test: synchronize multi-client resize assertion

tests/multi_client.rs

@@ -897,10 +897,17 @@ fn multi_client_disconnect_recalculates_to_next_smallest() {
 
     let size_with_three = read_pane_tty_size(&api_socket, &pane_id, Duration::from_secs(5));
 
+    drain_server_messages(&mut c100, Duration::from_millis(250));
+
     // Smallest client disconnects; effective size should increase to the next-smallest.
     send_client_detach(&mut c80);
     drop(c80);
 
+    assert!(
+        wait_for_frame(&mut c100, Duration::from_secs(2)),
+        "next-smallest client should receive resized-up frame"
+    );
+
     let deadline = Instant::now() + Duration::from_secs(8);
     let mut size_after_smallest_disconnect = None;
     while Instant::now() < deadline {