查看bootstrap-select examples不同的下拉菜单之间没有重叠。但是，在我的示例中，所有下拉菜单都重叠（Demo）。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="description" content="">
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<link rel="shortcut icon" href="../../docs-assets/ico/favicon.png">
<title>TEST</title>
<link rel="stylesheet" type="text/css" href="//netdna.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css" >
<link rel="stylesheet" type="text/css"href="//cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.6.2/css/bootstrap-select.min.css">
<link rel="stylesheet" href="/static/web.css">
<link href="http://getbootstrap.com/examples/non-responsive/non-responsive.css" rel="stylesheet">
</head>
<body>
<!-- Fixed navbar -->
<div class="navbar navbar-default navbar-fixed-top" role="navigation">
<div class="container">
<div class="navbar-header">
<button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="/search/">TEST</a>
</div>
<div class="navbar-collapse collapse" id="navbar-collapsible">
<ul class="nav navbar-nav navbar-left">
<li class="active"><a href="/search/">Home</a></li>
</ul>
<ul class="nav navbar-nav navbar-right">
<form class="navbar-form navbar-right" method="get" action="/snearch/" role="form">
<div class="form-group">
<input class="form-control" type="text" name="sid" placeholder="e.g. hello">
<button class="btn btn-default"><span class="glyphicon glyphicon-search" type="submit"></span>
</button>
</div>
</form>
</ul>
</div><!--/.nav-collapse -->
</div>
</div>
<div class="container">
<form class="form-horizontal" action="/snps/" method="POST" role="form">
<div style="display:none;"><input id="csrf_token" name="csrf_token" type="hidden" value="1413008961.84##d5f5a444ea1ef9d7ed3268e2e554bde2d2cc21e3"></div>
<div class="input-group">
<span class="input-group-addon">Test1:</span>
<select class="selectpicker form-control" data-live-search="true" id="test_select" name="test_name">
<option value="">--- Select One ---</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
</select>
</div>
<BR/>
<div class="input-group">
<span class="input-group-addon">Test2:</span>
<select class="selectpicker form-control" data-live-search="true" id="test2_select" name="test2_name">
<option value="">--- Select One ---</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
</select>
</div>
<BR/>
<div class="input-group">
<span class="input-group-addon">Test3:</span>
<select class="selectpicker form-control" data-live-search="true" id="test3_select" name="test3_name">
<option value="">--- Select One ---</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
</select>
</div>
<BR/>
<div class="input-group">
<span class="input-group-addon">test4:</span>
<select class="selectpicker show-menu-arrow form-control" data-live-search="true" id="test4_select" multiple data-max-options="2" name="test4_names">
<option value="">--- Select One ---</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
<option value="">Test</option>
</select>
</div>
<BR/>
<div class="input-group">
<span class="input-group-addon">Start position:</span>
<input class="form-control" id="start_pos" name="start_pos" placeholder="e.g. 200" type="number" value="">
<span class="input-group-addon">End position:</span>
<input class="form-control" id="end_pos" name="end_pos" placeholder="e.g. 1000" type="number" value="">
</div>
<BR/>
<div class="input-group">
<span class="input-group-btn">
<button class="btn btn-default" type="submit">Search</button>
</span>
</div>
</form>
</div> <!-- /container -->
<script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1.10.1/jquery.min.js"></script>
<script type="text/javascript" src="//netdna.bootstrapcdn.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.6.2/js/bootstrap-select.min.js"></script>
<script>
$('.selectpicker').selectpicker();
</script>
</body>
</html>
也许，我可以在任何地方使用 z-index ，但在bootstrap-select examples中他们不使用它。如何避免使用 z-index ？