X gets locked up after several switches to VT

[steelman]

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.

$ ss -n -p | grep <connection between Xorg and llightdm>
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))

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

Launchpad report: https://bugs.launchpad.net/lightdm/+bug/1690739
Debian bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=862389

Although the original reports are several years old I can still reproduce it on Debian 12 with lightdm 1.26.