SteamVR screenshots linked into ~/Pictures

Steam keeps VR screenshots (Steam button + trigger) in
~/.local/share/Steam/userdata/<account ID>/760/remote/250820/screenshots,
where Dolphin, Gwenview and the file pickers don't look.

screenshots (module screenshots, opt-in) links ~/Pictures/<name> (default
"SteamVR Screenshots") there. The account ID comes from steamUserId, or,
unset, at runtime: the Home Manager link points to
<runtimeDir>/steam-frame-nix/screenshots (tmpfs), which the oneshot user
service steam-frame-nix-screenshots points to the account last logged in
(loginusers.vdf MostRecent, else latest timestamp, else the only userdata
folder), on switch, at login and when loginusers.vdf or userdata changes.

install.sh cleanup knows the tmpfs link (--keep screenshots while used).
Flake checks: account detection on fake Steam dirs; cleanup of the link.

README.md

@@ -55,6 +55,7 @@ configuration, limitations and how it works.
 - [Jellyfin](docs/jellyfin.md) (`jellyfin.hardwareDecoding`): hardware video decoding in the Jellyfin Desktop Flatpak.
 - [Launchers](docs/launchers.md) (`launchers.<desktop ID>`): an app's own desktop entry (Flatpak, Nix package) with extra options, environment, wrappers and MIME defaults; `keyring` keeps its KDE wallet logins in both sessions (Electron too).
 - [Docker](docs/docker.md) (`docker`): rootless Docker as a user service, CLI working in both sessions.
+- [SteamVR screenshots](docs/screenshots.md) (`screenshots`): `~/Pictures/SteamVR Screenshots`, a link to Steam's folder of the current account.
 
 **Your own patches** of Steam's UI, and fixing patches after a Steam update: [UI patches](docs/ui-patches.md).
 
@@ -215,7 +216,7 @@ 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,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}`
+`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,screenshots}`
 (`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
@@ -326,6 +327,9 @@ which: [Repository layout](docs/development.md).
 | `steamFrame.docker.enable` | bool | `false` | Rootless Docker as a user service, CLI for both sessions, see [Docker](docs/docker.md). |
 | `steamFrame.docker.package` | package | `pkgs.docker` | Docker package (daemon and CLI). |
 | `steamFrame.docker.host` | str, read-only | `"unix://${runtimeDir}/docker.sock"` | The daemon's `DOCKER_HOST` (the CLI's default). |
+| `steamFrame.screenshots.enable` | bool | `false` | Link `~/Pictures/<name>` to the SteamVR screenshots, see [SteamVR screenshots](docs/screenshots.md). |
+| `steamFrame.screenshots.name` | str | `"SteamVR Screenshots"` | Name of the link in `~/Pictures`. |
+| `steamFrame.screenshots.steamUserId` | null or str (digits) | `null` | Steam account ID (folder in `~/.local/share/Steam/userdata`); `null`: the account last logged in, found at runtime. |
 | `steamFrame.cleanup.package` | package, read-only | | `steam-frame-nix-cleanup` (on `PATH` too), see [Changes outside Nix](#changes-outside-nix-exceptions). |
 
