gecko-dev/testing/web-platform/tests/css/css-position/position-sticky-offset-overflow.html

<!DOCTYPE html>
<title>Sticky positioning can cause overflow but must be accessible.</title>
<link rel="help" href="https://www.w3.org/TR/css-position-3/#sticky-pos" />
<meta name="assert" content="This test checks that a sticky positioned element
can cause overflow but must still be accessible through scrolling" />

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

<style>
.container {
  overflow: scroll;
  width: 100px;
  height: 100px;
}

.box {
  background-color: green;
  height: 50px;
  width: 50px;
}

.sticky {
  position: sticky;
  top: 200px; /* Forces the sticky position element below the overflow. */
}
</style>

<div id="scroller1" class="container">
  <div class="sticky box"></div>
</div>

<div id="scroller2" class="container">
  <div class="sticky box"></div>
</div>

<script>
test(() => {
    var scroller = document.getElementById('scroller1');
    var sticky = scroller.querySelector('.sticky');

    var stickyOffset = sticky.offsetTop -
        scroller.scrollTop - scroller.offsetTop;
    assert_equals(stickyOffset, 200);
    assert_equals(scroller.scrollHeight, 250);
}, 'sticky position offset should be able to cause overflow');

test(() => {
    var scroller = document.getElementById('scroller2');
    var sticky = scroller.querySelector('.sticky');

    scroller.scrollTop = 150;
    var stickyOffset = sticky.offsetTop -
        scroller.scrollTop - scroller.offsetTop;
    assert_equals(stickyOffset, 50);
    // Scroll height should be unaffected.
    assert_equals(scroller.scrollHeight, 250);

}, 'sticky position offset in overflow should be accessible');
</script>