X gets locked up after several switches to VT

[Łukasz Stelmach]

After I had switched from X to VT several times (maybe there are other
causes too, but I don't know them) X stopped updating windows. The mouse
cursor was moving and changing its shape as I moved it over different
windows. I could still switch between VT and X. On VT I found that,
apparently, lightdm wasn't handling data from X properly because there
was over 200kB worth of data waiting in X's send queue and 8k in receive
queue in lightdm.

--8<---------------cut here---------------start------------->8---
Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port
u_str ESTAB 0 213504 @/tmp/.X11-unix/X0 9689907 * 9691709 users:(("Xorg",pid=11405,fd=18))
u_str ESTAB 8896 0 * 9691709 * 9689907 users:(("lightdm",pid=10124,fd=11))
--8<---------------cut here---------------end--------------->8---

The numbers had been growing each time I switched between X and VT.

Debian bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=862389