title: First-child full-width in Flexbox
body: <p>How can I set the first-child of flexbox in full-width and all of the other childs set to <code>flex:1</code>(for split space)?</p>
  <p>Like this:</p>
  <p><a href="https://i.stack.imgur.com/bjGWl.png" rel="noreferrer"><img src="https://i.stack.imgur.com/bjGWl.png" alt="enter image description here" /></a></p>