如何在粘滞导航栏中设置平滑滚动？

<nav id="navbar">
  <!-- 导航栏内容 -->
</nav>

#navbar {
  position: fixed;
  height: 60px;
  transition: background-color 0.3s, color 0.3s;
}

#navbar.sticky {
  background-color: #fff; /* 导航栏背景色变化 */
  color: #333; /* 导航栏字体颜色变化 */
}

window.addEventListener('scroll', function() {
  var navbar = document.getElementById('navbar');
  var threshold = 200; // 滚动条位置的阈值

  if (window.pageYOffset > threshold) {
    navbar.classList.add('sticky');
  } else {
    navbar.classList.remove('sticky');
  }
});