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!". Waiting for the user manager to exit only keeps GDM's login screen up (10 s): no wait = about 1.4 s to GNOME.
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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