If there are so much text in a div (e.g. conversationList or conversation), I want that I can scroll down to read more. My problem is that div's wont resize themself. It's like they are all fixed. Please see my JSFiddle.
HTML
<body>
<header>
Text
</header>
<main>
<nav>
<a href="#">LINK 1</a><br>
<a href="#">LINK 2</a><br>
<a href="#">LINK 3</a><br>
<a href="#">LINK 4</a><br>
</nav>
<section id="pageTitle">
Title
</section>
<section id="conversationList">
List of Items
</section>
<section id="conversation">
Conversation
</section>
</main>
</body>
As was said in the comments, if you remove the position: fixed from your css for the "main" element it will let the section expand with its content.
Edit: Only remove it from main, not from header and you'll still have a fixed header.