"jumping" content in navbar in bootstrap - css

Question

Edit2:
Okay, the problem was, that I used a <div class="row"> around the <div class="topimage"> tag. Strange. Could someone explain why it is?
I'm pretty new to bootstrap and created a web page for learning purposes.
Everything is working pretty well so far, except the navbar.
This is my nav code:
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>
<div id="headertest ">
<nav class="navbar navbar-default navbar-fixed-top" role="navigation">
<div class="container">
<div class="row">
<div class="col-sm-4 col-sm-offset-1">
<form class="navbar-form navbar-left">
<button class="btn btn-default">ENTDECKEN</button>
<button class="btn btn-default">FINDEN</button>
<button class="btn btn-default">PLANEN</button>
<p class="font">
<b> Zurück zur Hafenseite |</b> <a style="text-decoration: none">Salsbury & Mysteriöses Stonehenge</a>
</p>
</form>
</div>
<div class="col-sm-2 ">
<div class="navbar-form">
<img src="../assets/aida.png" class="logo ">
</div>
</div>
<div class="col-sm-4">
<div class="navbar-form navbar-right">
<form action="#" style>
<div class="dropdown">
<img src="../assets/lupe.png" class="icons" />
<img src="../assets/telefon.png" class="icons" />
<button class="btn btn-default dropdown-toggle" type="button" id="dropdownMenu1" data-toggle="dropdown" aria-haspopup="true" aria-expanded="true">
Dropdown
<span class="caret"></span>
</button>
<ul class="dropdown-menu" aria-labelledby="dropdownMenu1">
<li>Erste Aktion</li>
<li>Zweite Aktion</li>
<li>Noch eine Aktion</li>
<li role="separator" class="divider"></li>
<li>Ein Link</li>
</ul>
</div>
</form>
</div>
</div>
</div>
</div>
</nav>
</div>
The problem is, if I use the navbar-fixed-top class, the content inside the navbar is jumping while I scroll.
Like the top margin is vanishing while I'm scrolling down and if I Scroll up, the margin (or whatever the problem is) just magically appears back.
Normal and scrolling up:
Scrolling down:
I hope you can see what I mean.
Does anybody know what I'm doing wrong?
(my css classes shouldn't be the problem. If I delete them, the content still keep "jumping")
Thank you
Edit:
I have an image as the first item (after the navbar) that seems to cause the problems.
If I delete it, the navbar is working as expected.
Css:
.topimage{
height: 600px;
width: 100%;
align-self: center;
background-image: url("./assets/testbild3.jpg");
background-repeat: no-repeat;
background-size: cover;
z-index: -1;
}
and how I implemented it:
<div class="row">
<headertest></headertest><!-- navbar component -->
</div>
<div class="row ">
<div class="topimage">
</div>
</div>
The fiddler link: (where it works without "jumping" navbar content)
https://jsfiddle.net/Biberium/g52cd6v8/