 Renamed options still work under their old names, with a warning:
@@ -363,6 +367,7 @@ lives in memory (the UI patches). These are written at runtime:
 | `/run/user/1000/systemd/user/steamvr.service.d/50-steam-frame-nix-debugger.conf`, `/run/user/1000/steam-frame-nix/steamvr-debugger-restore` | SteamVR debugger: puts the key back when SteamVR stops, without Nix | until reboot (tmpfs) | reboot; `steam-frame-nix-cleanup` while SteamVR is stopped and the debugger is off |
 | `~/.local/state/steam-frame-nix/ui-patches/<name>.json` | Saved choices of dashboard patches ([persistent state](docs/ui-patches.md#persistent-state)): window control bar placements (`frame-controls`), "Steam hidden" (`steam-close-button`). SteamOS's `steamvr.service` deletes `~/.cache/SteamVR` (the dashboard's own browser storage) on every SteamVR start. | until removed: kept when a patch is disabled (the choices come back when you enable it again) | `steam-frame-nix-cleanup --all`, `install.sh uninstall` |
 | `/run/user/1000/steam-frame-nix/applications/<id>.desktop` (with `.<id>.desktop.sum`, `.lock`) | [Launchers](docs/launchers.md) of Flatpaks and host files; `~/.local/share/applications/<id>.desktop` is a Home Manager link to it | until reboot (tmpfs), written again at login | reboot; the next switch or rewrite when the launcher or its app is gone; `steam-frame-nix-cleanup --all` |
+| `/run/user/1000/steam-frame-nix/screenshots` | [SteamVR screenshots](docs/screenshots.md#how-it-works) without `steamUserId`: link to the current account's folder; `~/Pictures/<name>` is a Home Manager link to it | until reboot (tmpfs), written again at login | reboot; `steam-frame-nix-cleanup` once unused |
 | mtime of `~/.local/share/applications` | [Launchers](docs/launchers.md#how-it-works): a running Steam rescans the "+" menu | only the directory's timestamp | nothing to remove |
 | mtime of `~/.local/share/icons/hicolor` | [icon fallbacks](docs/launcher-menu.md#icon-fallbacks): a running Steam rescans icons | only the directory's timestamp | nothing to remove |
 

docs/cleanup.md

@@ -26,6 +26,7 @@ is reported as "left alone" and never touched:
 | `jellyfin` | the hwdec shim entries in the Jellyfin Flatpak's user override `~/.local/share/flatpak/overrides/org.jellyfin.JellyfinDesktop` (nix-flatpak), an empty override file, and the shim copy of earlier versions in `~/.var/app/org.jellyfin.JellyfinDesktop` (marker `~/.local/state/steam-frame-nix/jellyfin-hwdec-shim`) |
 | `ui-state`: `~/.local/state/steam-frame-nix/ui-patches/<name>.json` | the dashboard patches' saved choices; `--all` only, never `--orphans`; stray `*.json.tmp` files |
 | `launchers`: `/run/user/1000/steam-frame-nix/applications` | the entries of [launchers](launchers.md) (tmpfs); `--all` only (switches remove those of removed launchers themselves) |
+| `screenshots`: `/run/user/1000/steam-frame-nix/screenshots` | a link to `*/userdata/*/760/remote/250820/screenshots` ([SteamVR screenshots](screenshots.md)); kept by `--orphans --keep screenshots` (while `steamUserId` is unset) |
 | `dirs` | `~/.local/state/steam-frame-nix` and `/run/user/1000/steam-frame-nix` when empty |
 
 ### Left by older versions

docs/development.md

@@ -64,6 +64,11 @@ modules/
                                of Flatpaks/host files in <runtimeDir>/steam-frame-nix/applications
     check.nix, fixtures/       test: real entries rewritten (fixtures/expected), generator runs
   docker.nix                   service: rootless dockerd (docker)
+  screenshots.nix              ~/Pictures link to the SteamVR screenshots (screenshots)
+  screenshots/
+    link.sh, package.nix       service steam-frame-nix-screenshots (+ .path), switch:
+                               <runtimeDir>/steam-frame-nix/screenshots -> account folder
+    check.nix                  test: account detection on fake Steam dirs
   firefox.nix                  Flatpak prefs extension and launcher (firefox)
   firefox/wrapper.nix          app: launcher wrapper (desktop profile, fullscreen fix)
   firefox/check.nix            test: wrapper against a fake flatpak
@@ -128,6 +133,7 @@ they are kept even where a file name says more:
 | `frame-controls` | `dashboard.frameControls` | `frame-controls` (SteamVR, state) | `steam-ui-patches` |
 | `steamvr-debugger` | `steamvrDebugger` | | `steamvr-webhelper-debugger` |
 | `launchers` | `launchers` | | `steam-frame-nix-launchers` (`.service`, `.path`) |
+| `screenshots` | `screenshots` | | `steam-frame-nix-screenshots` (`.service`, `.path`) |
 
