Clean up cancelled SSE wait tasks

tests/test_web.py

@@ -575,6 +575,39 @@ def test_render_stream_stops_when_shutdown_is_requested() -> None:
     asyncio.run(run())
 
 
+def test_render_stream_cleans_up_child_tasks_when_cancelled() -> None:
+    async def run() -> None:
+        queue = RefreshBroker().subscribe()
+        shutdown_event = asyncio.Event()
+
+        async def render() -> str:
+            return '<main id="morph">queue</main>'
+
+        stream = render_stream(
+            queue,
+            render,
+            render_on_connect=False,
+            shutdown_event=shutdown_event,
+        )
+        next_event = asyncio.create_task(anext(stream))
+        await asyncio.sleep(0)
+        next_event.cancel()
+
+        with pytest.raises(asyncio.CancelledError):
+            await next_event
+
+        await asyncio.sleep(0)
+
+        pending = tuple(
+            task
+            for task in asyncio.all_tasks()
+            if task is not asyncio.current_task() and not task.done()
+        )
+        assert pending == ()
+
+    asyncio.run(run())
+
+
 def test_render_dashboard_shows_dashboard_information_architecture(
     monkeypatch, tmp_path: Path
 ) -> None: