<!DOCTYPE html>
<style>
body {
  position: relative;
  width: 600px;
  height: 800px;
}

#container {
  background-color: red;
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  display: flex;
  flex-direction: row;
}

#top-bar {
  background-color: green;
  width: 20%;
}

#content {
  background-color: blue;
  flex-basis: 100%;
}
</style>


<div id="container">
  <div id="top-bar">
  Tests that percentage widths get resolved correctly when the flexbox is
  </div>
  <div id="content">
  absolutely positioned without an explicit width. You should see no red.
  </div>
</div>