fix: Chats doesn't sync for multi client users

server/src/websocket/handler.ts

@@ -705,7 +705,32 @@ async function handlePluginEvent(user: ConnectedUser, message: WsMessage, connec
   }
 }
 
-export async function handleWebSocketMessage(connectionId: string, message: WsMessage): Promise<void> {
+/**
+ * Tail of the in-flight message chain per connection.
+ *
+ * Messages from one client can arrive in the same tick (one TCP segment), but
+ * handlers like identify await async work. Without serialization a
+ * join_server can be evaluated while identify is still pending, get rejected
+ * as unauthenticated, and silently lose the room membership.
+ */
+const connectionMessageChains = new Map<string, Promise<void>>();
+
+export function handleWebSocketMessage(connectionId: string, message: WsMessage): Promise<void> {
+  const prior = connectionMessageChains.get(connectionId) ?? Promise.resolve();
+  const current = prior.then(() => processWebSocketMessage(connectionId, message));
+  const tail = current.catch(() => undefined);
+
+  connectionMessageChains.set(connectionId, tail);
+  void tail.then(() => {
+    if (connectionMessageChains.get(connectionId) === tail) {
+      connectionMessageChains.delete(connectionId);
+    }
+  });
+
+  return current;
+}
+
+async function processWebSocketMessage(connectionId: string, message: WsMessage): Promise<void> {
   const user = connectedUsers.get(connectionId);
 
   if (!user)