 Log of all patches:
 `journalctl --user -u steam-ui-patches -u steam-keyboard-patch -u vr-keyboard-relay`.

docs/screenshots.md

@@ -0,0 +1,54 @@
+# SteamVR screenshots
+
+`screenshots.*`, module `screenshots`. Options:
+[README, Options](../README.md#options).
+
+## Problem
+
+Steam keeps VR screenshots deep in its data directory, under the Steam
+account ID, where Dolphin, Gwenview and the file pickers don't look.
+
+## What you get
+
+`~/Pictures/SteamVR Screenshots` (`screenshots.name`), a link to Steam's
+folder of SteamVR screenshots (app 250820):
+`~/.local/share/Steam/userdata/<account ID>/760/remote/250820/screenshots`.
+Nothing is copied; the link goes away when you turn the option off.
+
+**Taking a screenshot:** hold the Steam button and click a trigger
+(SteamVR's `TakeScreenshot` chord). Steam writes the capture as a PNG to
+`/run/user/1000/Screenshot_*.png` (tmpfs, temporary), then imports it as a
+1920x1080 JPG into the folder above (small copies in `thumbnails/`, listed
+in `…/760/screenshots.vdf`). The JPG is the only copy that stays.
+
+## Configuration
+
+```nix
+steamFrame.screenshots.enable = true;
+# steamFrame.screenshots.steamUserId = "80511808";   # optional
+```
+
+The account ID is found at runtime: the account last logged in to Steam
+(`MostRecent`, else the latest `timestamp` in
+`~/.local/share/Steam/config/loginusers.vdf`), else the only folder in
+`~/.local/share/Steam/userdata`; it follows when you switch accounts.
+`steamUserId` (the folder name, `ls ~/.local/share/Steam/userdata`) fixes
+it instead: a plain Home Manager link, no service.
+
+## Caveats
+
+- Steam creates the folder with the first screenshot: until then the link
+  points nowhere.
+- Only SteamVR's folder is linked; screenshots Steam files under another app
+  are in `…/760/remote/<app ID>/screenshots`.
+- Without `steamUserId`, when no account is found there is no link (the
+  service warns in `journalctl --user -u steam-frame-nix-screenshots`).
+
+## How it works
+
+Without `steamUserId`, `~/Pictures/<name>` is a Home Manager link to
+`/run/user/1000/steam-frame-nix/screenshots`, a link that the oneshot user
+service `steam-frame-nix-screenshots` points to the account's folder: on
+switch, at login and when `loginusers.vdf` or `userdata` changes (path
+unit). Its lifetime and cleanup:
+[Changes outside Nix](../README.md#changes-outside-nix-exceptions).

flake.nix

@@ -37,6 +37,7 @@
       jellyfin = ./modules/jellyfin.nix;
       launchers = ./modules/launchers.nix;
       docker = ./modules/docker.nix;
+      screenshots = ./modules/screenshots.nix;
     };
     # Former attribute names, kept so existing imports keep working (not in
     # `default`, which imports each module once under its current name).
@@ -67,8 +68,8 @@
     # Tests of the VR keyboard (text model, corrector, swipe decoder on the
     # default German + English dictionary), the extra keys' xdotool
     # allowlist, the Jellyfin mpv shim, the Firefox wrapper, the launchers,
-    # install.sh cleanup, the applications.menu link and the portal config:
-    # nix flake check
+    # install.sh cleanup, the applications.menu link, the portal config and
+    # the screenshots account detection: nix flake check
     checks = forSystems (pkgs: {
       applications-menu = import ./modules/applications-menu/check.nix { inherit pkgs; };
       cleanup = import ./modules/cleanup/check.nix { inherit pkgs; };
@@ -76,6 +77,7 @@
       jellyfin = import ./modules/jellyfin/check.nix { inherit pkgs; };
       launchers = import ./modules/launchers/check.nix { inherit pkgs; };
       portal = import ./modules/portal/check.nix { inherit pkgs; };
+      screenshots = import ./modules/screenshots/check.nix { inherit pkgs; };
       vr-keyboard = import ./modules/vr-keyboard/check.nix { inherit pkgs; };
       vr-keyboard-extra-keys = import ./modules/vr-keyboard-extra-keys/check.nix { inherit pkgs; };
     });

