full js solution (vibe coded)

src/open_cups/app.py

@@ -2,8 +2,10 @@
 
 import qrcode
 import streamlit as st
+import streamlit.components.v1 as components_v1
 from streamlit_autorefresh import st_autorefresh
 
+from open_cups import leave_server
 from open_cups.plots import show_room_statistics, show_status_history_chart
 from open_cups.state_provider import (
     ClientState,
@@ -223,6 +225,7 @@ def show_active_room_host(host_state: HostState) -> None:
     st.divider()
 
     show_open_questions(host_state)
+    _inject_leave_beacon(host_state._session_id)
 
 
 def show_active_room_client(client_state: ClientState) -> None:
@@ -254,12 +257,26 @@ def handle_question_submit() -> None:
         )
 
     show_open_questions(client_state)
+    _inject_leave_beacon(client_state._session_id)
+
+
+def _inject_leave_beacon(session_id: str) -> None:
+    components_v1.html(
+        f"""<script>
+    window.parent.addEventListener('pagehide', () =>
+        navigator.sendBeacon('http://localhost:8502', '{session_id}')
+    );
+    </script>""",
+        height=0,
+    )
 
 
 def run() -> None:
+
     st_autorefresh(interval=AUTOREFRESH_INTERVAL_MS, key="data_refresh")
 
     state_provider = StateProvider()
+    leave_server.start(state_provider.context.application_state)
     cleanup = state_provider.get_cleanup(USER_REMOVAL_TIMEOUT_SECONDS)
     cleanup.cleanup_all()
 

src/open_cups/leave_server.py

@@ -0,0 +1,41 @@
+import threading
+from http.server import BaseHTTPRequestHandler, HTTPServer
+
+from open_cups.application_state import ApplicationState
+
+_started = False
+
+
+def start(application_state: ApplicationState, port: int = 8502) -> None:
+    global _started
+    if _started:
+        return
+    _started = True
+
+    def handler_factory(*args):  # type: ignore[no-untyped-def]
+        return _Handler(*args, app_state=application_state)
+
+    thread = threading.Thread(
+        target=lambda: HTTPServer(("", port), handler_factory).serve_forever(),
+        daemon=True,
+    )
+    thread.start()
+
+
+class _Handler(BaseHTTPRequestHandler):
+    def __init__(self, *args, app_state: ApplicationState, **kwargs) -> None:  # type: ignore[no-untyped-def]
+        self._app_state = app_state
+        super().__init__(*args, **kwargs)
+
+    def do_POST(self) -> None:
+        length = int(self.headers.get("Content-Length", 0))
+        session_id = self.rfile.read(length).decode().strip()
+        room = self._app_state.get_session_room(session_id)
+        if room:
+            room.add_question("system", f"User {session_id[:8]}… left")
+        self.send_response(200)
+        self.send_header("Access-Control-Allow-Origin", "*")
+        self.end_headers()
+
+    def log_message(self, *args: object) -> None:
+        pass