一個btn-group里面不能同時兩個按鈕屬性都包含data-toggle="dropdown", 否則任何一個按鈕都可以點開菜單dropdown-menu
正確的按鈕組下拉菜單代碼如下：
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap 按鈕組下拉菜單</title>
<link rel="stylesheet" href="bootstrap-3.3.7-dist\css\bootstrap.css">
<script src="jquery-3.4.1.min.js"></script>
<script src="bootstrap-3.3.7-dist\js\bootstrap.min.js"></script>
</head>
<body>
<div class="btn-group">
<Button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown">
Home
<span class="caret"></span>
</Button>
<ul class="dropdown-menu" >
<li role="presentation"><a href="#">action1 </a></li>
<li role="presentation"><a href="#">action2 </a></li>
<li role="presentation"><a href="#">action3 </a></li>
</ul>
</div>
</body>
</html>