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<!DOCTYPE html>
<title>Layout Instability: layout shift when content expanded above the viewport</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="resources/test-adapter.js"></script>
<script src="resources/util.js"></script>
<style>
body {
margin: 0;
/* To avoid browser automatic scroll offset adjustment for the expansion. */
overflow-anchor: none;
}
</style>
<div id="expander" style="height: 50vh"></div>
<div id="shifted" style="width: 300px; height: 300vh; background: blue"></div>
<script>
promise_test(async () => {
const watcher = new ScoreWatcher;
// Wait for the initial render to complete.
await waitForAnimationFrames(2);
const viewHeight = document.documentElement.clientHeight;
window.scrollTo(0, viewHeight);
await waitForAnimationFrames(2);
// Expander expands to push |shifted| down.
expander.style.height = '150vh';
const expectedScore1 = computeExpectedScore(300 * viewHeight, viewHeight);
// Observer fires after the frame is painted.
cls_expect(watcher, {score: 0});
await watcher.promise;
cls_expect(watcher, {score: expectedScore1});
// Expander expands to push |shifted| out of viewport.
expander.style.height = '200vh';
const expectedScore2 = expectedScore1 +
computeExpectedScore(0.5 * 300 * viewHeight, 0.5 * viewHeight);
await watcher.promise;
cls_expect(watcher, {score: expectedScore2});
}, "Layout shift when content expanded above the viewport");
</script>
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