Add a headless test mode to ft-screens and a throwaway test desktop

ft-screens --no-vr runs without SteamVR and leaves the input relay alone;
--control names its control socket; "toplevels" lists KWin's windows with
their titles and sizes; "input" feeds pointer events as if from a panel.
screens/test/headless.sh starts it with a bare nested KWin next to the
running desktop, and loads KWin scripts, runs apps, and takes screenshots.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

screens/compositor.c

@@ -12,8 +12,13 @@
 //     buffers are in pixels, so a panel position in pixels is divided by the screen's KWin
 //     scale first ("scale <screen> <s>", from ft-layout).
 //
-// Usage: ft-screens [--socket NAME] [--screen WxH@METRES]... [-- COMMAND ARGS...]
+// Usage: ft-screens [--socket NAME] [--control NAME] [--no-vr] [--screen WxH@METRES]...
+//                   [-- COMMAND ARGS...]
 //   --socket    Wayland socket name in $XDG_RUNTIME_DIR (default ft-screens-0)
+//   --control   the control socket's abstract name (default ft_screens)
+//   --no-vr     run without SteamVR, for tests next to the running desktop: no panels, no
+//               input, and nothing sent to the input relay. Commands still work, and
+//               "toplevels" shows what KWin opened.
 //   --screen    one per screen, in KWin's order (default: 3440x1440@2.4)
 //   COMMAND     run with WAYLAND_DISPLAY set to our socket (e.g. the Frametop session)
 // Runs in the dev container (wlroots 0.20); KWin connects from the host.
@@ -68,8 +73,10 @@ struct screen {
     struct wlr_xdg_toplevel *toplevel;
     struct wlr_buffer *held;      // on the panel now; unlocked when the next one arrives
     bool frame_pending;           // a commit waits for its frame callback
+    unsigned commits;             // buffers committed, and the last one's size ("toplevels")
+    int buffer_width, buffer_height;
     struct wlr_xdg_toplevel_decoration_v1 *decoration;  // answered on the first commit
-    struct wl_listener commit, destroy, decoration_destroy;
+    struct wl_listener commit, destroy, decoration_destroy, set_title;
 };
 
 // Per client buffer: forget its import when it goes away.
@@ -102,6 +109,7 @@ struct server {
     int relay_fd;         // unbound, so the relay can't reply into our control socket
     uint32_t relay_sent;  // when the relay last heard from us (ms)
     unsigned ticks;
+    bool vr;  // connected to SteamVR (not --no-vr)
 };
 
 static uint32_t now_ms(void) {
@@ -153,6 +161,8 @@ static void screen_commit(struct wl_listener *l, void *data) {
     }
     if (!buffer) return;
     sc->frame_pending = true;
+    ++sc->commits;
+    sc->buffer_width = buffer->width, sc->buffer_height = buffer->height;
     if (buffer == sc->held) return;
     struct wlr_dmabuf_attributes a;
     if (!wlr_buffer_get_dmabuf(buffer, &a)) {
@@ -187,9 +197,17 @@ static void screen_destroy(struct wl_listener *l, void *data) {
     sc->server->screens[sc->index] = NULL;
     wl_list_remove(&sc->commit.link);
     wl_list_remove(&sc->destroy.link);
+    wl_list_remove(&sc->set_title.link);
     free(sc);
 }
 
+// KWin titles each window "KDE Wayland Compositor <output name>", with "- Output disabled"
+// after it while that output is off.
+static void screen_title(struct wl_listener *l, void *data) {
+    struct screen *sc = wl_container_of(l, sc, set_title);
+    wlr_log(WLR_INFO, "screen %d: \"%s\"", sc->index + 1, sc->toplevel->title ? sc->toplevel->title : "");
+}
+
 static void new_toplevel(struct wl_listener *l, void *data) {
     struct server *s = wl_container_of(l, s, new_toplevel);
     struct wlr_xdg_toplevel *toplevel = data;
@@ -211,6 +229,8 @@ static void new_toplevel(struct wl_listener *l, void *data) {
     wl_signal_add(&toplevel->base->surface->events.commit, &sc->commit);
     sc->destroy.notify = screen_destroy;
     wl_signal_add(&toplevel->events.destroy, &sc->destroy);
+    sc->set_title.notify = screen_title;
+    wl_signal_add(&toplevel->events.set_title, &sc->set_title);
 }
 
