Portal: KDE file dialog for Flatpak apps in the Steam session

Neither Valve backend (gamescope, holo) implements FileChooser, so sandboxed
apps fell back to an in-sandbox dialog without the home dir (e.g. Element's
Attachments). session.portalFix.fileChooser (on) links kde.portal into the
Steam session's portal dir and selects kde for FileChooser;
xdg-desktop-portal-kde is D-Bus activated with DISPLAY=:0. Flake check
for the portal config.

README.md

@@ -26,7 +26,7 @@ configuration, limitations and how it works.
 **Session** ([docs/session.md](docs/session.md)):
 
 - [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.
+- [Portal fix](docs/session.md#portal-fix) (`session.portalFix`, on): apps in the Steam session can open links, and Flatpak apps get KDE's file dialog.
 - [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.
@@ -232,6 +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.portalFix.fileChooser` | bool | `true` | KDE's file dialog as the Steam session's FileChooser portal (Flatpak apps can open and save files outside their sandbox). Needs `portalFix.enable`. |
 | `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). |

docs/development.md

@@ -50,6 +50,7 @@ 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)
+  portal/check.nix             test: KDE FileChooser on by default, absent when disabled
   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)

docs/session.md

@@ -40,21 +40,37 @@ launcher's `hostEnv`; a hand-written entry uses the prefix `session.busEnv`:
 
 ## Portal fix
 
-`session.portalFix.enable`, module `portal`. On by default.
+`session.portalFix.enable` and `session.portalFix.fileChooser`, module
+`portal`. Both on by default.
 
 **Problem:** the Frame image (SteamOS 0.3.0, build 20260922) points the Steam
 session's `xdg-desktop-portal` at `/usr/share/xdg-desktop-portal/gamescope-portals`,
 which lacks `gamescope-portals.conf`: no backend, no OpenURI, so no app in
-the Steam session can open links.
+the Steam session can open links. Its two backends (gamescope, holo) also
+have no FileChooser, so Flatpak apps fall back to a dialog inside their
+sandbox: e.g. Element's Attachments shows an empty file selector without
+your home dir.
 
-**What you get:** a working OpenURI portal in the Steam session; the
-desktop's portal is unaffected.
+**What you get:** a working OpenURI portal in the Steam session and, with
+`fileChooser`, KDE's file dialog there (a normal window in VR) with access
+to all your files; the app gets only the files you pick. The desktop's
+portal is unaffected.
 
-**Remove when** SteamOS ships `gamescope-portals.conf`.
+**Configuration:** takes effect when the portal restarts (next Steam session
+start, or `systemctl --user restart xdg-desktop-portal`) and the app
+restarts (Electron apps check for the portal at startup). To keep the old
+dialog: `steamFrame.session.portalFix.fileChooser = false;`.
+
+**Remove when** SteamOS ships `gamescope-portals.conf` (and a FileChooser
+backend, for `fileChooser`).
 
 **How it works:** a portal dir in `~/.local/share` linking Valve's `.portal`
 files plus a config (`default=holo;gamescope`), and a drop-in on
-`xdg-desktop-portal.service`.
+`xdg-desktop-portal.service`. `fileChooser` adds a link to
+`/usr/share/xdg-desktop-portal/portals/kde.portal` and
+`org.freedesktop.impl.portal.FileChooser=kde`; the preinstalled
+`xdg-desktop-portal-kde` starts on demand (D-Bus activation) with the Steam
+session's `DISPLAY=:0`.
 
