关于 Bootstrap 4 - Navbar 顶部放置

Question

新手上路，请多包涵

我正在使用 Bootstrap 4。

我对 Navbar“Fixed top”类有疑问。 http://v4-alpha.getbootstrap.com/components/navbar/#placement

导航栏“固定顶部”类涵盖内容。

像下面这个demo，导航栏覆盖 <div class="jumbotron"> ，我想让导航栏不覆盖，怎么办？

我不知道如何防止导航栏覆盖内容，因为设备的大小是可见的。

 <!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
    <meta http-equiv="x-ua-compatible" content="ie=edge">

    <link href="https://cdn.bootcss.com/bootstrap/4.0.0-alpha.2/css/bootstrap.min.css" rel="stylesheet">
    <link href="https://cdn.bootcss.com/tether/1.3.2/css/tether.min.css" rel="stylesheet">
</head>
<body>
<nav class="navbar navbar-fixed-top navbar-light bg-faded">
    <a class="navbar-brand" href="#">Navbar</a>
    <ul class="nav navbar-nav">
        <li class="nav-item active">
            <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
        </li>
    </ul>
    <form class="form-inline pull-xs-right">
        <input class="form-control" type="text" placeholder="Search">
        <button class="btn btn-success-outline" type="submit">Search</button>
    </form>
</nav>
<div class="jumbotron">
    <h1 class="display-3">Hello, world!</h1>
    <p class="lead">This is a simple hero unit, a simple jumbotron-style component for calling extra attention to featured content or information.</p>
    <hr class="m-y-2">
    <p>It uses utility classes for typography and spacing to space content out within the larger container.</p>
    <p class="lead">
        <a class="btn btn-primary btn-lg" href="#" role="button">Learn more</a>
    </p>
</div>

<script src="https://cdn.bootcss.com/jquery/2.2.4/jquery.min.js"></script>
<script src="https://cdn.bootcss.com/tether/1.3.2/js/tether.min.js"></script>
<script src="https://cdn.bootcss.com/bootstrap/4.0.0-alpha.2/js/bootstrap.min.js"></script>
</body>
</html>