Processes stuck in STOP state on 12.0-BETA3

On a fresh install of 12.0-BETA3, I'm seeing processes get stuck in the
STOP state.  Neither SIGCONT nor SIGKILL has any effect.  And it's
reproducible.  All I have to do is:
1) Login to my login manager, which starts xfce
2) Start firefox
3) kill X
4) Login again
5) ps -ax  | egrep '\<T\>'

And I see three firefox processes.  It's not just firefox either; I first
saw 0ad spontaneously enter the STOP state, without first killing X.

Anybody have any ideas?  The bug seems reproducible, so I'll try anything.

-Alan