It seems that if we have a RD server with a lot of users on it, we can't scroll the dropdown for user sessions to get to all of them.  It seems the dropdown is sized to the vertical resolution of the monitor, and if you've got more users than will fit there, you're outta luck.  But it is a design flaw that should be fixed to allow you to scroll the dropdown to see and select any user.

I've opened ticket #762933 about this but wanted to see if anyone else can verify this issue or if they found a workaround.