页面中间的溢出div在主体开始滚动之前完成滚动

div {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  z-index: 999;
  /* 其他样式 */
}

window.addEventListener('scroll', function() {
  var div = document.querySelector('div');
  var scrollTop = window.pageYOffset || document.documentElement.scrollTop;
  var scrollPosition = scrollTop + window.innerHeight;
  
  if (scrollPosition >= div.offsetTop) {
    div.style.top = scrollPosition - div.offsetTop + 'px';
  }
});