<!doctype html>
<html>
<head>
  <link rel="help" href="https://www.w3.org/TR/css-position-3/#def-cb">
  <link rel="help" href="https://crbug.com/970171">
  <script src="/resources/testharness.js"></script>
  <script src="/resources/testharnessreport.js"></script>
<style>
  #container {
    position: relative;
    border: 1px solid black;
  }
  .narrow {
    width: 200px;
    height: 300px;
  }
  .wide {
    width: 300px;
    height: 200px;
  }
  #target {
    background: green;
    position: absolute;
    width: 50px;
    height: 30px;
    left: 50%;
    top: 50%;
    margin-left: -25px;
    margin-top: -15px;
  }
</style>
</head>
<body>
<div id="container" class="narrow">
  <button id="target"></button>
</div>
<script>
  test( t => {
    let container = document.querySelector("#container");
    let target = document.querySelector("#target");
    document.body.offsetTop;
    // start off narrow
    let narrow_left = target.offsetLeft;
    let narrow_top = target.offsetTop;
    // make it wide
    container.classList.toggle("narrow");
    container.classList.toggle("wide");
    document.body.offsetTop;
    // make it narrow again
    container.classList.toggle("narrow");
    container.classList.toggle("wide");
    document.body.offsetTop;
    assert_equals(target.offsetLeft, narrow_left);
    assert_equals(target.offsetTop, narrow_top);
  }, "absolute positioned button with percentage top gets positioned");
</script>
</body>
</html>