install.sh

@@ -74,7 +74,7 @@ Commands:
                      or before removing it.
         --orphans    what the configuration no longer uses (run by the Home
                      Manager module on every switch); keeps saved choices.
-        --keep       still in use (with --orphans): debugger
+        --keep       still in use (with --orphans): debugger, screenshots
         --dry-run    only print what would be done
         --quiet      print only actions, deferrals and warnings
       SteamVR's VRWebHelper.DebuggerEnabled can't be changed while SteamVR
@@ -410,6 +410,9 @@ EOF
 #   launchers  entries of steamFrame.launchers in
 #              $OUTER_RUNTIME_DIR/steam-frame-nix/applications (tmpfs; --all
 #              only: the switch's own run removes those of removed launchers).
+#   screenshots  the link $OUTER_RUNTIME_DIR/steam-frame-nix/screenshots
+#              (tmpfs) to a SteamVR screenshot folder
+#              (*/userdata/<id>/760/remote/250820/screenshots).
 #   dirs       $SFN_STATE and $OUTER_RUNTIME_DIR/steam-frame-nix when empty.
 #
 # --orphans keeps what the current configuration still uses (--keep ...);
@@ -839,6 +842,19 @@ clean_launchers() {
   c_rmdir "$d"
 }
 
+# --- screenshots link (tmpfs) ---
+
+clean_screenshots() { # keep
+  local l=$SFN_RUNTIME/screenshots
+  (( $1 )) && return 0
+  [[ -L $l ]] || return 0
+  if [[ $(readlink "$l") == */userdata/*/760/remote/250820/screenshots ]]; then
+    c_rm "$l" "link to the SteamVR screenshots"
+  else
+    c_left "$l (not a link to SteamVR screenshots)"
+  fi
+}
+
 # --- dashboard patch state ---
 
 clean_ui_state() { # all
@@ -859,7 +875,7 @@ clean_ui_state() { # all
 }
 
 cmd_cleanup() {
-  local mode='' keep_debugger=0 keeps=0 k
+  local mode='' keep_debugger=0 keep_screenshots=0 keeps=0 k
   while (( $# )); do
     case $1 in
       --all) mode=all; shift ;;
@@ -871,7 +887,8 @@ cmd_cleanup() {
         k=$2; shift 2; keeps=$((keeps + 1))
         case $k in
           debugger) keep_debugger=1 ;;
-          *) die "cleanup: unknown artifact '$k' for --keep (debugger)" ;;
+          screenshots) keep_screenshots=1 ;;
+          *) die "cleanup: unknown artifact '$k' for --keep (debugger, screenshots)" ;;
         esac ;;
       -h|--help) usage; exit 0 ;;
       *) die "cleanup: unknown option '$1' (see --help)" ;;
@@ -891,6 +908,7 @@ cmd_cleanup() {
   clean_jellyfin
   clean_ui_state "$([[ $mode == all ]] && echo 1 || echo 0)"
   c_rmdir "$SFN_STATE"
+  clean_screenshots "$keep_screenshots"
   if [[ $mode == all ]]; then clean_launchers; c_rmdir "$SFN_RUNTIME"; fi
   (( CLEAN_ACTIONS || ${#CLEAN_DEFERRED[@]} || CLEAN_QUIET )) || info "nothing to clean up"
   return 0

