阻止select离开div：

div {
  height: 200px;
  overflow: auto;
}

<div id="container">
  <select id="select">
    <option value="option1">Option 1</option>
    <option value="option2">Option 2</option>
    <option value="option3">Option 3</option>
    <!-- 更多选项... -->
  </select>
</div>

<script>
  var container = document.getElementById('container');
  var select = document.getElementById('select');

  select.addEventListener('click', function() {
    var selectRect = select.getBoundingClientRect();
    var containerRect = container.getBoundingClientRect();

    if (selectRect.bottom > containerRect.bottom) {
      select.style.top = (containerRect.bottom - selectRect.height) + 'px';
    }
  });
</script>