resize-observer/scrollbars-2.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>
  #scrollContainer {
    width: 100px;
    height: 100px;
    /* Should be bigger than any reasonable scrollbar */
    padding: 30px;
    border: 10px solid blue;
    overflow: scroll;
    background: #818182;
  }

</style>
<div id="scrollContainer"></div>
<script>
  promise_test(async function() {
    let count = 0;

    const scrollContainer = document.getElementById('scrollContainer');
    // 20 to account for the border.
    const scrollbarSize = scrollContainer.offsetWidth - scrollContainer.clientWidth - 20;
    let size = await new Promise(resolve => {
      const observer = new ResizeObserver(entries => {
        resolve(entries[0].contentBoxSize[0]);
      });
      observer.observe(scrollContainer);
    });

    assert_equals(size.inlineSize, 100 - scrollbarSize);
    assert_equals(size.blockSize, 100 - scrollbarSize);
  });
</script>