 // KWin asks for server-side decorations for its screens; we draw none. It asks before its
@@ -294,6 +314,7 @@ static void handle_vr_event(const struct ft_event *e, void *data) {
 // gamescope's focus) doesn't get the keys too. Without word from us for a few seconds,
 // the relay gives the keyboards back, so a closed desktop doesn't keep them.
 static void keys_update(struct server *s) {
+    if (!s->vr) return;  // a test instance leaves the running desktop's keyboards alone
     const bool desktop = s->keys_clicked && ft_vr_screens_shown();
     const uint32_t t = now_ms();
     if (desktop == s->keys_desktop && t - s->relay_sent < 1000) return;
@@ -366,6 +387,9 @@ static void handle_key(struct server *s, uint32_t code, int value, char *reply,
 
 // Control socket: abstract datagram @ft_screens. Here: "size <screen> <w> <h>" (a new
 // resolution, live), "scale <screen> <s>" (KWin's scale for it, from ft-layout),
+// "toplevels" (-> "ok <count>" and a line per KWin window: "<screen> <w>x<h> <commits>
+// <title>"), "input <screen> move|down|up|leave [x y [left|right|middle]]" (pointer input
+// as if from that panel, x and y in its pixels; for tests, mostly with --no-vr),
 // "key <code> <value>", and "click <overlay key>" (from the pointer
 // helper: a mouse click landed on that panel, "-" for none); the rest is in vr.cpp
 // (ft_vr_command).
@@ -402,6 +426,34 @@ static int control_readable(int fd, uint32_t mask, void *data) {
             handle_key(s, code, value, reply, sizeof reply);
             len = sizeof from;
             continue;  // no reply: keys are fire-and-forget
+        } else if (strncmp(buf, "input ", 6) == 0) {
+            char what[8] = "", button[8] = "left";
+            double x = 0, y = 0;
+            const int got = sscanf(buf, "input %d %7s %lf %lf %7s", &index, what, &x, &y, button);
+            struct ft_event e = {.screen = index - 1, .x = x, .y = y, .button = BTN_LEFT};
+            if (strcmp(button, "right") == 0) e.button = BTN_RIGHT;
+            else if (strcmp(button, "middle") == 0) e.button = BTN_MIDDLE;
+            if (got >= 2 && strcmp(what, "leave") == 0) e.type = FT_LEAVE;
+            else if (got >= 4 && strcmp(what, "move") == 0) e.type = FT_MOTION;
+            else if (got >= 4 && (strcmp(what, "down") == 0 || strcmp(what, "up") == 0))
+                e.type = FT_BUTTON, e.pressed = what[0] == 'd';
+            else index = 0;
+            if (index < 1 || index > MAX_SCREENS || !s->screens[index - 1]) {
+                snprintf(reply, sizeof reply, "error input <screen> move|down|up|leave [x y [button]]");
+            } else {
+                handle_vr_event(&e, s);
+                snprintf(reply, sizeof reply, "ok");
+            }
+        } else if (strcmp(buf, "toplevels") == 0) {
+            int n = 0, at = 0;
+            for (int i = 0; i < MAX_SCREENS; ++i) n += s->screens[i] != NULL;
+            at = snprintf(reply, sizeof reply, "ok %d", n);
+            for (int i = 0; i < MAX_SCREENS && at < (int)sizeof reply; ++i) {
+                const struct screen *sc = s->screens[i];
+                if (!sc) continue;
+                at += snprintf(reply + at, sizeof reply - at, "\n%d %dx%d %u %s", i + 1, sc->buffer_width,
+                               sc->buffer_height, sc->commits, sc->toplevel->title ? sc->toplevel->title : "");
+            }
         } else if (strncmp(buf, "click ", 6) == 0) {
             // A click on another panel takes typing to Steam. Our own panels (the screens,
             // their controls) leave it: a click on a screen arrives as a panel event.
@@ -418,13 +470,18 @@ static int control_readable(int fd, uint32_t mask, void *data) {
     return 0;
 }
 
