Bootstrap .dropup类
也可以将菜单构建为下拉菜单，而不是下拉菜单。为此，只需将.dropup添加到父.btn-group容器中。
您可以尝试运行以下代码集菜单以进行下拉：
示例
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap Example</title>
<link href = "/bootstrap/css/bootstrap.min.css" rel = "stylesheet">
<script src = "/scripts/jquery.min.js"></script>
<script src = "/bootstrap/js/bootstrap.min.js"></script>
</head>
<body>
<p>Dropup</p>
<div class = "row" style = "margin-left:50px; margin-top:120px">
<div class = "btn-group dropup">
<button type = "button" class = "btn btn-primary dropdown-toggle" data-toggle = "dropdown">
Subject
<span class = "caret"></span>
</button>
<ul class = "dropdown-menu" role = "menu">
<li><a href = "#">Programming</a></li>
<li><a href = "#">Web Dev</a></li>
<li><a href = "#">Database</a></li>
<li><a href = "#">Networking</a></li>
</ul>
</div>
</div>
</body>
</html>