fix(desktop): file dialog on the GTK backend, one portal config

useNautilus = false assigns FileChooser to gtk and drops the nautilus D-Bus
registration; the GTK3 dialog then follows the same settings.ini as thunar.

The deployed niri-portals.conf is gone: /etc/xdg-desktop-portal/ ranks below
/etc/xdg/, where programs.niri writes the same file. Its one extra key,
Inhibit=none, moves to desktop.nix.

DECISIONS.md

@@ -1,5 +1,61 @@
 # Decisions
 
+## 2026-09-25 – One portal config, and the file dialog on the GTK backend
+
+- **Who**: Dominik, ClaudeCode
+- **Why**: Firefox's save dialog was drawn in a style that belongs to nothing
+  else on the system — Adwaita buttons and colours — while thunar, the file
+  manager, was themed. Two causes, one behind the other.
+
+  The dialog is not Firefox's own: under Wayland it is requested through
+  `org.freedesktop.impl.portal.FileChooser`, and that interface was unassigned,
+  so the `default=gnome;gtk` line handed it to `xdg-desktop-portal-gnome`. That
+  backend delegates to nautilus, which nixpkgs' niri module registers with D-Bus
+  although the binary is on no `PATH`. Nautilus is GTK4 and libadwaita, and
+  libadwaita draws its own style without reading `gtk-theme-name` from
+  `settings.ini` — so no amount of theming in `modules/theme.nix` could have
+  reached it.
+
+  Behind that: the portal assignment was written in two places. `desktop.nix`
+  deployed moonarch's `niri-portals.conf` to `/etc/xdg-desktop-portal/`, and
+  `programs.niri` had NixOS write its own from `xdg.portal.config.niri` to
+  `/etc/xdg/xdg-desktop-portal/`. `portals.conf(5)` searches every directory in
+  `$XDG_CONFIG_DIRS` — `/etc/xdg` among them — before the build-time sysconfdir
+  `/etc`, so the module's file outranked the deployed one and the file this
+  repo maintained decided nothing. Both carried the same four keys; moonarch's
+  stated a fifth, `Inhibit=none;`, whose effect from the lower-ranked file was
+  never established.
+- **Tradeoffs**: (1) **Installing nautilus** would have made the dialog a
+  deliberate choice rather than an accident, but it stays unthemeable for the
+  reason above, and it is a second file manager next to thunar. (2) **Keeping
+  moonarch's file as the source** and generating `xdg.portal.config.niri` from
+  it would hold the two distros together, at the price of parsing an ini file
+  in Nix — more code than the assignment itself, for five keys of which four
+  already match what the module sets. (3) **Leaving the deployed copy in place**
+  and only fixing the dialog was rejected: it would have left a file in the tree
+  that looks like the place to change the portal assignment and is not.
+- **How**: The `environment.etc` entry for `niri-portals.conf` is gone, and with
+  it the `etc` binding in the `let` block, which had no other reader. The whole
+  assignment now comes from `xdg.portal.config.niri`: four keys from
+  `programs.niri`, `FileChooser = "gtk"` from setting
+  `programs.niri.useNautilus = false`, and
+  `org.freedesktop.impl.portal.Inhibit = "none"` restated in `desktop.nix` with
+  moonarch's reasoning carried over in the comment. The option rather than a
+  config key for the file chooser: only the option also drops the nautilus D-Bus
+  registration, and setting the key in both places would collide.
+
+  Verified on the thinkpad host: the generated
+  `/etc/xdg/xdg-desktop-portal/niri-portals.conf` carries all six lines, no
+  second portal config is deployed anywhere, nautilus is out of
+  `services.dbus.packages`, and the system builds.
+
+  `xdg-desktop-portal-gtk` builds the dialog with GTK3 and reads
+  `/etc/xdg/gtk-3.0/settings.ini`, the same file thunar follows. Screencasting
+  keeps the gnome backend: niri serves `org.gnome.Mutter.ScreenCast` itself and
+  that backend is the only caller, which is why it stays in `extraPortals`.
+  Assigning the file chooser to gtk is also what the niri wiki recommends for a
+  setup without nautilus.
+
 ## 2026-09-24 – One topic per module, split out of desktop.nix
 
