How to simulate 'position:fixed' behavior

Question

As it was already answered (How can I have a position: fixed; behaviour for a flexbox sized element?) absolutely/fixed positioned boxes are taken out of the normal flow of Flexbox-aligned elements. But how can I at least simulate position: fixed behavior of, say, width: 300px; height: 100vw element?

Here is a demo (http://codepen.io/anon/pen/ZGmmzR) of initial layout with sidebar on the left and content block on the right. I would like nav act like position: fixed element following user's scroll across the page. I know how to do it without Flexbox. In the first place I would consider pure CSS solution without using JavaScript. Thank you!

Answer 1

Edit:

A solution that somehow feels less hacky could be to make the container (.wrapper) as tall as the screen, and only add scrolling to the main <section> element:

.wrapper {
    display: flex;
    height: 100vh;
}

section {
    flex: 1 1 auto;
    overflow: auto;
}

http://codepen.io/Sphinxxxx/pen/WjwbEO

Original answer:

You could simply put everything inside <nav> in a wrapper (here: nav-wrapper), and then fix that wrapper:

...
.nav-wrapper {
    position: fixed;
    top: 0; left: 0;
}

<nav role="navigation">
    <div class="nav-wrapper">
        ...
    </div>
</nav>

http://codepen.io/anon/pen/PqXYGM