modules/cleanup/check.nix

@@ -57,6 +57,8 @@ pkgs.runCommand "cleanup-check" { nativeBuildInputs = [ cleanup pkgs.jq ]; } ''
   echo '[Desktop Entry]' > $root/run/steam-frame-nix/applications/im.riot.Riot.desktop
   echo x > $root/run/steam-frame-nix/applications/.im.riot.Riot.desktop.sum
   : > $root/run/steam-frame-nix/applications/.lock
+  ln -s $HOME/.local/share/Steam/userdata/1/760/remote/250820/screenshots \
+    $root/run/steam-frame-nix/screenshots               # steamFrame.screenshots
   echo old > $root/run/systemd/user/steamvr.service.d/50-steam-frame-nix-debugger.conf
   echo keep > $root/run/systemd/user/steamvr.service.d/other.conf
   ln -s /nix/store/00000000000000000000000000000000-breeze-icons-6.30.0/share/icons/breeze/apps/64/utilities-terminal.svg $I/utilities-terminal.svg
@@ -191,6 +193,17 @@ pkgs.runCommand "cleanup-check" { nativeBuildInputs = [ cleanup pkgs.jq ]; } ''
   exec 8<&-                                            # Firefox closed
   res=$(steam-frame-nix-cleanup --orphans)
   gone $F/desktop/user.js
+  # screenshots link: kept while used, a foreign link left alone
+  mkdir -p $root/run/steam-frame-nix
+  ln -s /x/userdata/1/760/remote/250820/screenshots $root/run/steam-frame-nix/screenshots
+  res=$(steam-frame-nix-cleanup --orphans --keep screenshots)
+  there $root/run/steam-frame-nix/screenshots
+  res=$(steam-frame-nix-cleanup --orphans)
+  gone $root/run/steam-frame-nix/screenshots
+  ln -s /elsewhere $root/run/steam-frame-nix/screenshots
+  res=$(steam-frame-nix-cleanup --orphans)
+  has "$res" "left alone: $root/run/steam-frame-nix/screenshots"
+  rm -r $root/run/steam-frame-nix
   ! steam-frame-nix-cleanup --all --keep debugger 2>/dev/null || fail "--all --keep accepted"
   ! steam-frame-nix-cleanup --orphans --keep bogus 2>/dev/null || fail "unknown --keep accepted"
   echo "C ok"

modules/screenshots.nix

@@ -0,0 +1,73 @@
+# ~/Pictures/<name>: a link to Steam's folder of SteamVR screenshots
+# (docs/screenshots.md). With steamUserId a plain Home Manager link; without,
+# it points to <session.runtimeDir>/steam-frame-nix/screenshots (tmpfs),
+# which steam-frame-nix-screenshots (screenshots/link.sh) points to the
+# current account's folder: on switch, at login and when loginusers.vdf or
+# userdata changes (path unit).
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.steamFrame.screenshots;
+  session = config.steamFrame.session;
+  steam = "${config.home.homeDirectory}/.local/share/Steam";
+  detect = cfg.enable && cfg.steamUserId == null;
+  script = import ./screenshots/package.nix { inherit lib pkgs steam; inherit (session) runtimeDir; };
+  exe = lib.getExe script;
+in {
+  imports = [ ./cleanup.nix ];
+
+  options.steamFrame.screenshots = {
+    enable = lib.mkEnableOption "a link to the SteamVR screenshots in ~/Pictures";
+    name = lib.mkOption {
+      type = lib.types.str;
+      default = "SteamVR Screenshots";
+      description = "Name of the link in ~/Pictures.";
+    };
+    steamUserId = lib.mkOption {
+      type = lib.types.nullOr (lib.types.strMatching "[1-9][0-9]*");
+      default = null;
+      example = "80511808";
+      description = ''
+        Steam account ID: the folder name in ~/.local/share/Steam/userdata.
+        `null`: the account last logged in, found at runtime.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable (lib.mkMerge [
+    {
+      home.file."Pictures/${cfg.name}".source = config.lib.file.mkOutOfStoreSymlink
+        (if detect then "${session.runtimeDir}/steam-frame-nix/screenshots"
+         else "${steam}/userdata/${cfg.steamUserId}/760/remote/250820/screenshots");
+    }
+    (lib.mkIf detect {
+      steamFrame.cleanup.keep = [ "screenshots" ];
+      home.packages = [ script ];
+
+      systemd.user.services.steam-frame-nix-screenshots = {
+        Unit.Description = "steam-frame-nix: link to the SteamVR screenshots of the current Steam account";
+        Service = {
+          Type = "oneshot";
+          ExecStart = exe;
+        };
+        Install.WantedBy = [ "default.target" ];
+      };
+      systemd.user.paths.steam-frame-nix-screenshots = {
+        Unit.Description = "steam-frame-nix: update the screenshots link when the Steam account changes";
+        Path = {
+          PathChanged = [ "${steam}/config/loginusers.vdf" "${steam}/userdata" ];
+          Unit = "steam-frame-nix-screenshots.service";
+        };
+        Install.WantedBy = [ "default.target" ];
+      };
+      steamFrame.session.services.start = [ "steam-frame-nix-screenshots.path" ];
+
+      home.activation.steamFrameScreenshots = lib.hm.dag.entryAfter [ "linkGeneration" "steamFrameNixCleanup" ] ''
+        if [[ -v DRY_RUN ]]; then
+          echo "would run: ${exe}"
+        else
+          ${exe}
+        fi
+      '';
+    })
+  ]);
+}

