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Test Info:

<html class="reftest-wait">
<title>Layout changes on percentage-based scroll timeline</title>
<meta name="assert" content="Scroll timeline should properly handle
layout changes on percentage-based scroll offset">
<link rel="match" href="animation-ref.html">
<script src="/web-animations/testcommon.js"></script>
<script src="/common/reftest-wait.js"></script>
<style>
#box {
width: 100px;
height: 100px;
background-color: green;
}
#covered {
width: 100px;
height: 100px;
background-color: red;
color: red;
}
#scroller {
overflow: auto;
height: 100px;
width: 100px;
will-change: transform;
}
#contents {
height: 500px;
width: 100%;
}
#spacer {
height: 80px;
}
.invisible {
display: none;
}
</style>
<div id="box"></div>
<div id="covered">Scrolling Test</div>
<div id="scroller">
<div id="contents"></div>
<div id="spacer" class="invisible"></div>
</div>
<script>
const box = document.getElementById('box');
const effect = new KeyframeEffect(box,
[
{ transform: 'translateY(0)', opacity: 1 },
{ transform: 'translateY(200px)', opacity: 0 }
]
);
const scroller = document.getElementById('scroller');
const timeline = new ScrollTimeline({
source: scroller
});
const animation = new Animation(effect, timeline);
animation.play();
animation.ready.then(_ => {
// Moves the scroller to the end point (240px).
const maxScroll = scroller.scrollHeight - scroller.clientHeight;
scroller.scrollTop = maxScroll * 0.6;
// Makes sure that the animation runs on compositor with current scroll offset
waitForAnimationFrames(2).then(_ => {
// Adds 80px to scroll height which pushes scroll progress back to 50%.
const spacer = document.getElementById('spacer');
spacer.classList.remove('invisible');
// Makes sure that the change is propagated to the compositor.
waitForAnimationFrames(2).then(_ => {
takeScreenshot();
});
});
});
</script>