css/css-conditional/container-queries/auto-scrollbars.html

<!doctype html>
<title>CSS Container Queries Test: scrollbar stability for @container queries and overflow:auto</title>
<link rel="help" href="https://drafts.csswg.org/css-conditional-5/#container-queries">
<link rel="help" href="https://drafts.csswg.org/css-overflow-3/#scrollbar-layout">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="support/cq-testcommon.js"></script>
<style>
  #scroller {
    height: 100px;
    width: 100px;
    overflow-y: auto;
  }
  #container {
    container-type: inline-size;
  }
  #inner {
    height: 100px;
    border-bottom: 1px solid red;
  }
  @container (max-width: 99px) {
    #inner {
      height: 50px;
    }
  }
</style>
<div id="precondition" style="width:100px;height:100px;overflow:scroll"></div>
<div id="scroller">
  <div id="container">
    <div id="inner"></div>
  </div>
</div>
<script>
  setup(() => {
    assert_implements_size_container_queries();
  });

  const has_overlay_scrollbars = precondition.clientWidth === 100;
  let initialScrollerWidth = scroller.clientWidth;
  test(() => {
    if (has_overlay_scrollbars) return;
    assert_equals(getComputedStyle(inner).height, "50px",
                  "Layout with a scrollbar means the container query applies");
  }, "Initial layout - expecting a scrollbar without overflowing content instead of overflowing content without a scrollbar");

  test(() => {
    if (has_overlay_scrollbars) return;
    inner.style.borderBottomWidth = "2px";
    assert_equals(scroller.clientWidth, initialScrollerWidth, "Scrollbar visibility is consistent after reflow.");
    assert_equals(getComputedStyle(inner).height, "50px",
                  "Layout with a scrollbar means the container query applies");
  }, "Same result after a reflow");

</script>