The new user selector was always showing space for a vertical scrollbar,

which looked bad on any OS that shows scrollbars inline. This was due to

an overflow-y: scroll rule.
This change makes the scrollbar only appear when necessary, using auto

instead. If there isn't enough room for the list, the scrollbar will

kick in, but otherwise it will be hidden.

Looked at the user selector and saw that the contents of the list were

flush with the sides. No scrollbar space was being reserved.
Forced the height of the list to be too small for the contents, and saw

the scrollbar appear.