如何在滚动时将表单元素固定在显示屏的底部？

.fixed-bottom {
  position: fixed;
  bottom: 0;
}

<form class="fixed-bottom">
  <!-- 表单元素的内容 -->
</form>

window.addEventListener('scroll', function() {
  var form = document.querySelector('.fixed-bottom');
  var scrollHeight = document.documentElement.scrollHeight;
  var clientHeight = document.documentElement.clientHeight;
  var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
  
  if (scrollHeight - scrollTop - clientHeight <= 0) {
    form.classList.add('fixed-bottom');
  } else {
    form.classList.remove('fixed-bottom');
  }
});