resize-observer/scrollbars.html

<!DOCTYPE html>
<title>ResizeObserver content-box size and scrollbars</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1733042">
<style>
  #outer {
    position: relative;
    width: 100px;
    height: 200px;
    overflow: auto;
    background: #818182;
  }

  #inner {
    position: absolute;
    top: 0;
    left: 0;
    width: 10px;
    height: 10px;
    background: #0a6fc0;
  }
</style>
<div id="outer">
  <div id="inner"></div>
</div>
<script>
  async function animationFrame() {
    return new Promise(r => requestAnimationFrame(r));
  }

  // This test is expected to fail with overlay scrollbars.
  promise_test(async function() {
    let count = 0;

    const outer = document.getElementById('outer');
    const inner = document.getElementById('inner');
    const observer = new ResizeObserver(entries => {
      count++;
    });

    observer.observe(outer);

    inner.style.top = '1000px';

    await animationFrame();
    await animationFrame();

    inner.style.top = 0;

    await animationFrame();
    await animationFrame();

    assert_equals(count, 2, "ResizeObserver should subtract scrollbar sizes from content-box rect");
  });
</script>