-static int open_control_socket(void) {
+static int open_control_socket(const char *name) {
     int fd = socket(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0);
     struct sockaddr_un addr = {.sun_family = AF_UNIX};
-    const char name[] = "ft_screens";
-    memcpy(addr.sun_path + 1, name, sizeof name - 1);
-    if (bind(fd, (struct sockaddr *)&addr, offsetof(struct sockaddr_un, sun_path) + 1 + sizeof name - 1) != 0) {
-        wlr_log(WLR_ERROR, "can't bind @ft_screens (another ft-screens running?)");
+    const size_t len = strlen(name);
+    if (len == 0 || len >= sizeof addr.sun_path - 1) {
+        wlr_log(WLR_ERROR, "bad control socket name");
+        close(fd);
+        return -1;
+    }
+    memcpy(addr.sun_path + 1, name, len);
+    if (bind(fd, (struct sockaddr *)&addr, offsetof(struct sockaddr_un, sun_path) + 1 + len) != 0) {
+        wlr_log(WLR_ERROR, "can't bind @%s (another ft-screens running?)", name);
         close(fd);
         return -1;
     }
@@ -467,11 +524,16 @@ static bool setup_dmabuf(struct server *s) {
 int main(int argc, char **argv) {
     struct server s = {0};
     for (int i = 0; i < MAX_SCREENS; ++i) s.scale[i] = 1;
-    const char *socket_name = "ft-screens-0";
+    const char *socket_name = "ft-screens-0", *control_name = "ft_screens";
     char **command = NULL;
+    s.vr = true;
     for (int i = 1; i < argc; ++i) {
         if (strcmp(argv[i], "--socket") == 0 && i + 1 < argc) {
             socket_name = argv[++i];
+        } else if (strcmp(argv[i], "--control") == 0 && i + 1 < argc) {
+            control_name = argv[++i];
+        } else if (strcmp(argv[i], "--no-vr") == 0) {
+            s.vr = false;
         } else if (strcmp(argv[i], "--screen") == 0 && i + 1 < argc && s.n_config < MAX_SCREENS) {
             struct config *c = &s.config[s.n_config];
             c->metres = 0;
@@ -485,7 +547,10 @@ int main(int argc, char **argv) {
             command = &argv[i + 1];
             break;
         } else {
-            fprintf(stderr, "usage: %s [--socket NAME] [--screen WxH@METRES]... [-- COMMAND ARGS...]\n", argv[0]);
+            fprintf(stderr,
+                    "usage: %s [--socket NAME] [--control NAME] [--no-vr] [--screen WxH@METRES]... "
+                    "[-- COMMAND ARGS...]\n",
+                    argv[0]);
             return 2;
         }
     }
@@ -493,7 +558,8 @@ int main(int argc, char **argv) {
 
     setvbuf(stdout, NULL, _IOLBF, 0);  // vr.cpp prints to stdout; keep it in order with the log
     wlr_log_init(WLR_INFO, NULL);
-    if (!ft_vr_init()) return 1;
+    if (s.vr && !ft_vr_init()) return 1;
+    if (!s.vr) wlr_log(WLR_INFO, "--no-vr: running without SteamVR");
 
     s.display = wl_display_create();
     s.loop = wl_display_get_event_loop(s.display);
@@ -530,7 +596,7 @@ int main(int argc, char **argv) {
     snprintf(path, sizeof path, "%s/%s", getenv("XDG_RUNTIME_DIR") ? getenv("XDG_RUNTIME_DIR") : "/tmp", socket_name);
     wlr_log(WLR_INFO, "listening on %s; %d screen(s) configured", path, s.n_config);
 
