title: CSS - Issue with fill-avaliable
body: <p>I have a sidebar on the left which is fixed (width: 400px) and content on the right which should contain the rest of the space. While using <code>fill-available</code>: the sidebar changes its width to 309px.</p>
  
  <p>Why doesn't <code>fill-available</code> work properly? Is there a possibility to set content in the remained space?</p>
  
  <p>Please find the codepen here: <a href="https://codepen.io/ullaakut/pen/eQxbvY" rel="nofollow noreferrer">https://codepen.io/ullaakut/pen/eQxbvY</a></p>
  
  <p>Thanks!</p>