<!DOCTYPE html>
<title>Layout Instability: shift offscreen with scroll anchoring and natural scroll</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<style>
#scroller {
overflow: scroll;
left: 20px;
top: 20px;
width: 200px;
height: 200px;
}
#spacer {
height: 3000px;
}
#ch {
position: relative;
background: yellow;
left: 10px;
top: 100px;
width: 150px;
height: 150px;
}
#offscreenElement {
width: 300px;
height: 300px;
background: lightblue;
}
#onscreenElement {
width: 300px;
height: 300px;
background: lightgreen;
}
</style>
<div id="scroller">
<div id="offscreenElement"></div>
<div id="spacer"></div>
<div id="onscreenElement"></div>
</div>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="resources/util.js"></script>
<script>
promise_test(async () => {
const watcher = new ScoreWatcher;
// Wait for the initial render to complete.
await waitForAnimationFrames(2);
// Scroll to show #onscreenElement.
scroller.scrollTop = 3250;
await waitForAnimationFrames(1);
// Resize #offscreernElement and scroll a bit.
// Visually, #onscreenElement will move by 20px.
offscreenElement.style.height = '250px';
scroller.scrollBy(0, 20);
await waitForAnimationFrames(3);
// There should be no reported layout shift, because to the user it looks
// like a natural scroll by 20px.
assert_equals(watcher.score, 0);
}, "Offscreen shift with scroll annchoring and natural scroll not counted.");
</script>
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