向上弹出式菜单

给父元素添加 .dropup 类就能使触发的下拉菜单朝上方打开。

<div class="btn-group dropup">
  <button type="button" class="btn btn-default">Dropup</button>
  <button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown" aria-expanded="false">
    <span class="caret"></span>
    <span class="sr-only">Toggle Dropdown</span>
  </button>
  <ul class="dropdown-menu" role="menu">
    Dropdown menu links 
  </ul>
</div>