Stacked collapsible navbar in bootstrap 4

Question

I would like to create a collapsible navbar. Everything works, but the nav items aren't stacked under each other like it should be. They are horizontally next to each other.

Code:

<nav class="navbar navbar-light bg-faded" role="navigation">
    <button class="navbar-toggler hidden-sm-up" type="button" data-toggle="collapse" data-target="#collapsing-navbar">
        &#9776;
    </button>

    <div class="collapse navbar-toggleable-xs" id="collapsing-navbar">   
        <a class="navbar-brand" href="/">Logo</a>

        <ul class="nav navbar-nav">
            <li class="nav-item">
                <a href="/messages" class="nav-link">Messages</a>
            </li>

            <li class="nav-item">
                <a href="/logout" class="nav-link">Logout</a>
            </li>
        </ul>
    </div>
</nav>

Screenshot:

Answer 1

Bootstrap 4 Alpha 6 (since alpha 6 version stacked menu is built in - no need to use extra CSS):

<nav class="navbar navbar-toggleable-md navbar-light bg-faded">
  <button class="navbar-toggler navbar-toggler-right" 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>
  <a class="navbar-brand" href="#">Navbar</a>

  <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">
        <a class="nav-link disabled" href="#">Disabled</a>
      </li>
    </ul>
    <form class="form-inline my-2 my-lg-0">
      <input class="form-control mr-sm-2" type="text" placeholder="Search">
      <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
    </form>
  </div>
</nav>

CODEPEN