如何实现饿了么点餐左侧菜单的滑动和固定效果？

<div class="container">
  <div class="left-menu">
    <ul>
      <li>菜单项1</li>
      <li>菜单项2</li>
      <li>菜单项3</li>
      <!-- ...更多菜单项 -->
    </ul>
  </div>
  <div class="right-content">
    <!-- 右侧内容区域 -->
    <p>这是一个很长的内容区域，用于测试菜单固定效果。</p>
    <p>...</p>
  </div>
</div>

.container {
  display: flex;
}

.left-menu {
  width: 100px;
  background-color: #f0f0f0;
  overflow-y: auto; /* 允许垂直滚动 */
}

.left-menu ul {
  list-style: none;
  padding: 0;
  margin: 0;
}

.left-menu li {
  padding: 10px;
  border-bottom: 1px solid #ddd;
}

.right-content {
  flex-grow: 1;
  padding: 20px;
}

.fixed-menu {
  position: fixed;
  top: 0;
  width: 100px;
  background-color: #f0f0f0;
  z-index: 100; /* 确保菜单位于其他内容之上 */
}

window.addEventListener('scroll', function() {
  const leftMenu = document.querySelector('.left-menu');
  const menuRect = leftMenu.getBoundingClientRect();

  if (menuRect.top <= 0) {
    leftMenu.classList.add('fixed-menu');
  } else {
    leftMenu.classList.remove('fixed-menu');
  }
});