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<!DOCTYPE html>
<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#avoiding-cycles">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/web-animations/testcommon.js"></script>
<style>
#scroller {
overflow: hidden;
width: 100px;
height: 100px;
scroll-timeline: timeline;
}
#contents {
height: 200px;
}
@keyframes expand {
from { width: 100px; }
to { width: 200px; }
}
#element {
width: 0px;
animation: expand 10s linear;
animation-timeline: timeline;
}
/* Ensure stable expectations if feature is not supported */
@supports not (animation-timeline:foo) {
#element { animation-play-state: paused; }
}
</style>
<div id=scroller>
<div id=contents></div>
<div id=element></div>
</div>
<script>
promise_test(async (t) => {
// The scroll timeline is initially inactive until the first frame.
assert_equals(getComputedStyle(element).width, '0px');
await waitForNextFrame();
scroller.scrollTop = 50;
// Scrolling position should not yet be reflected in the animation,
// since the new scroll position has not yet been sampled.
assert_equals(getComputedStyle(element).width, '100px');
await waitForNextFrame();
assert_equals(getComputedStyle(element).width, '150px');
}, 'Scroll position is sampled once per frame');
</script>
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