title: Github-like Issue creation Write/Preview Layout
body: <p>For the life of mine I can't figure out how to achieve the following.</p>
  <p>I need to create a form with support for preview of the typed content just like the Github new issue creation form as below</p>
  <p><a href="https://i.stack.imgur.com/i6zud.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/i6zud.png" alt="enter image description here" /></a>
  What I want to replicate is the curved border around the Write/Preview pair with the Write and Preview demarcated just as shown in this image.</p>
  <p>How can this be achieved in CSS?</p>
  <p>Thanks</p>