 1.8.0RC3 on the Apple Mac with the Carbon widget set.
As soon as the app starts, if I press the tab key just the once, I see the list box receive focus, and I see messages saying First Button Enter AND Second Button Enter.
I don't know what this means, having just this minute woken up, but someone more skilled than I will hopefully understand what on earth is going on!