diff --git a/install.sh b/install.sh
index 7ee703a..ff310d2 100755
--- a/install.sh
+++ b/install.sh
@@ -43,8 +43,14 @@ if [[ "${ans,,}" == "y" ]]; then
     SERVICE_SRC="$SCRIPT_DIR/steam-backlog-enforcer.service"
     SERVICE_DST="/etc/systemd/system/steam-backlog-enforcer.service"
 
-    # Set the correct working directory and PYTHONPATH in the service file.
-    sed "s|WorkingDirectory=.*|WorkingDirectory=$SCRIPT_DIR|; s|PYTHONPATH=.*|PYTHONPATH=$SCRIPT_DIR|" \
+    # Set the correct working directory, PYTHONPATH, and desktop user (the
+    # user whose Steam/X11 session the service should drive) in the service
+    # file. install.sh requires root, so it's invoked via `sudo` and
+    # SUDO_USER holds the real desktop user.
+    DESKTOP_USER="${SUDO_USER:-$USER}"
+    sed "s|WorkingDirectory=.*|WorkingDirectory=$SCRIPT_DIR|; \
+         s|PYTHONPATH=.*|PYTHONPATH=$SCRIPT_DIR|; \
+         s|__DESKTOP_USER__|$DESKTOP_USER|" \
         "$SERVICE_SRC" > "$SERVICE_DST"
 
     systemctl daemon-reload
diff --git a/steam-backlog-enforcer.service b/steam-backlog-enforcer.service
index 65dbe33..976781a 100644
--- a/steam-backlog-enforcer.service
+++ b/steam-backlog-enforcer.service
@@ -12,6 +12,8 @@ RestartSec=5
 Environment=PYTHONUNBUFFERED=1
 Environment=PYTHONPATH=/opt/steam-backlog-enforcer
 Environment=HOME=/home/kuhy
+Environment=STEAM_ENFORCER_DESKTOP_USER=__DESKTOP_USER__
+Environment=DISPLAY=:0
 # Hardening: enforcer must not be easily killed.
 OOMScoreAdjust=-900
 
diff --git a/steam_backlog_enforcer/library_hider.py b/steam_backlog_enforcer/library_hider.py
index ec8f53d..16ea96f 100644
--- a/steam_backlog_enforcer/library_hider.py
+++ b/steam_backlog_enforcer/library_hider.py
@@ -31,7 +31,12 @@ import websockets
 
 logger = logging.getLogger(__name__)
 
-_CDP_PORT = 8080
+_CDP_PORT = 9222
+# NOTE: was 8080, which collided with a different local service (Open WebUI)
+# already bound to 0.0.0.0:8080. requests to 127.0.0.1:8080 resolved to that
+# service instead of steamwebhelper's CDP endpoint (which only bound
+# [::1]:8080), so CDP detection silently never worked. 9222 is the
+# conventional Chrome DevTools debug port and was confirmed free.
 _CDP_TIMEOUT = 120
 _STEAM_STARTUP_WAIT = 45
 
@@ -183,9 +188,24 @@ def _wait_for_collections_ready() -> bool:
     return False
 
 
+def _resolve_desktop_user() -> str | None:
+    """Resolve which desktop user owns the Steam/X11 session.
+
+    Prefers the explicit STEAM_ENFORCER_DESKTOP_USER (set by the systemd
+    unit, which has no SUDO_USER/USER of its own since it is started
+    directly by systemd rather than via `sudo`), then falls back to
+    SUDO_USER/USER for interactive `sudo` invocations.
+    """
+    return (
+        os.environ.get("STEAM_ENFORCER_DESKTOP_USER")
+        or os.environ.get("SUDO_USER")
+        or os.environ.get("USER")
+    )
+
+
 def _shutdown_steam() -> None:
     """Send ``steam -shutdown`` and wait for the process to exit."""
-    real_user = os.environ.get("SUDO_USER") or os.environ.get("USER")
+    real_user = _resolve_desktop_user()
     try:
         _run_as_user(["steam", "-shutdown"], real_user)
     except FileNotFoundError:
@@ -205,7 +225,7 @@ def _shutdown_steam() -> None:
 
 def _launch_steam_with_debug() -> None:
     """Launch Steam with CEF debugging enabled."""
-    real_user = os.environ.get("SUDO_USER") or os.environ.get("USER")
+    real_user = _resolve_desktop_user()
     _run_as_user(
         [
             "steam",
diff --git a/steam_backlog_enforcer/tests/test_library_hider_part2.py b/steam_backlog_enforcer/tests/test_library_hider_part2.py
index bad1c3d..15681f0 100644
--- a/steam_backlog_enforcer/tests/test_library_hider_part2.py
+++ b/steam_backlog_enforcer/tests/test_library_hider_part2.py
@@ -9,6 +9,7 @@ from unittest.mock import MagicMock, patch
 from steam_backlog_enforcer.library_hider import (
     _SPAWNED,
     _reap_spawned,
+    _resolve_desktop_user,
     _run_as_user,
     hide_other_games,
     restart_steam,
@@ -19,6 +20,48 @@ from steam_backlog_enforcer.library_hider import (
 PKG = "steam_backlog_enforcer.library_hider"
 
 
+class TestResolveDesktopUser:
+    """Tests for _resolve_desktop_user."""
+
+    def test_prefers_steam_enforcer_desktop_user(self) -> None:
+        """The systemd unit's explicit var wins over SUDO_USER/USER."""
+        with patch.dict(
+            os.environ,
+            {
+                "STEAM_ENFORCER_DESKTOP_USER": "kuhy",
+                "SUDO_USER": "someone_else",
+                "USER": "root",
+            },
+        ):
+            assert _resolve_desktop_user() == "kuhy"
+
+    def test_falls_back_to_sudo_user(self) -> None:
+        """Interactive `sudo` invocations have no explicit var set."""
+        env = os.environ.copy()
+        env.pop("STEAM_ENFORCER_DESKTOP_USER", None)
+        env["SUDO_USER"] = "alice"
+        env["USER"] = "root"
+        with patch.dict(os.environ, env, clear=True):
+            assert _resolve_desktop_user() == "alice"
+
+    def test_falls_back_to_user(self) -> None:
+        """A direct, non-sudo invocation has neither var set."""
+        env = os.environ.copy()
+        env.pop("STEAM_ENFORCER_DESKTOP_USER", None)
+        env.pop("SUDO_USER", None)
+        env["USER"] = "kuhy"
+        with patch.dict(os.environ, env, clear=True):
+            assert _resolve_desktop_user() == "kuhy"
+
+    def test_returns_none_when_nothing_set(self) -> None:
+        env = os.environ.copy()
+        env.pop("STEAM_ENFORCER_DESKTOP_USER", None)
+        env.pop("SUDO_USER", None)
+        env.pop("USER", None)
+        with patch.dict(os.environ, env, clear=True):
+            assert _resolve_desktop_user() is None
+
+
 class TestSteamIsInstalled:
     """Tests for steam_is_installed."""