Xephyr broke in 21.1.3 due to the `XDG_SEAT` change
Arch Linux bug report: https://bugs.archlinux.org/task/73274
After upgrading xorg-server from 21.1.2 to 21.1.3 Xephyr stopped working properly — it prints lots of messages complaining about input devices from the host (which it should not try to use at all):
unrecognised device identifier: /dev/input/event2
unrecognised device identifier: /dev/input/event4
unrecognised device identifier: /dev/input/event5
unrecognised device identifier: /dev/input/event1
unrecognised device identifier: /dev/input/event0
unrecognised device identifier: /dev/input/event10
unrecognised device identifier: /dev/input/event11
unrecognised device identifier: /dev/input/event12
unrecognised device identifier: /dev/input/event13
unrecognised device identifier: /dev/input/event14
Pointer option key (_source) of value (server/udev) not assigned!
Pointer option key (major) of value (13) not assigned!
Pointer option key (minor) of value (82) not assigned!
Pointer option key (config_info) of value (udev:/sys/devices/pci0000:00/0000:00:08.1/0000:05:00.4/usb3/3-2/3-2:1.1/0003:046D:C52F.0003/0003:046D:4057.0005/input/input33/event18) not assigned!
couldn't find driver for pointer device "Logitech B330/M330/M3" (/dev/input/event18)
Pointer option key (_source) of value (server/udev) not assigned!
Pointer option key (major) of value (13) not assigned!
Pointer option key (minor) of value (34) not assigned!
Pointer option key (config_info) of value (udev:/sys/devices/pci0000:00/0000:00:08.1/0000:05:00.4/usb3/3-2/3-2:1.1/0003:046D:C52F.0003/0003:046D:4057.0005/input/input33/mouse2) not assigned!
couldn't find driver for pointer device "Logitech B330/M330/M3" (/dev/input/mouse2)Then the Xephyr window appears, but any input in it does not work.
Apparently the problematic change is b27eaa72 (ultimately coming from !762 (merged)) — executing unset XDG_SEAT before starting Xephyr returns things back to normal (there are no messages about the host input devices, and input in Xephyr works properly). So at least I have a workaround, but this should somehow be fixed, so that using Xephyr does not require undocumented actions like unsetting some magic environment variables.