 ## Applications menu
 

flake.nix

@@ -66,14 +66,15 @@
 
     # 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, install.sh cleanup and the
-    # applications.menu link: nix flake check
+    # Firefox wrapper, the launchers, install.sh cleanup, the
+    # applications.menu link and the portal config: 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; };
       launchers = import ./modules/launchers/check.nix { inherit pkgs; };
+      portal = import ./modules/portal/check.nix { inherit pkgs; };
       vr-keyboard = import ./modules/vr-keyboard/check.nix { inherit pkgs; };
     });
 

modules/portal.nix

@@ -3,8 +3,11 @@
 # gamescope-portals.conf, so no backend is selected and there is no OpenURI
 # (links don't open). We point it at our own dir: Valve's .portal files + a
 # config. Remove once SteamOS ships the .conf.
+# Neither Valve backend has a FileChooser, so sandboxed apps fall back to an
+# in-sandbox dialog that can't see home; fileChooser adds KDE's for it.
 { config, lib, ... }:
 let
+  cfg = config.steamFrame.session.portalFix;
   sys = "/usr/share/xdg-desktop-portal/gamescope-portals";
   dir = "xdg-desktop-portal/gamescope-portals";
 in {
@@ -15,23 +18,40 @@ in {
       [ "steamFrame" "session" "portalFix" "enable" ])
   ];
 
-  options.steamFrame.session.portalFix.enable = lib.mkOption {
-    type = lib.types.bool;
-    default = true;
-    description = ''
-      Give the Steam session's xdg-desktop-portal a config so apps there can
-      open links (OpenURI).
-    '';
+  options.steamFrame.session.portalFix = {
+    enable = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = ''
+        Give the Steam session's xdg-desktop-portal a config so apps there can
+        open links (OpenURI).
+      '';
+    };
+    fileChooser = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = ''
+        Use KDE's file dialog (xdg-desktop-portal-kde) as the Steam session's
+        FileChooser portal, so sandboxed apps (Flatpak) can open and save
+        files outside their sandbox. Needs `enable`.
+      '';
+    };
   };
 
-  config = lib.mkIf config.steamFrame.session.portalFix.enable {
+  config = lib.mkIf cfg.enable {
     xdg.dataFile."${dir}/holo.portal".source =
       config.lib.file.mkOutOfStoreSymlink "${sys}/holo.portal";
     xdg.dataFile."${dir}/gamescope.portal".source =
       config.lib.file.mkOutOfStoreSymlink "${sys}/gamescope.portal";
+    xdg.dataFile."${dir}/kde.portal" = lib.mkIf cfg.fileChooser {
+      source = config.lib.file.mkOutOfStoreSymlink
+        "/usr/share/xdg-desktop-portal/portals/kde.portal";
+    };
     xdg.dataFile."${dir}/gamescope-portals.conf".text = ''
       [preferred]
       default=holo;gamescope
+    '' + lib.optionalString cfg.fileChooser ''
+      org.freedesktop.impl.portal.FileChooser=kde
     '';
 
     # Outer (systemd) portal only; the nested desktop's keeps kde-portals.conf.

modules/portal/check.nix

@@ -0,0 +1,36 @@
+# Evaluates the portal module against a stub of Home Manager's options: on by
+# default the Steam session's portal config picks KDE's FileChooser, with
+# `fileChooser = false` it has neither the kde.portal link nor that line.
+{ pkgs }:
+let
+  inherit (pkgs) lib;
+  dir = "xdg-desktop-portal/gamescope-portals";
+  eval = cfg: (lib.evalModules {
+    modules = [
+      ../portal.nix
+      {
+        options.xdg.dataFile = 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:${p}";
+        };
+      }
+      cfg
+    ];
+  }).config.xdg.dataFile;
+  on = eval { };
+  off = eval { steamFrame.session.portalFix.fileChooser = false; };
+  kde = "org.freedesktop.impl.portal.FileChooser=kde";
+in
+assert on."${dir}/kde.portal".source
+  == "out-of-store:/usr/share/xdg-desktop-portal/portals/kde.portal";
+assert lib.hasInfix kde on."${dir}/gamescope-portals.conf".text;
+assert !(off ? "${dir}/kde.portal");
+assert !(lib.hasInfix kde off."${dir}/gamescope-portals.conf".text);
+assert off ? "${dir}/holo.portal";
+pkgs.runCommand "portal-check" { } "touch $out"