wpt/css/css-transitions/transitioncancel-003.html at master · web-platform-tests/wpt · GitHub

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<!doctype html>
<html>
<head>
<meta charset=utf-8>
<title>CSS Transitions Test: Changing transition properties mid-transition</title>
<meta name="assert" content="When transition properties are changed mid-transition, the original transition completes and new transition parameters apply to subsequent changes">
<link rel="help" href="https://drafts.csswg.org/css-transitions/#starting">

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

</head>
<body>
<div id="log"></div>

<script>
promise_test(async t => {
  const div = addDiv(t, {
    style: 'transition: width 5000ms; width: 0px;'
  });

  // Flush initial state
  getComputedStyle(div).width;
  // Start first transition to 100px
  div.style.width = '100px';
  // Wait until transition starts running
  await new Promise(resolve => {
    div.addEventListener('transitionrun', resolve, { once: true });
  });

  // MID-TRANSITION CHANGE: Switch to 0s duration + 5000ms delay
  div.style.transition = 'width 0s 5000ms';
  assert_not_equals(
    getComputedStyle(div).width,
    '100px',
    'Width should not reach 100px yet'
  );

  // Trigger new width change mid-transition
  div.style.width = '200px';

  assert_not_equals(
    getComputedStyle(div).width,
    '200px',
    'Width should not change to 0px immediately'
  );

  await waitForTransitionEnd(div);

  // Final computed style
  assert_equals(
    getComputedStyle(div).width,
    '200px',
    'Final width should be 200px'
  );
}, 'Mid-transition transition changes affect subsequent transitions');
</script>

</body>
</html>