title: react-tranisition-group use flex-box for react-router transition
body: <p>I'm following the tutorial to add transition for <code>react-router</code> page use <code>react-transition-group</code>. 
  My page content all wrapped into the container, and I use flex-box to center the container.</p>
  
  <p>However, it leads to transition cannot apply properly. </p>
  
  <p>I expect page transition should be in the container, either from left to right or right to left.
  I'm trying to use <code>position:relative</code> in container and <code>position:absolute</code> for transition class, seems doesn't work at all</p>
  
  <p><a href="https://codesandbox.io/s/transition-in-container-0z17n?fontsize=14&amp;hidenavigation=1&amp;theme=dark" rel="nofollow noreferrer">code is here</a></p>
  
  <p>Any ideas are welcome!</p>
  
  <p>cheers.</p>