modules/screenshots/check.nix

@@ -0,0 +1,58 @@
+# Checks of steam-frame-nix-screenshots (link.sh) on fake Steam and runtime
+# dirs: account from loginusers.vdf (MostRecent, else latest timestamp),
+# else the only userdata folder, else no link (an old one removed); an
+# unchanged target isn't rewritten; no outer runtime dir is a no-op.
+{ pkgs }:
+let
+  script = import ./package.nix {
+    inherit (pkgs) lib; inherit pkgs;
+    runtimeDir = "/nonexistent"; steam = "/nonexistent";
+  };
+in
+pkgs.runCommand "screenshots-check" { nativeBuildInputs = [ script ]; } ''
+  set -euo pipefail
+  fail() { echo "FAIL: $*" >&2; exit 1; }
+  export STEAM_FRAME_NIX_RUNTIME_DIR=$PWD/run STEAM_FRAME_NIX_STEAM_DIR=$PWD/steam
+  S=$STEAM_FRAME_NIX_STEAM_DIR L=$STEAM_FRAME_NIX_RUNTIME_DIR/steam-frame-nix/screenshots
+  target() { echo "$S/userdata/$1/760/remote/250820/screenshots"; }
+  expect() { [ "$(readlink $L)" = "$(target $1)" ] || fail "$2: $(readlink $L || echo no link)"; }
+
+  steam-frame-nix-screenshots 2>&1 | grep -q "doesn't exist" || fail "no runtime dir"
+  [ ! -e run ] || fail "runtime dir created"
+  mkdir -p run $S/config $S/userdata/0 $S/userdata/80511808
+
+  # the only account folder ("0" is not an account)
+  steam-frame-nix-screenshots; expect 80511808 "userdata"
+
+  # MostRecent wins over a later timestamp
+  cat > $S/config/loginusers.vdf <<'VDF'
+"users"
+{
+	"76561198040777536"
+	{
+		"AccountName"		"a"
+		"MostRecent"		"1"
+		"timestamp"		"100"
+	}
+	"76561197960265729"
+	{
+		"AccountName"		"b"
+		"MostRecent"		"0"
+		"timestamp"		"200"
+	}
+}
+VDF
+  steam-frame-nix-screenshots; expect 80511808 "MostRecent"
+  # unchanged: not rewritten
+  res=$(steam-frame-nix-screenshots 2>&1); [ -z "$res" ] || fail "rewrote: $res"
+
+  # no MostRecent (current Steam): latest timestamp
+  sed -i '/MostRecent/d' $S/config/loginusers.vdf
+  steam-frame-nix-screenshots; expect 1 "timestamp"
+
+  # nothing to go by: link removed
+  rm $S/config/loginusers.vdf; mkdir $S/userdata/42
+  steam-frame-nix-screenshots 2>&1 | grep -q "no Steam account" || fail "no warning"
+  [ ! -L $L ] || fail "link kept"
+  touch $out
+''

