css/css-position/position-absolute-container-dynamic.html

<!DOCTYPE html>
<title>CSS Position Absolute: dynamic changes to containing block width</title>
<link rel="author" href="mailto:atotic@google.com">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<link rel="help" href="https://www.w3.org/TR/css-position-3/#abs-non-replaced-width">
<meta name="assert" content="abspos descendant responds to containing block size change">
<style>

#container {
  position: relative;
  width: 50px;
  height: 100px;
  background: red;
}
#target {
  position: absolute;
  left: 0;
  width: 100%;
  height: 100%;
  background: green;
}
</style>
<div id="container">
  <div style="display:flex;">
    <div id="target"></div>
  </div>
</div>
<script>
document.body.offsetTop;
test(() => {
  document.getElementById("container").style.width = "100px";
  assert_equals(document.querySelector("#target").offsetWidth, 100);
}, '#target used container width when computing size');
</script>