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

<!doctype html>
<title>Container Queries - Scrollbars do not cause transitions</title>
<link rel="help" href="https://drafts.csswg.org/css-transitions/#starting">
<link rel="help" href="https://drafts.csswg.org/css-conditional-5/#animated-containers">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="support/cq-testcommon.js"></script>
<style>
  #scrollable {
    overflow: auto;
    width: 100px;
    height: 100px;
  }
  #container {
    container-type: inline-size;
  }
  #target {
    background-color: black;
  }

  /* Matches with or without a scrollbar: */
  @container (max-width: 100px) {
    #target {
      background-color: blue;
    }
  }

  /* Matches only when there's a scrollbar: */
  @container (max-width: 99px) {
    #target {
      background-color: green;
      font-size: 10px;
      transition: 2s steps(2, start) background-color;
    }
  }
</style>
<div id=scrollable>
  <div id=container>
    <div id=target>
      Foo bar foo bar foo
      Foo bar foo bar foo
      Foo bar foo bar foo
      Foo bar foo bar foo
      Foo bar foo bar foo
    </div>
  </div>
</div>
<script>
  setup(() => assert_implements_size_container_queries());

  test(() => {
    // Whether or not a scrollbar appeared is out of scope for this test.
    // The only thing we care about is that no transition was triggered.
    // Therefore we allow both 'green' and 'blue', but not any other values.
    let has_scrollbar = target.offsetWidth < 100;
    let expected = has_scrollbar ? 'rgb(0, 128, 0)' : 'rgb(0, 0, 255)';
    assert_equals(getComputedStyle(target).backgroundColor, expected);
  }, 'Scrollbars do not cause a transition of background-color');
</script>