title: relative positioning of flexbox and fixed children
body: <p>i have some trubles with positioning elements while using flexbox.
  Here is my example close to real situation i have <a href="https://jsfiddle.net/383p73L1/4/" rel="nofollow noreferrer">https://jsfiddle.net/383p73L1/4/</a></p>
  
  <p>The problem is <code>display: fixed</code> element takes all content width instead of his parents width, that has <code>position: relative</code>. How is this possible and what i missed?</p>