使固定侧边栏在到达页脚时开始滚动

<div class="content">
  <!-- 页面内容 -->
</div>

<div class="sidebar">
  <!-- 侧边栏内容 -->
</div>

.sidebar {
  position: fixed;
  top: 50px; /* 侧边栏距离页面顶部的距离 */
  width: 200px; /* 侧边栏宽度 */
}

.content {
  /* 页面内容样式 */
}

window.addEventListener('scroll', function() {
  var sidebar = document.querySelector('.sidebar');
  var content = document.querySelector('.content');
  var footer = document.querySelector('footer');
  
  var sidebarHeight = sidebar.offsetHeight;
  var contentHeight = content.offsetHeight;
  var footerOffset = footer.offsetTop;
  var windowHeight = window.innerHeight;
  var scrollTop = window.pageYOffset || document.documentElement.scrollTop;
  
  if (scrollTop + windowHeight >= footerOffset) {
    sidebar.style.position = 'absolute';
    sidebar.style.top = (contentHeight - sidebarHeight) + 'px';
  } else {
    sidebar.style.position = 'fixed';
    sidebar.style.top = '50px';
  }
});