Why is my bootstrap navbar not displaying inline?

This is because in Bootstrap the css for li is this:

.nav>li {
  position: relative;
  display: block;
}

But for me the menu seems to appear correctly. See snippet, but only on a full page. On mobile devices it will behave like a block element since the screen doesn't have room to show them inline.

EDIT: Updated the snippit to work as requested (see comments below this answer)

.navbar .container-fluid>.navbar-header {
  float: left;
  margin-right: 10px;
}
.navbar .navbar-nav {
  float: left;
  margin: 5px;
}
.nav>li {
  float: left;
}
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css" rel="stylesheet">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js"></script>
<nav class="navbar navbar-inverse">
  <div class="container-fluid">

    <div class="navbar-header">
      <a class="navbar-brand">Rebar</a>
    </div>

    <div style="display: inline-block;">
      <ul class="nav navbar-nav">
        <li><a href="#">Link</a>
        </li>
        <li><a href="#">Link</a>
        </li>
        <li><a href="#">Link</a>
        </li>
      </ul>
    </div>
  </div>
</nav>

Currently on Bootstrap 4. The flex layout seems to be columnar by default in my ecosystem as well.

The most direct override for this use case:

.navbar-nav { 
    flex-direction: row;
}