Steamify on GNOME (GDM): prototype plan

Started 2026-10-14. Goal: Steamify's SteamOS conversion on a GNOME desktop (GDM), detected automatically; KDE keeps working. First a prototype to see it work, then something solid. Product branch: feature/gnome-prototype in steamify-cachyos (from main = 2.12.0). Notes live here, never in the product repo.

Why it needs work

CachyOS's steam-set-session only handles plasma-login-manager/SDDM (silent no-op on GDM) and steamos-session-select ends the session with a KDE-only qdbus6 org.kde.Shutdown call. GDM logs in by itself only when the daemon starts, so a session switch needs a GDM restart. Prior art, unverified: https://git.upriser.nl/theupriser/cachyos-gamescope-boot-gnome (setup-gamescope-boot-gnome.sh: AccountsService Session=, replaced tools plus pacman hook, relogin by restarting gdm).

Prototype design (written, not yet run)

lib/desktop.sh: desktop_kind (gnome|kde from XDG_CURRENT_DESKTOP, else GDM = gnome), desktop_session_file, choose_login_manager (gdm on GNOME).
lib/login-manager.sh: gdm branch in gaming_status/enable/disable; configure_gdm_autologin (/etc/gdm/custom.conf), gdm_fix_install (our tools in /usr/local/share/steamify-gdm, copied over the real paths, pacman hook 95-steamify-gdm.hook, sudoers rule for systemctl restart gdm.service).
patches/steamify-gdm-{steam-set-session,session-select}.sh, patches/steamify-gdm.hook: the GDM-aware tools. Session goes into /var/lib/AccountsService/users/<user> (Session= and XSession=: which one GDM honours is OPEN, check in the VM). select restarts GDM directly instead of the old repo's shutdown.target unit.
Menu: theme and single hidden on GNOME (Plasma only). Packages: kconfig + accountsservice instead of plasma-keyboard on GNOME (kreadconfig6 is used by the undo journal). KWIN_IM_SHOW_ALWAYS only on KDE. Boot-into-desktop unit uses the detected session. Shortcut also goes to ~/.local/share/applications (GNOME shows no desktop icons).

Test VM

VM_DESKTOP=gnome added to dev scripts/vminstall.sh + share/vminstall-post.sh (uncommitted): CI=1 VM_DESKTOP=gnome VM_STEAMIFY=skip VM_DIR=~/vms/gnome-vm VM_SSH_KEY=~/vms/steamify-vm/vmkey REPO=<steamify-cachyos> scripts/vminstall.sh --iso ~/vms/iso-dl/steamify-cachyos-2.12.0-dev-261014-x86_64.iso (PATH with ~/vms/shim, sg kvm -c). Plain CachyOS GNOME, Steamify applied by hand afterwards.

Steps

GNOME VM installed (~/vms/gnome-vm; the headless installer ignores "desktop":"gnome", so vminstall-post.sh installs cachyos-gnome-settings + gnome + gdm).
Run steamify.sh --defaults --options gaming in the VM (repo on 9p /mnt); fix what breaks (kconfig, packages, shellcheck).
Check: custom.conf, AccountsService, hook, sudoers, our tools in place.
(gamescope itself dies in the VM: Vulkan/DRM scanout errors, also without GNOME; GDM did start it) Reboot: GDM autologs into the gamescope session (loginctl session desktop/type; gamescope may not render in the VM, a running session is enough).
[~] (gamescope->GNOME and GNOME->gamescope verified with a stand-in gamescope session (Exec=sleep infinity in /usr/share/wayland-sessions/gamescope-session.desktop, original kept as .real); reboot persistence, boot option and the shortcut click not yet) steamos-session-select desktop -> GNOME session; Return to Gaming Mode shortcut -> gamescope again; reboot persistence; boot option.
Turn off: everything restored (custom.conf backup, original tools).
KDE regression: existing suites still pass on the KDE VM.
Commit dev-repo changes (VM_DESKTOP) to main; product branch stays a prototype, no release without the user.

Later (solid version)

Tests: GNOME suite in the dev repo, TESTPLAN rows; ISO with a GNOME variant.
GNOME theme based on the Vapor Plasma theme (user idea: colours, wallpaper, icons; shell/GTK parts new). Single user mode and the other KDE-only parts (powerdevil in steam-machine, first-login, update notifier) need GNOME versions or stay hidden.
Decide: keep replacing CachyOS's two files, or get GDM support upstream into gamescope-session-cachyos.
Real hardware: GDM restart behaviour, Steam's Switch to Desktop button.

Findings so far (2026-10-14)

GDM logs in by itself only when it starts. A switch = set the session in AccountsService, loginctl terminate-user, systemctl restart gdm (patches/steamify-gdm-relogin.sh, run through sudo from inside the session, as a transient unit because ending the user kills the caller). Without ending the user first, GNOME refuses: "A graphical session is already running!". Starting the new login at once fails (the session dies in 0.2 s), so the helper stops GDM first (no login screen), ends the user, waits for the user manager to go (about 10 s in the VM), then starts GDM. Shortcut click (gtk-launch) and steamos-session-select desktop verified both ways with the stand-in session. Open: the ~10 s (what keeps the user manager up? Steam?), and what the screen shows during it.
Both Session= and XSession= are written to AccountsService; GDM picked the right session. (Which one is needed is untested: could drop one.)
cachyos-gamescope-autologin.service resets the session to gamescope when a desktop session ends (Deck behaviour), so after GNOME the next login is gamescope again.
A failed gamescope session (VM) lands on GDM's login screen; SDDM relogin loops instead. Real hardware untested.
Menu: theme and single user mode hidden on GNOME.
ssh into the VM dies at every switch (terminate-user): reconnect.

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