Function onVisibility

frontend/src/App.tsx:755–759

()

  useEffect(() => {
    const onVisibility = () => {
      if (document.hidden && tracking) {
        setMessage("你刚才离开了挑战页，期间不会记录。回到页面后继续。");
      }
    };
    document.addEventListener("visibilitychange", onVisibility);
    return () => document.removeEventListener("visibilitychange", onVisibility);
  }, [tracking]);