引导程序4下拉问题内导航栏固定顶部

我花了最后一天的时间试图弄清楚为什么视口更改时导航栏中的下拉菜单不起作用。

如果你通过电话(或开发人员工具(查看这个通用示例，并将方向旋转为横向，你会发现下拉列表上的链接是不可访问的。

我正试图将滚动添加到.溢出的下拉列表：滚动！重要而没有成功。我还试着把它放在上面，固定好位置，但它不起作用。

我想要的结果是，如果菜单在视图上展开，你可以滚动屏幕(甚至下拉菜单本身(来查看其他链接。

请注意，当我从导航中删除类"fixed top"时，它都会按预期工作，所以我想这是有问题的

代码：

<!doctype html>
<html>
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no" />
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">
</head>
<body style="padding-top: 70px;">
<nav class="navbar navbar-expand-lg navbar-light bg-light fixed-top">
<a class="navbar-brand" href="#">Navbar</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav mr-auto">
<li class="nav-item active">
<a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Link</a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Dropdown
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>          
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="#">Something else here</a>
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>          
</div>
</li>
</ul>
</div>
</nav>
<main>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
</main>
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js" integrity="sha384-ZMP7rVo3mIykV+2+9J3UJ46jBk0WLaUAdn689aCwoqbBJiSnjAK/l8WvCWPIPm49" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js" integrity="sha384-ChfqqxuZUCnJSK3+MXmPNIyE6ZbWh2IMqE241rYiqJxyMiZ6OW/JmZQ5stwEULTy" crossorigin="anonymous"></script>
</body>
</html>

var fixed = document.getElementById('fixed');
fixed.addEventListener('touchmove', function(e) {
e.preventDefault();
}, false);

.dropdown-menu {
max-height: 70vh;
overflow-y: auto;
}
@media (max-width: 990px) {
.navbar.fixed-top {
overflow-y: auto;
max-height: 100vh;
}
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.5/popper.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.3/js/bootstrap.min.js"></script>
<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.3/css/bootstrap.min.css" rel="stylesheet" />
<nav id="#fixed" class="navbar navbar-expand-lg navbar-light bg-light fixed-top">
<a class="navbar-brand" href="#">Navbar</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav mr-auto">
<li class="nav-item active">
<a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Link</a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Dropdown
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="#">Something else here</a>
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
</div>
</li>
</ul>
</div>
</nav>
<main>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
<p>Example</p>
</main>

使用css设置导航栏的最大高度：

@media (max-width: 990px){
.navbar.fixed-top{
overflow-y: auto;
max-height: 100vh;
}   
}

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