css/css-overflow/overflow-codependent-scrollbars.html

<!DOCTYPE html>
<link rel="author" href="mailto:atotic@google.com">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<link rel="help" href="https://www.w3.org/TR/css-overflow-3/#scrollable">
<meta name="assert" content="What happens when appearance of one scrollbar triggers the other one?">
<style>
.container {
  width: 100px;
  height: 100px;
  overflow: auto;
  background: #DDD;
  --too-big: 120px;
  --slightly-smaller: 95px;
}
.target {
  width: 120px;
  height: 95px;
  background: rgba(0,255,0,0.3);
}
</style>
<!--  -->
<div class="container">
  <div class="target" style="width:var(--too-big);height:var(--slightly-smaller)" ></div>
</div>
<div class="container">
  <div class="target" style="height:var(--too-big);width:var(--slightly-smaller)" ></div>
</div>

<script>
test(() => {
  Array.from(document.querySelectorAll(".container")).forEach( el => {
    let verticalScrollbar = el.offsetWidth - el.clientWidth;
    let horizontalScrollbar = el.offsetHeight - el.clientHeight;
    assert_equals(verticalScrollbar, horizontalScrollbar, "both scrollbars are visible.");
  });
}, 'appearance of one scrollbar caused the other scrollbar to appear.');
</script>