We upgraded from 6.0.9 to 6.0.12p1 recently.
A user reports, and I confirm:
When I type a name, the list comes up as expected, and if the first entry is the one you want, it works as expected: return/semicolon/comma/etc cause the selected address to be placed in To/Cc. But if you want one of the other entries, using the keyboard to complete the address is broken (clicking with the mouse always works):
a) arrow down and hit comma or semicolon: "@domain.com" is appended to what you've typed so far. E.g. if I've typed "nancy" and arrow down to her correct entry, what I get is email@example.com.
b) arrow down and hit return: does not autocompletes, and selection is reset to the first entry. At this point you can arrow down to the desired entry and then things work like it should (and like it does for the first entry): semicolon/comma/return/etc cause the selected entry to autocomplete.
I get the same result with IE 8 (Win), Safari (Mac) and FF (both Win and Mac).
The user is sure that everything was fine in 6.0.9. It's possible that it was fine for the brief period we were running 6.0.12, i.e., that it broke with 6.0.12p1.