Applications menu for KDE apps in the Steam session

SteamOS only ships /etc/xdg/menus/plasma-applications.menu; the nested
desktop sets XDG_MENU_PREFIX=plasma-, the Steam session doesn't, so KDE
apps started there (Dolphin from the "+" menu) built an empty app
database: "no installed application can open" every file.

session.applicationsMenu (module applications-menu, on by default) links
~/.config/menus/applications.menu to Plasma's menu. Flake check: the link
is generated by default and absent when disabled.

README.md

@@ -3,10 +3,10 @@
 [Home Manager](https://github.com/nix-community/home-manager) modules for the
 Valve Steam Frame (SteamOS, `aarch64-linux`, standalone home-manager). They
 work around quirks of the Frame's [two graphical sessions](#two-sessions)
-(portal, keyboard layout, clipboard, KDE wallet, Firefox), enable hardware
-video decoding in Jellyfin, run rootless Docker, and extend Steam's and
-SteamVR's UIs at runtime (VR keyboard, "+" menu, dashboard windows, Steam
-close button, window curvature, window controls).
+(portal, KDE app menu, keyboard layout, clipboard, KDE wallet, Firefox),
+enable hardware video decoding in Jellyfin, run rootless Docker, and extend
+Steam's and SteamVR's UIs at runtime (VR keyboard, "+" menu, dashboard
+windows, Steam close button, window curvature, window controls).
 
 Everything is declarative: files are links into the Nix store, UI patches
 live in memory. The few things that have to be written elsewhere at runtime
@@ -15,8 +15,8 @@ their lifetime and what removes them; `steam-frame-nix-cleanup` removes every
 one of them (on each switch what the configuration no longer uses, `--all`
 for everything).
 
-All options live under `steamFrame.*`. The portal fix and clipboard sync are
-on by default; everything else is opt-in.
+All options live under `steamFrame.*`. The portal fix, the applications menu
+and clipboard sync are on by default; everything else is opt-in.
 
 ## Features
 
@@ -27,6 +27,7 @@ configuration, limitations and how it works.
 
 - [Session settings](docs/session.md#session-settings-and-services) (`session.*`): outer bus and user services, for launchers and the other modules.
 - [Portal fix](docs/session.md#portal-fix) (`session.portalFix`, on): apps in the Steam session can open links.
+- [Applications menu](docs/session.md#applications-menu) (`session.applicationsMenu`, on): KDE apps in the Steam session (Dolphin from the "+" menu) know the installed apps.
 - [Keyboard layout](docs/session.md#keyboard-layout) (`keyboard.layout`, `keyboard.variant`): XKB layout for the Steam session.
 - [Clipboard sync](docs/session.md#clipboard-sync) (`clipboardSync`, on): one clipboard for the Steam session and the nested desktop.
 
@@ -214,7 +215,7 @@ home-manager switch --flake .#steamos  # manual setup, from the flake's director
 In your own flake, add the input as above and
 `steam-frame-nix.homeManagerModules.default` to the modules. `default`
 imports all modules; single ones:
-`homeManagerModules.{session,portal,keyboard-layout,vr-keyboard-extra-keys,vr-keyboard,hidden-apps,steam-ui-patches,launcher-menu,steamvr-debugger,cleanup,dashboard-windows,steam-close-button,window-curvature,frame-controls,clipboard-sync,firefox,jellyfin,launchers,docker}`
+`homeManagerModules.{session,portal,applications-menu,keyboard-layout,vr-keyboard-extra-keys,vr-keyboard,hidden-apps,steam-ui-patches,launcher-menu,steamvr-debugger,cleanup,dashboard-windows,steam-close-button,window-curvature,frame-controls,clipboard-sync,firefox,jellyfin,launchers,docker}`
 (`steam-keyboard-patch` and `keyring` still work as the former names of
 `vr-keyboard-extra-keys` and `launchers`). Every module imports `cleanup` (see
 [Changes outside Nix](#changes-outside-nix-exceptions)); which file is
@@ -231,6 +232,7 @@ which: [Repository layout](docs/development.md).
 | `steamFrame.session.services.restart` | list of str | `[ ]` | User units restarted on every switch. |
 | `steamFrame.session.services.stop` | list of str | `[ ]` | User units stopped on switch if running (e.g. of a disabled feature). |
 | `steamFrame.session.portalFix.enable` | bool | `true` | Working portal config (OpenURI) for the Steam session. |
+| `steamFrame.session.applicationsMenu.enable` | bool | `true` | `~/.config/menus/applications.menu` linked to Plasma's, for KDE apps in the Steam session. |
 | `steamFrame.keyboard.layout` | null or str | `null` | XKB layout for the Steam session, e.g. `"de"`; `null`: US. |
 | `steamFrame.keyboard.variant` | null or str | `null` | XKB variant for the Steam session, e.g. `"nodeadkeys"`; see [Keyboard layout](docs/session.md#keyboard-layout). |
 | `steamFrame.keyboard.vr.extraKeys.enable` | bool | `false` | VR keyboard with Esc/Ctrl/Alt, arrows, real chords, AltGr/non-ASCII. |