 - **Who**: Dominik, ClaudeCode

README.md

@@ -280,6 +280,7 @@ NixOS module owns the destination:
 | `gtklock/config.ini` | generated, not copied: background and module paths are store paths. Deployed to `/etc/xdg`, which gtklock reads — no `$HOME` seed |
 | `stasis/stasis.rune` | copied unchanged and seeded into `$HOME`; its four action lines name `moonarch-lock`, which `moonarch-scripts` installs |
 | `waypaper/config.ini` | wallpaper path, rewritten and seeded into `$HOME` |
+| `xdg-desktop-portal/niri-portals.conf` | not deployed at all: `programs.niri` writes the same file from `xdg.portal.config.niri` to `/etc/xdg`, which `portals.conf(5)` ranks above the `/etc` copy. The one key moonarch states beyond the module's four is set in `desktop.nix` |
 | fontconfig defaults | `/etc/fonts/conf.d` is owned by the NixOS module |
 
 `kanshi/config` is not deployed and kanshi is not installed: niri configures its

modules/desktop.nix

@@ -5,8 +5,6 @@
 
 let
   xdg = "${moonarchSrc}/defaults/xdg";
-  etc = "${moonarchSrc}/defaults/etc";
-
   # waypaper calls `swww`/`swww-daemon`, but the maintained successor ships as
   # `awww`. The symlinks keep the shared config working unchanged.
   swww-compat = pkgs.runCommand "swww-compat" { } ''
@@ -70,9 +68,13 @@ in
     # No stasis entry: /etc/stasis/ is only its fallback for a missing user
     # config, and the seed below means there never is one.
 
-    # Configs that live outside defaults/xdg/ in moonarch as well.
-    "xdg-desktop-portal/niri-portals.conf".source =
-      "${etc}/xdg-desktop-portal/niri-portals.conf";
+    # No xdg-desktop-portal entry: moonarch's niri-portals.conf would be
+    # deployed to /etc/xdg-desktop-portal/, and portals.conf(5) searches that
+    # path — the build-time sysconfdir — only after every directory in
+    # $XDG_CONFIG_DIRS. /etc/xdg is one of those, and it is where NixOS writes
+    # the same file from xdg.portal.config.niri, which programs.niri defines.
+    # The deployed copy would rank below it and decide nothing. What it carries
+    # is set under Portals below instead.
   };
 
   # --- Compositor ---
@@ -97,10 +99,44 @@ in
   ];
 
   # --- Portals ---
+  #
+  # The whole assignment lives here, in the one file NixOS writes. programs.niri
+  # defines xdg.portal.config.niri with default=gnome;gtk and Access,
+  # FileChooser, Notification and Secret — the same four moonarch's
+  # niri-portals.conf carries, which is why that file is not deployed; see the
+  # note in environment.etc above. Only what moonarch states beyond them is
+  # restated below.
   xdg.portal = {
     enable = true;
     extraPortals = with pkgs; [ xdg-desktop-portal-gnome xdg-desktop-portal-gtk ];
+
+    # Carried over from moonarch's file verbatim, its reasoning included:
+    # xdg-desktop-portal-gtk reports the Inhibit interface as success even
+    # though nothing implements it under niri, which makes Firefox skip the
+    # native Wayland idle-inhibit. With no backend the browser falls back to
+    # zwp_idle_inhibit, which niri honors, so windowed video keeps the screen
+    # awake. programs.niri leaves this interface unassigned, so stating it here
+    # does not collide with the module's definition.
+    config.niri."org.freedesktop.impl.portal.Inhibit" = "none";
   };
+
+  # The file dialog every portal client opens — Firefox's save dialog among
+  # them. The option defaults to true, which registers nautilus with D-Bus and
+  # leaves org.freedesktop.impl.portal.FileChooser unassigned, so the
+  # default=gnome;gtk line hands it to xdg-desktop-portal-gnome and the dialog
+  # is nautilus' own: GTK4 and libadwaita, which draw their own style and never
+  # read gtk-theme-name out of settings.ini. That is why the dialog came up in
+  # Adwaita while thunar, a GTK3 program reading /etc/xdg/gtk-3.0/settings.ini,
+  # was themed.
+  #
+  # Off, the module assigns the interface to gtk and drops the nautilus D-Bus
+  # registration; xdg-desktop-portal-gtk then builds the dialog with GTK3 from
+  # that same settings.ini. It is set through the option rather than through
+  # config.niri above because only the option covers the D-Bus half — and
+  # setting the key in both places would collide. Screencasting is unaffected:
+  # niri serves org.gnome.Mutter.ScreenCast itself and only the gnome backend
+  # calls it, which is why that one stays in extraPortals.
+  programs.niri.useNautilus = false;
   environment.systemPackages = with pkgs; [
     # XDG plumbing, before anything that calls into it. xdg-terminal-exec
     # resolves a caller to whichever TerminalEmulator entry the XDG data