mozilla-release/testing/web-platform/tests/fullscreen/rendering/fullscreen-root-block-scroll-manual.html

<!DOCTYPE html>
<style>
  body {
    height: 10000px;
    background: grey;
  }
</style>
<title>fullscreen root block scrolling</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/resources/testdriver.js"></script>
<script src="/resources/testdriver-vendor.js"></script>
<div>This page tests that entering fullscreen doesn't adjust the scroll offset</div>
<script>
async_test(t => {
  document.scrollingElement.scrollTop = 300;
  document.onfullscreenchange = t.step_func_done(() => {
    assert_equals(document.fullscreenElement, document.documentElement);
    assert_not_equals(document.scrollingElement.scrollTop, 0);
  });
  document.documentElement.addEventListener('click', e => {
      document.documentElement.requestFullscreen();
  }, {once: true});
  test_driver.click(document.documentElement);
});
</script>