docs: spec for mibook boot-time terminal/desktop choice

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01CfozKLQdUh5TzqyjSigLUx
2026-07-27 09:27:37 +02:00
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+# mibook: choose terminal-only vs desktop at boot
+
+## Goal
+
+Let mibook boot to a text console by default, with KDE fully installed and
+launchable on demand. The user decides after boot whether they need the
+graphical desktop, without rebuilding the system.
+
+## Behavior
+
+- mibook boots to a text console (`multi-user.target`). SDDM and Plasma do
+  **not** start automatically.
+- KDE (SDDM + Plasma 6) remains fully installed.
+- Running `desktop` in the terminal starts the SDDM display manager, which
+  presents the graphical login screen; logging in there enters a Plasma 6
+  session.
+- No reverse command. To return to a text console, stop the display manager
+  (`sudo systemctl stop display-manager`) or reboot.
+- Other machines that enable the `kde-desktop` profile are unaffected; their
+  desktop still starts on boot.
+
+## Implementation
+
+### `modules/environments/kde-desktop/default.nix`
+
+Add an opt-out option to the existing profile:
+
+- New option `my.profiles.kde-desktop.startOnBoot`, a boolean defaulting to
+  `true`. The default preserves the current behavior for every consumer.
+- The existing `config` block (SDDM, Plasma 6, user packages) stays enabled
+  whenever the profile is enabled, regardless of `startOnBoot`.
+- When `startOnBoot = false`, additionally apply:
+  - `systemd.services.display-manager.wantedBy = lib.mkForce [ ];` so SDDM is
+    defined but not pulled in by any boot target.
+  - `systemd.defaultUnit = "multi-user.target";` so the boot target is the
+    text console.
+  - A `desktop` command provided via
+    `pkgs.writeShellScriptBin "desktop" "exec sudo systemctl start display-manager.service"`,
+    added to `environment.systemPackages`.
+
+### `machines/mibook/environments.nix`
+
+Set `kde-desktop.startOnBoot = false;` alongside the existing
+`kde-desktop.enable = true;`.
+
+## Testing / verification
+
+- `nix build '.#nixosConfigurations.mibook.config.system.build.toplevel'`
+  builds without error.
+- After `nixos-rebuild switch` on mibook: system boots to a TTY, no SDDM;
+  running `desktop` brings up the SDDM login and a working Plasma session.
+- Confirm jupiter (or any non-mibook consumer) is unchanged: its evaluated
+  `systemd.defaultUnit` / display-manager wantedBy are not altered.
+
+## Trade-offs
+
+- `desktop` relies on `sudo`; the user has sudo access, so no extra
+  configuration is required.
+- Chose "boot to terminal, launch on demand" over a GRUB boot-menu entry
+  because it is more robust across NixOS generations and needs no fragile
+  static kernel entries.