如何让dropdown on open移动下面的dropdown

<select id="dropdown1">
  <option value="option1">Option 1</option>
  <option value="option2">Option 2</option>
  <option value="option3">Option 3</option>
</select>

<select id="dropdown2">
  <option value="option1">Option 1</option>
  <option value="option2">Option 2</option>
  <option value="option3">Option 3</option>
</select>

#dropdown1 {
  position: relative;
}

#dropdown2 {
  position: relative;
  top: 30px; /* 初始位置 */
}

document.getElementById("dropdown1").addEventListener("click", function() {
  var dropdown2 = document.getElementById("dropdown2");
  dropdown2.style.top = (dropdown2.offsetHeight + 30) + "px"; /* 移动距离为下方dropdown的高度加上偏移量 */
});