当移动页面在顶部时，Div垂直居中，但当我们向下滚动页面时，出现错误

.parent {
  display: flex;
  align-items: center;
  justify-content: center;
}

.parent {
  display: grid;
  place-items: center;
}

window.addEventListener('scroll', function() {
  var div = document.getElementById('myDiv');
  var scrollTop = window.pageYOffset || document.documentElement.scrollTop;
  if (scrollTop > threshold) {
    div.style.top = '50%';
    div.style.transform = 'translateY(-50%)';
  } else {
    div.style.top = '0';
    div.style.transform = 'translateY(0)';
  }
});