向下滚动时，可调整大小停止工作

<div id="fixed-element">这是一个固定的元素</div>
<div id="content">这是页面的内容</div>

#fixed-element {
  position: fixed;
  top: 50px;
  left: 50px;
  width: 200px;
  height: 100px;
  background-color: #f1f1f1;
}

#content {
  height: 2000px;
}

window.addEventListener('scroll', function() {
  var fixedElement = document.getElementById('fixed-element');
  var content = document.getElementById('content');
  
  var scrollTop = window.pageYOffset || document.documentElement.scrollTop;
  var contentTop = content.offsetTop;
  var fixedElementHeight = fixedElement.offsetHeight;
  
  if (scrollTop > contentTop && scrollTop < contentTop + content.offsetHeight - fixedElementHeight) {
    fixedElement.style.position = 'fixed';
    fixedElement.style.top = '50px';
  } else if (scrollTop >= contentTop + content.offsetHeight - fixedElementHeight) {
    fixedElement.style.position = 'absolute';
    fixedElement.style.top = (contentTop + content.offsetHeight - fixedElementHeight) + 'px';
  } else {
    fixedElement.style.position = 'static';
  }
});