feat(desktop): nwg-displays writes niri outputs, kanshi dropped

The port swapped moonarch's wdisplays-persistent for the nixpkgs wdisplays,
which cannot save -- so the kanshi config stayed empty and its unit was
skipped on every boot. nwg-displays supports niri and writes the layout to
~/.config/niri/monitor.kdl, which niri applies itself.

niri prefers a home config and does not merge, so config.kdl is seeded with
an include of /etc/niri/config.kdl, like moonarch does for stasis.rune.

DECISIONS.md

@@ -1,5 +1,40 @@
 # Decisions
 
+## 2026-09-08 – nwg-displays replaces wdisplays; kanshi dropped for niri's own outputs
+
+- **Who**: Dominik, ClaudeCode
+- **Why**: Monitors could not be set persistently at all. moonarch pairs
+  `wdisplays-persistent` (AUR fork `sfs-pra/wdisplays`, see moonarch `README.md:25` and
+  its `DECISIONS.md:178`) with kanshi: the fork saves straight into the kanshi config.
+  The port took the nixpkgs `wdisplays` instead — upstream, with no save function — and
+  never recorded the difference. The result was a GUI that applies a layout and forgets
+  it, an always-empty `~/.config/kanshi/config`, and a `kanshi.service` whose
+  `ConditionPathExists` therefore skipped it on every boot.
+- **Tradeoffs**: Packaging the fork would match moonarch exactly and keeps kanshi's
+  per-combination profiles, but it is unmaintained and would be ours to carry.
+  `nwg-displays` is in nixpkgs, actively developed, and supports niri as a first-class
+  target alongside sway and Hyprland; it writes `~/.config/niri/monitor.kdl` and adds the
+  `include` for it. That makes kanshi redundant — niri applies `output` blocks itself, on
+  hotplug and with live reload — so it goes, together with the hardening set it had just
+  been given. What is given up is kanshi's profiles *per attached combination*: niri has
+  one setting per output. Fixed monitors do not care; several docks with different
+  arrangements would, and then kanshi has to come back.
+  The seed is the same mechanism moonarch chose for stasis (its `DECISIONS.md`,
+  2026-04-22): niri has no config hierarchy either, so system-default-plus-user-override
+  cannot be modelled — the home file is seeded once and then belongs to the user.
+- **How**: `modules/desktop.nix` — `wdisplays` → `nwg-displays`, `kanshi` dropped from
+  `environment.systemPackages`, new `niriUserConfig` in the `let` block holding
+  `include "/etc/niri/config.kdl"`, seeded through the existing
+  `systemd.user.tmpfiles.rules` block with `C` so later edits survive. The stale comment
+  about the undeployed kanshi config is gone from `environment.etc`, the paragraph in
+  `README.md` rewritten. `modules/services.nix` — `systemd.user.services.kanshi` removed
+  and `wlsunset`'s `after` reduced to `graphical-session.target`.
+  Verified: niri 26.04 does load `include` (an include pointing at a deliberately broken
+  file makes `niri validate` fail on that file, so it is parsed, not ignored);
+  `nwg-displays` 0.4.4 logs `Running on niri` and sees both outputs; without
+  `~/.config/niri/` it exits 0 with no window, and it creates `monitor.kdl` but no
+  `config.kdl` — which is what the seed is for.
+
 ## 2026-08-24 – power-profiles-daemon replaces auto-cpufreq
 
 - **Who**: Dominik, ClaudeCode

README.md

@@ -257,11 +257,13 @@ NixOS module owns the destination:
 | `waypaper/config.ini` | wallpaper path, rewritten and seeded into `$HOME` |
 | fontconfig defaults | `/etc/fonts/conf.d` is owned by the NixOS module |
 
-`kanshi/config` is not deployed: the file in the config source is empty, and
-monitor profiles are machine specific — they belong in
-`~/.config/kanshi/config`. A missing config file is what kanshi treats as an
-error (`failed to parse config file`), which with `Restart=on-failure` would
-spin, so the service starts only once that file exists.
+`kanshi/config` is not deployed and kanshi is not installed: niri configures its
+outputs itself, and `nwg-displays` writes the layout it is given to
+`~/.config/niri/monitor.kdl`. niri prefers a home config over the deployed one
+and does not merge them, so `~/.config/niri/config.kdl` is seeded with an
+`include` of `/etc/niri/config.kdl` — the home config then adds the monitor
+layout instead of replacing everything. nwg-displays also refuses to start
+without that file.
 
