get-computed-style-in-callback.html

<!doctype HTML>
<script src="../resources/testharness.js"></script>
<script src="../resources/testharnessreport.js"></script>

<div id="sizeChangingDiv"></div>

<script>
test(() => {
  document.body.offsetTop;
  const resizeObserver = new ResizeObserver(() => assert_equals(getComputedStyle(document.body).height, "123px"));
  resizeObserver.observe(sizeChangingDiv);
  sizeChangingDiv.style.height = "123px";
}, "getComputedStyle gets up-to-date style when called from within a ResizeObserver callback");
</script>

chromium/third_party/blink/web_tests/resize-observer/get-computed-style-in-callback.html