-    const int control = open_control_socket();
+    const int control = open_control_socket(control_name);
     if (control < 0) return 1;
     s.relay_fd = socket(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0);
     wl_event_loop_add_fd(s.loop, control, WL_EVENT_READABLE, control_readable, &s);

screens/test/headless.sh

@@ -0,0 +1,160 @@
+#!/usr/bin/env bash
+# A throwaway desktop for experiments, next to the running one: ft-screens --no-vr (its own
+# Wayland socket, control socket @ft_screens_test, and runtime folder) with a bare nested
+# KWin inside it, on its own D-Bus and config folder. Nothing touches SteamVR, the input
+# relay, or the real desktop's settings. Run it from the PC (it runs itself on the Frame)
+# or on the Frame. Build ft-screens first (screens/build.sh).
+#
+#   headless.sh start [SCREENS] [SPARES]   (default 2 screens, 1920x1080 and 1080x1920, and
+#                                          3 spare 800x600 outputs, disabled once KWin is up)
+#   headless.sh stop
+#   headless.sh ask '<ft-screens command>'  e.g. toplevels, "input 1 down 400 20"
+#   headless.sh kd <kscreen-doctor args>    e.g. -o, output.WL-2.enable
+#   headless.sh run <command> [args]        start an app in the test KWin (in the background)
+#   headless.sh script <file.js> [name]     load a KWin script into it and run it
+#   headless.sh unscript <name>
+#   headless.sh js '<javascript>'           run a one-off KWin script; its print()s come back
+#   headless.sh shot [OUTPUT]               a JPEG of one output (default WL-0); from the PC it
+#                                          lands in captures/ and its path is printed
+#   headless.sh log [kwin|screens|apps] [N] the last N lines (default 30)
+set -euo pipefail
+here=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+. "$here/../../scripts/_env.sh"
+
+if [ "$FRAME_LOCAL" != 1 ]; then
+  "$REPO_ROOT/scripts/sync.sh" >/dev/null
+  if [ "${1:-}" = shot ]; then
+    mkdir -p "$REPO_ROOT/captures"
+    out=$REPO_ROOT/captures/ft-test-${2:-WL-0}-$(date +%H%M%S).jpg
+    on_frame "screens/test/headless.sh $(printf '%q ' "$@")" > "$out"
+    echo "$out"
+  else
+    on_frame "screens/test/headless.sh $(printf '%q ' "$@")"
+  fi
+  exit
+fi
+
+rt=/run/user/$(id -u)/ft-test
+cfg=/tmp/ft-test-config
+bus=unix:path=$rt/bus
+logs=/tmp/ft-test
+mkdir -p "$logs"
+
+kwin_env() {
+  env XDG_RUNTIME_DIR="$rt" XDG_CONFIG_HOME="$cfg" DBUS_SESSION_BUS_ADDRESS="$bus" \
+    WAYLAND_DISPLAY=wayland-0 QT_QPA_PLATFORM=wayland "$@"
+}
+
+ask() {
+  python3 - "$1" <<'EOF'
+import socket, sys
+s = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM)
+s.bind("")
+s.settimeout(2)
+s.sendto(sys.argv[1].encode(), "\0ft_screens_test")
+print(s.recv(8192).decode())
+EOF
+}
+
+kd() { kwin_env kscreen-doctor "$@" 2>&1 | sed 's/\x1b\[[0-9;]*m//g'; }
+
+stop() {
+  pkill -f -- "--control ft_screens_test" 2>/dev/null || true
+  [ -f "$rt/dbus.pid" ] && kill "$(cat "$rt/dbus.pid")" 2>/dev/null || true
+  sleep 0.5
+}
+
+load_script() {  # file name
+  local id
+  id=$(kwin_env gdbus call --session -d org.kde.KWin -o /Scripting \
+         -m org.kde.kwin.Scripting.loadScript "$1" "$2" | tr -dc '0-9-')
+  [ "$id" -ge 0 ] 2>/dev/null || { echo "loadScript failed ($id)" >&2; return 1; }
+  kwin_env gdbus call --session -d org.kde.KWin -o "/Scripting/Script$id" -m org.kde.kwin.Script.run >/dev/null
+}
+
+unload_script() {
+  kwin_env gdbus call --session -d org.kde.KWin -o /Scripting -m org.kde.kwin.Scripting.unloadScript "$1" >/dev/null
+}
+
+case "${1:-}" in
+  start)
+    screens=${2:-2} spares=${3:-3}
+    stop
+    rm -rf "$rt" "$cfg"
+    mkdir -p "$rt" "$cfg"
