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Test Info:

<!DOCTYPE html>
<title>Layout Instability: simple block movement is detected</title>
<link rel="help" href="" />
<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>
#shifter { position: relative; width: 300px; height: 200px; background: blue; }
<div id="shifter"></div>
const watcher = new ScoreWatcher;
promise_test(async () => {
// Wait for the initial render to complete.
await waitForAnimationFrames(2);
// Modify the position of the div.
document.querySelector("#shifter").style = "top: 160px";
// An element of size (300 x 200) has shifted by 160px.
const expectedScore = computeExpectedScore(300 * (200 + 160), 160);
// Observer fires after the frame is painted.
cls_expect(watcher, {score: 0});
await watcher.promise;
cls_expect(watcher, {score: expectedScore});
}, 'Simple block movement.');