ㄧ.下拉式選單的變化

<div class="component" id="demo">
  <div class="btn-group dropup" style="position: absolute; bottom: 30%;">
    <a href="#" class="btn btn-success">Dropdown</a>
    <button class="btn btn-success dropdown-toggle dropdown-toggle-split" data-toggle="dropdown" data-offset="15">
      <span class="sr-only">Dropdown</span> 
    </button>
    <div class="dropdown-menu dropdown-menu-left">
      <div class="dropdown-header">Web language</div>
      <a href="#" class="dropdown-item">HTML</a>
      <a href="#" class="dropdown-item">CSS</a>
      <a href="#" class="dropdown-item">JavaScript</a>
      <div class="dropdown-divider"></div>
      <a href="#" class="dropdown-item">More</a>
    </div>
  </div>
</div>


.demo.component{
  padding: 50px;
}

重點：

1.在.btn-group這一層添加.dropup以及style="position: absolute; bottom: 30%;"
2.在.dropdown-menu這層添加.dropdown-menu-left， 左空白對齊
3.data-offset="15"，如果是.dropdown-menu-left則是往左推移

codepen
參考：bootstrap4