+    chmod 700 "$rt"
+    args=(--screen 1920x1080@1.6 --screen 1080x1920@0.9)
+    for ((i = 2; i < screens; i++)); do args+=(--screen 1920x1080@1.6); done
+    args=("${args[@]:0:$((screens * 2))}")
+    for ((i = 0; i < spares; i++)); do args+=(--screen 800x600); done
+    cd "$REPO_ROOT"
+    nohup "$HOME/.local/bin/distrobox" enter "$FRAME_BOX" -- env XDG_RUNTIME_DIR="$rt" \
+      ./screens/build/ft-screens --no-vr --socket ft-test-0 --control ft_screens_test "${args[@]}" \
+      > "$logs/screens.log" 2>&1 &
+    for _ in $(seq 100); do [ -S "$rt/ft-test-0" ] && break; sleep 0.2; done
+    [ -S "$rt/ft-test-0" ] || { echo "ft-screens didn't start:"; tail "$logs/screens.log"; exit 1; }
+    dbus-daemon --session --address="$bus" --fork --print-pid > "$rt/dbus.pid"
+    # KWin's screenshot interface without a permission check, so tests can look.
+    nohup env XDG_RUNTIME_DIR="$rt" XDG_CONFIG_HOME="$cfg" DBUS_SESSION_BUS_ADDRESS="$bus" \
+      WAYLAND_DISPLAY=ft-test-0 KWIN_SCREENSHOT_NO_PERMISSION_CHECKS=1 QT_LOGGING_RULES="kwin_scripting=true" QT_FORCE_STDERR_LOGGING=1 \
+      kwin_wayland --output-count $((screens + spares)) --width 800 --height 600 --no-lockscreen \
+      > "$logs/kwin.log" 2>&1 &
+    for _ in $(seq 100); do [ -S "$rt/wayland-0" ] && break; sleep 0.2; done
+    [ -S "$rt/wayland-0" ] || { echo "KWin didn't start:"; tail "$logs/kwin.log"; exit 1; }
+    sleep 1
+    off=()
+    for ((i = screens; i < screens + spares; i++)); do off+=("output.WL-$i.disable"); done
+    [ ${#off[@]} -eq 0 ] || kd "${off[@]}" >/dev/null
+    ask toplevels
+    ;;
+  stop) stop ;;
+  ask) ask "$2" ;;
+  kd) shift; kd "$@" ;;
+  run)
+    shift
+    nohup env XDG_RUNTIME_DIR="$rt" XDG_CONFIG_HOME="$cfg" DBUS_SESSION_BUS_ADDRESS="$bus" \
+      WAYLAND_DISPLAY=wayland-0 QT_QPA_PLATFORM=wayland "$@" >> "$logs/apps.log" 2>&1 &
+    echo "started $1 (pid $!)"
+    ;;
+  script) load_script "$(realpath "$2")" "${3:-$(basename "$2" .js)}" ;;
+  unscript) unload_script "$2" ;;
+  js)
+    f=$(mktemp /tmp/ft-test-XXXX.js)
+    printf '%s\n' "$2" > "$f"
+    name=ft-test-js-$$
+    mark=$(wc -l < "$logs/kwin.log")
+    load_script "$f" "$name"
+    sleep 0.5
+    unload_script "$name" || true
+    rm -f "$f"
+    tail -n +$((mark + 1)) "$logs/kwin.log" | sed -n 's/^js: //p'
+    ;;
+  shot)
+    raw=$(mktemp /tmp/ft-test-shot.XXXXXX)
+    meta=$(kwin_env python3 - "${2:-WL-0}" "$raw" <<'PY'
+import os, sys, dbus
+name, raw = sys.argv[1:3]
+bus = dbus.bus.BusConnection(os.environ['DBUS_SESSION_BUS_ADDRESS'])
+shot = dbus.Interface(bus.get_object('org.kde.KWin', '/org/kde/KWin/ScreenShot2'), 'org.kde.KWin.ScreenShot2')
+r, w = os.pipe()
+fd = dbus.types.UnixFd(w)
+os.close(w)
+opts = dbus.Dictionary({'include-cursor': True, 'native-resolution': True}, signature='sv')
+res = shot.CaptureScreen(name, opts, fd)
+del fd  # our copy of the write end, so the read ends when KWin is done
+with os.fdopen(r, 'rb') as src, open(raw, 'wb') as dst:
+    while chunk := src.read(1 << 20):
+        dst.write(chunk)
+print(int(res['width']), int(res['height']), int(res['stride']), int(res['format']))
+PY
+)
+    read -r width height stride format <<<"$meta"
+    layout=bgra  # QImage formats 4-6: (A)RGB32, stored B,G,R,A; 16-18: RGBA8888
+    case $format in 16|17|18) layout=rgba ;; esac
+    "$HOME/.local/bin/distrobox" enter "$FRAME_BOX" -- magick -size "$((stride / 4))x$height" -depth 8 \
+      "$layout:$raw" -crop "${width}x${height}+0+0" +repage -alpha off -resize '1280x1280>' -quality 80 jpg:-
+    rm -f "$raw"
+    ;;
+  log) tail -n "${3:-30}" "$logs/${2:-kwin}.log" ;;
+  *) sed -n '2,20p' "$0"; exit 2 ;;
+esac