modules/screenshots/link.sh

@@ -0,0 +1,54 @@
+# steam-frame-nix-screenshots (the module prepends runtime, steam): points
+# <runtime>/steam-frame-nix/screenshots (tmpfs), where ~/Pictures/<name>
+# links, to the SteamVR screenshot folder of the Steam account last logged
+# in:
+# 1. the user marked MostRecent "1" in config/loginusers.vdf, else the one
+#    with the latest "timestamp" (SteamID64 - 76561197960265728 = account
+#    ID, the folder in userdata);
+# 2. else the only account folder in userdata;
+# 3. else no link (warning).
+# The link is replaced atomically and only when its target changes.
+link=$runtime/steam-frame-nix/screenshots
+log() { echo "steam-frame-nix-screenshots: $*" >&2; }
+
+if [[ ! -d $runtime ]]; then
+  log "$runtime doesn't exist (outer session not running?); skipped"
+  exit 0
+fi
+
+id=''
+if [[ -f $steam/config/loginusers.vdf ]]; then
+  id64=$(awk '
+    { gsub(/"/, " ") }
+    $1 == "{" { depth++; next }
+    $1 == "}" { depth--; next }
+    depth == 1 && $1 ~ /^[0-9]+$/ && NF == 1 { user = $1; next }
+    depth == 2 && tolower($1) == "mostrecent" && $2 == "1" { recent = user }
+    depth == 2 && tolower($1) == "timestamp" && $2 + 0 > best { best = $2 + 0; latest = user }
+    END { print recent != "" ? recent : latest }
+  ' "$steam/config/loginusers.vdf")
+  if [[ $id64 =~ ^[0-9]{17}$ ]] && (( id64 > 76561197960265728 )); then
+    id=$(( id64 - 76561197960265728 ))
+  fi
+fi
+if [[ -z $id ]]; then
+  dirs=()
+  for d in "$steam"/userdata/*/; do
+    d=${d%/}; d=${d##*/}
+    [[ $d =~ ^[1-9][0-9]*$ ]] && dirs+=("$d")
+  done
+  (( ${#dirs[@]} == 1 )) && id=${dirs[0]}
+fi
+
+if [[ -z $id ]]; then
+  log "no Steam account found in $steam (log in to Steam, or set steamFrame.screenshots.steamUserId); no link"
+  [[ -L $link ]] && rm -f "$link"
+  exit 0
+fi
+
+target=$steam/userdata/$id/760/remote/250820/screenshots
+[[ -L $link && $(readlink "$link") == "$target" ]] && exit 0
+mkdir -p "${link%/*}"
+ln -sfn "$target" "$link.tmp.$$"
+mv -fT "$link.tmp.$$" "$link"
+log "$link -> $target"

modules/screenshots/package.nix

@@ -0,0 +1,11 @@
+# steam-frame-nix-screenshots: link.sh with the outer runtime dir and the
+# Steam directory (both overridable by environment, for the check).
+{ lib, pkgs, runtimeDir, steam }:
+pkgs.writeShellApplication {
+  name = "steam-frame-nix-screenshots";
+  runtimeInputs = [ pkgs.coreutils pkgs.gawk ];
+  text = ''
+    runtime=''${STEAM_FRAME_NIX_RUNTIME_DIR:-${lib.escapeShellArg runtimeDir}}
+    steam=''${STEAM_FRAME_NIX_STEAM_DIR:-${lib.escapeShellArg steam}}
+  '' + builtins.readFile ./link.sh;
+}