docs/development.md

@@ -50,6 +50,8 @@ modules/
   cleanup/package.nix          build: install.sh as a command (cleanup, steamvr-debugger-arm)
   cleanup/check.nix            test: install.sh cleanup on fake home/runtime dirs
   portal.nix                   Steam session portal config (session.portalFix)
+  applications-menu.nix        applications.menu link for KDE apps (session.applicationsMenu)
+  applications-menu/check.nix  test: the link on by default, absent when disabled
   keyboard-layout.nix          gamescope-session drop-in with XKB_DEFAULT_* (keyboard.layout)
   clipboard-sync.nix           app: KDE autostart of clipboard-sync (clipboardSync)
   hidden-apps.nix              Hidden=true desktop entries (launcherMenu.hiddenApps)

docs/session.md

@@ -1,9 +1,10 @@
 # Session fixes
 
-Four fixes for the Frame's [two graphical sessions](../README.md#two-sessions):
-session settings and services, the portal, the keyboard layout and the
-clipboard. Options: [README, Options](../README.md#options) (`session.*`,
-`keyboard.layout`, `keyboard.variant`, `clipboardSync.*`).
+Five fixes for the Frame's [two graphical sessions](../README.md#two-sessions):
+session settings and services, the portal, the applications menu, the
+keyboard layout and the clipboard. Options:
+[README, Options](../README.md#options) (`session.*`, `keyboard.layout`,
+`keyboard.variant`, `clipboardSync.*`).
 
 ## Session settings and services
 
@@ -55,6 +56,29 @@ desktop's portal is unaffected.
 files plus a config (`default=holo;gamescope`), and a drop-in on
 `xdg-desktop-portal.service`.
 
+## Applications menu
+
+`session.applicationsMenu.enable`, module `applications-menu`. On by default;
+`false` turns it off.
+
+**Problem:** KDE apps build their app database (ksycoca) from
+`applications.menu`, but SteamOS only ships
+`/etc/xdg/menus/plasma-applications.menu`. The nested desktop sets
+`XDG_MENU_PREFIX=plasma-`, the Steam session doesn't: KDE apps started there
+(e.g. Dolphin from the "+" menu) know no apps at all ("no installed
+application can open …").
+
+**What you get:** KDE apps in the Steam session see every installed app,
+Flatpaks included, e.g. for "Open with". The nested desktop is unaffected (it
+reads Plasma's menu through its prefix).
+
+**Remove when** the Steam session sets `XDG_MENU_PREFIX` or SteamOS ships
+`applications.menu`.
+
+**How it works:** `~/.config/menus/applications.menu` is a Home Manager link
+to `/etc/xdg/menus/plasma-applications.menu`. If SteamOS drops that file,
+the link dangles, which is the same as no menu.
+
 ## Keyboard layout
 
 `keyboard.layout`, `keyboard.variant`, module `keyboard-layout`.

flake.nix

@@ -14,6 +14,7 @@
     modules = {
       session = ./modules/session.nix;
       portal = ./modules/portal.nix;
+      applications-menu = ./modules/applications-menu.nix;
       keyboard-layout = ./modules/keyboard-layout.nix;
       vr-keyboard-extra-keys = ./modules/vr-keyboard-extra-keys.nix;
       vr-keyboard = ./modules/vr-keyboard.nix;
@@ -65,8 +66,10 @@
 