screens/test/observe.js

@@ -0,0 +1,22 @@
+// A KWin script for experiments (screens/test/headless.sh script ...): prints what happens
+// to windows, for checking the script API floating windows rely on.
+function desc(w) {
+    const g = w.frameGeometry;
+    return String(w.internalId).slice(1, 9) + " " + w.resourceClass + " pid=" + w.pid + " popup=" + w.popupWindow +
+        " transient=" + w.transient + " normal=" + w.normalWindow + " dialog=" + w.dialog +
+        " out=" + (w.output ? w.output.name : "?") + " geo=" + g.x + "," + g.y + " " + g.width + "x" + g.height +
+        " '" + w.caption + "'";
+}
+function watch(w) {
+    print("added " + desc(w));
+    w.interactiveMoveResizeStarted.connect(() => print("move-start move=" + w.move + " resize=" + w.resize + " " + desc(w)));
+    w.interactiveMoveResizeStepped.connect(g => print("move-step " + g.x + "," + g.y + " " + g.width + "x" + g.height));
+    w.interactiveMoveResizeFinished.connect(() => print("move-end " + desc(w)));
+    w.outputChanged.connect(() => print("output " + desc(w)));
+    w.fullScreenChanged.connect(() => print("fullscreen " + w.fullScreen + " " + desc(w)));
+    w.minimizedChanged.connect(() => print("minimized " + w.minimized));
+}
+workspace.windowList().forEach(watch);
+workspace.windowAdded.connect(watch);
+workspace.windowRemoved.connect(w => print("removed " + desc(w)));
+workspace.windowActivated.connect(w => print("activated " + (w ? w.resourceClass : "none")));

