The receiver does not need a window manager: SDL3's KMSDRM backend
renders straight to the kernel display pipeline, which is the right
setup for a small-board appliance. But a systemd system service has
no controlling terminal, and SDL's KMSDRM backend expects one for
its VT handling — so the headless path would have failed at boot.
Give the unit StandardInput=tty with TTYPath=/dev/tty1 (harmless in
desktop mode), and document the headless recipe in the unit, the
install script output, and the RUNBOOK: Raspberry Pi OS Lite with
Console Autologin, no desktop enabled (a compositor would hold the
DRM master), and consoleblank=0.
Ship systemd/screencast-receiver.service, a template the install
script substitutes (binary path, run user) and enables so the
receiver starts at boot after the network and avahi-daemon, with
restart-on-failure and a raised start limit for slow-booting boards.
SIGTERM gives the graceful shutdown that withdraws the mDNS
announcement — systemctl stop never triggers the stale-record trap
that kill -9 does. Headless consoles render via KMSDRM (the script
adds the run user to video/render/input); desktop autologin sessions
uncomment two documented Environment lines.
The install script gains SC_RECEIVER_SERVICE=0 to opt out.
Validated: systemd-analyze verify on the substituted unit (only the
expected missing /usr/local/bin path on this dev machine), and a live
lifecycle test as a user unit: the service-managed receiver announced
itself (--discover found it), systemctl stop produced a clean exit
(Result=success, no restart) and the mDNS announcement was withdrawn
immediately.
fegger