Comment # 5 on bug 1035770 from Egbert Eich

As I said. the issue arises as ssh uses unix sockets for remote connections:
AFAIR, the XShm extensions detects a remote condition for TCP sockets, however,
not for unix sockets. Therefore, it doesn't bail out early: the problem
originates from ssh tunneling a remote connection using unix sockets. The issue
could be fixed on the ssh side by disallowing Xshm, this would however require
ssh to track the X traffic - and intercept it.
I do know that the xcb backend is able to run without Xshm. So the best bet
would be to do a test during init of the xcb plugin and back off - ie disable
the use of the Xshm extension.