screens/vr.cpp

@@ -206,6 +206,7 @@ constexpr double kRollStep = 5;      // degrees per scroll notch on the roll but
 constexpr float kChromeIdle = 0.55f; // the controls' opacity without a laser on them
 constexpr long kControlsLinger = 35; // ticks (~0.4 s) the controls stay after a laser leaves
 long g_tick = 0;                     // ft_vr_poll calls
+bool g_vr = false;                   // connected to SteamVR (ft-screens --no-vr runs without it)
 constexpr vr::TrackedDeviceIndex_t kNone = vr::k_unTrackedDeviceIndexInvalid;
 
 struct Screen {
@@ -1150,6 +1151,7 @@ bool ft_vr_init(void) {
         std::fprintf(stderr, "openvr: no IVRIPCResourceManagerClient (SteamVR too old?)\n");
         return false;
     }
+    g_vr = true;
     RefreshPoses();
     // The catcher (see UpdateCatcher): clear and invisible, but the laser lands on it, and
     // it keeps SteamVR's laser mouse on while it's up.
@@ -1165,6 +1167,7 @@ bool ft_vr_init(void) {
 }
 
 void ft_vr_shutdown(void) {
+    if (!g_vr) return;
     if (g_catcher != vr::k_ulOverlayHandleInvalid) vr::VROverlay()->DestroyOverlay(g_catcher);
     g_catcher = vr::k_ulOverlayHandleInvalid;
     for (auto &[i, s] : g_screens)
@@ -1179,14 +1182,20 @@ void ft_vr_shutdown(void) {
 }
 
 int ft_vr_modifiers(uint32_t format, uint64_t *out, int max) {
+    if (!g_vr) {  // --no-vr: nothing imports the buffers, so any layout KWin can draw
+        if (max < 1) return 0;
+        out[0] = 0;  // DRM_FORMAT_MOD_LINEAR
+        return 1;
+    }
     uint32_t n = uint32_t(max);
     if (!vr::VRIPCResourceManager()->GetDmabufModifiers(vr::VRApplication_Overlay, format, &n, out)) return 0;
     return int(n < uint32_t(max) ? n : uint32_t(max));
 }
 
-bool ft_vr_screens_shown(void) { return ModeVisible(); }
+bool ft_vr_screens_shown(void) { return g_vr && ModeVisible(); }
 
 void ft_vr_screen_create(int index, double metres, int count) {
+    if (!g_vr) return;
     Screen &s = g_screens[index];
     s.metres = metres;
     char key[64], name[64];
@@ -1237,6 +1246,7 @@ void ft_vr_screen_destroy(int index) {
 }
 
 bool ft_vr_screen_present(int index, const void *key, const struct ft_dmabuf *b) {
+    if (!g_vr) return false;
     auto sit = g_screens.find(index);
     if (sit == g_screens.end()) return false;
     Screen &s = sit->second;
@@ -1284,6 +1294,7 @@ void ft_vr_forget(const void *key) {
 }
 
 void ft_vr_poll(void (*handle)(const struct ft_event *, void *), void *data) {
+    if (!g_vr) return;
     RefreshPoses();
     for (auto &[index, s] : g_screens) {
         vr::VREvent_t ev;
@@ -1438,6 +1449,7 @@ void ft_vr_poll(void (*handle)(const struct ft_event *, void *), void *data) {
 // (size <screen> <w> <h> and key <code> <value> are handled in compositor.c.) Screens are
 // numbered from 1 here, like everywhere the user sees them.
 void ft_vr_command(const char *cmd, char *reply, int size) {
+    if (!g_vr) return (void)std::snprintf(reply, size, "error no SteamVR (--no-vr)");
     RefreshPoses();
     int n;
     double x, y, z, yaw, pitch, roll, w;