     # Tests of the VR keyboard (text model, corrector, swipe decoder on the
     # default German + English dictionary), the Jellyfin mpv shim, the
-    # Firefox wrapper, the launchers and install.sh cleanup: nix flake check
+    # Firefox wrapper, the launchers, install.sh cleanup and the
+    # applications.menu link: nix flake check
     checks = forSystems (pkgs: {
+      applications-menu = import ./modules/applications-menu/check.nix { inherit pkgs; };
       cleanup = import ./modules/cleanup/check.nix { inherit pkgs; };
       firefox = import ./modules/firefox/check.nix { inherit pkgs; };
       jellyfin = import ./modules/jellyfin/check.nix { inherit pkgs; };

modules/applications-menu.nix

@@ -0,0 +1,27 @@
+# Workaround (SteamOS 0.3.0, build 20260922): KDE apps build their app
+# database (ksycoca) from "applications.menu", but SteamOS only ships
+# /etc/xdg/menus/plasma-applications.menu. The nested desktop sets
+# XDG_MENU_PREFIX=plasma-, the Steam session doesn't, so KDE apps started
+# there (Dolphin from the "+" menu) know no apps. We link the user's
+# applications.menu to Plasma's. A missing host file leaves a dangling link,
+# which KDE treats like no file. Remove once the Steam session sets the prefix.
+{ config, lib, ... }:
+let
+  sys = "/etc/xdg/menus/plasma-applications.menu";
+in {
+  imports = [ ./cleanup.nix ];
+
+  options.steamFrame.session.applicationsMenu.enable = lib.mkOption {
+    type = lib.types.bool;
+    default = true;
+    description = ''
+      Link `~/.config/menus/applications.menu` to Plasma's menu, so KDE apps
+      in the Steam session find the installed apps.
+    '';
+  };
+
+  config = lib.mkIf config.steamFrame.session.applicationsMenu.enable {
+    xdg.configFile."menus/applications.menu".source =
+      config.lib.file.mkOutOfStoreSymlink sys;
+  };
+}

modules/applications-menu/check.nix

@@ -0,0 +1,31 @@
+# Evaluates the applications-menu module against a stub of Home Manager's
+# options: on by default the link to Plasma's menu is generated, disabled it
+# is absent.
+{ pkgs }:
+let
+  inherit (pkgs) lib;
+  eval = cfg: (lib.evalModules {
+    modules = [
+      ../applications-menu.nix
+      {
+        options.xdg.configFile = lib.mkOption {
+          type = lib.types.attrsOf (lib.types.attrsOf lib.types.anything);
+          default = { };
+        };
+        options.lib = lib.mkOption { type = lib.types.attrsOf lib.types.anything; };
+        config = {
+          _module.check = false;   # Home Manager options the stub doesn't declare
+          _module.args.pkgs = pkgs;
+          lib.file.mkOutOfStoreSymlink = p: "out-of-store:${p}";
+        };
+      }
+      cfg
+    ];
+  }).config.xdg.configFile;
+  on = eval { };
+  off = eval { steamFrame.session.applicationsMenu.enable = false; };
+in
+assert on."menus/applications.menu".source
+  == "out-of-store:/etc/xdg/menus/plasma-applications.menu";
+assert !(off ? "menus/applications.menu");
+pkgs.runCommand "applications-menu-check" { } "touch $out"

template/home.nix

@@ -13,7 +13,8 @@
   # home.packages = with pkgs; [ htop ripgrep ];
 
   # Steam Frame fixes, see https://github.com/lhns/steam-frame-nix
-  # (session.portalFix and clipboardSync are on by default).
+  # (session.portalFix, session.applicationsMenu and clipboardSync are on
+  # by default).
   # steamFrame = {
   #   keyboard.layout = "de";               # XKB layout, Steam session
   #   keyboard.vr.extraKeys.enable = true;  # Esc/Ctrl/Alt/arrows in VR