 Programs that read only from `$HOME` and have no system-wide fallback get a
 copy seeded through `systemd.user.tmpfiles` with the `C` directive, which never

modules/desktop.nix

@@ -36,6 +36,20 @@ let
       --replace-fail /usr/share/moonarch/wallpaper-blur.jpg ${wallpaperBlur}
   '';
 
+  # niri prefers ~/.config/niri/config.kdl over /etc/niri/config.kdl and does not
+  # merge the two, so a home config would otherwise replace the whole deployed
+  # one. This seed pulls the system config back in through `include`, which niri
+  # 26.04 really does load (verified by pointing an include at a broken file and
+  # watching `niri validate` fail on it).
+  #
+  # It exists for nwg-displays, which writes the output layout to
+  # ~/.config/niri/monitor.kdl and adds an include for it here. Without a
+  # config.kdl in the home directory nwg-displays refuses to start at all
+  # ("Couldn't find niri config directory", exit 0, no window).
+  niriUserConfig = pkgs.writeText "niri-config.kdl" ''
+    include "/etc/niri/config.kdl"
+  '';
+
   # PipeWire searches $PIPEWIRE_CONFIG_DIR, then $XDG_CONFIG_HOME/pipewire, then
   # its compiled-in /etc/pipewire — never /etc/xdg (verified in PipeWire's
   # src/pipewire/conf.c). The shared file therefore cannot be deployed the way
@@ -93,11 +107,6 @@ in
     "xdg/quickshell/moonarch".source = "${xdg}/quickshell/moonarch";
     "xdg/foot/foot.ini".source = "${xdg}/foot/foot.ini";
     "xdg/fastfetch/config.jsonc".source = "${xdg}/fastfetch/config.jsonc";
-    # defaults/xdg/kanshi/config is an empty placeholder. Not
-    # deployed: monitor profiles are machine specific and belong in
-    # ~/.config/kanshi/config. An empty file parses fine; a missing one is what
-    # kanshi reports as "failed to parse config file", which is why the unit in
-    # modules/services.nix waits for the home config to exist.
     "xdg/Kvantum/kvantum.kvconfig".source = "${xdg}/Kvantum/kvantum.kvconfig";
     "xdg/qt6ct/qt6ct.conf".source = "${xdg}/qt6ct/qt6ct.conf";
     "xdg/gtk-3.0/settings.ini".source = "${xdg}/gtk-3.0/settings.ini";
@@ -286,7 +295,6 @@ in
 
     # Compositor extras
     xwayland-satellite
-    kanshi
 
     # Bar, notifications, clipboard.
     # Neither waybar nor swaync are installed: the Quickshell bar owns both
@@ -361,7 +369,7 @@ in
     fwupd
     upower
     polkit_gnome
-    wdisplays
+    nwg-displays
     wl-color-picker
 
     stasis
@@ -399,6 +407,9 @@ in
     # exists, so the moonarch config has to be in place before it starts.
     "C %h/.config/stasis/stasis.rune 0644 - - - ${xdg}/stasis/stasis.rune"
     "C %h/.config/swaylock/config 0644 - - - ${swaylockConfig}"
+    # nwg-displays needs this file to exist before it will start, and niri would
+    # otherwise ignore the monitor.kdl it writes next to it.
+    "C %h/.config/niri/config.kdl 0644 - - - ${niriUserConfig}"
     "C %h/.zshrc 0644 - - - ${userZshrc}"
   ];
 }

modules/services.nix

@@ -112,30 +112,6 @@ in
   # --- Firewall, replaces the ufw calls in post-install.sh ---
   networking.firewall.enable = true;
 
-  # --- Session services ---
-  #
-  # Rebuilt rather than copied: the upstream units reference /usr/bin/,
-  # which does not exist under Nix.
-
-  systemd.user.services.kanshi = {
-    description = "Dynamic display configuration";
-    partOf = [ "graphical-session.target" ];
-    after = [ "graphical-session.target" ];
-    wantedBy = [ "graphical-session.target" ];
-    # Without a config kanshi exits with a parse error and, with
-    # Restart=on-failure, spins. Monitor profiles are per machine.
-    unitConfig.ConditionPathExists = "%h/.config/kanshi/config";
-    # The config is read, never written, so ProtectSystem=strict is enough and
-    # no exception for %h is needed. Not exercised at runtime here: this host
-    # has no kanshi config, so the condition skips the unit.
-    serviceConfig = {
-      Type = "simple";
-      ExecStart = lib.getExe pkgs.kanshi;
-      Restart = "on-failure";
-      RestartSec = 3;
-    } // waylandClientHardening;
-  };
-
   # --- System services from defaults/etc/systemd/system/ ---
   #
   # Rebuilt rather than copied, same reason as the user units: the originals
@@ -166,6 +142,11 @@ in
     } // sysfsWriterHardening;
   };
 
+  # --- Session services ---
+  #
+  # Rebuilt rather than copied: the upstream units reference /usr/bin/,
+  # which does not exist under Nix.
+
   # The bar, launcher and every popout. A unit rather than a spawn-at-startup so
   # a crash does not leave the session without a bar until the next login.
   systemd.user.services.quickshell = {
@@ -262,7 +243,7 @@ in
   systemd.user.services.wlsunset = {
     description = "Wlsunset night light (blue light filter)";
     partOf = [ "graphical-session.target" ];
-    after = [ "graphical-session.target" "kanshi.service" ];
+    after = [ "graphical-session.target" ];
     wantedBy = [ ];
     serviceConfig = {
       Type = "simple";