testing/web-platform/tests/layout-instability/shift-into-viewport.html


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<!DOCTYPE html>
<title>Layout Instability: shift into viewport</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<style>

body { margin: 0; }
#j { position: absolute; width: 600px; height: 200px; top: 100%; background: blue; }

</style>
<div id='j'></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);

  // Move div partially into viewport.
  document.querySelector("#j").style.top =
      document.documentElement.clientHeight - 200 + "px";

  // The element moves from outside the viewport to within the viewport, which
  // should generate a shift.
  // (width 600) * (height 0 + move 200)
  const expectedScore = computeExpectedScore(600 * 200, 200);

  await watcher.promise;
  assert_equals(watcher.score, expectedScore);
}, "Shift into viewport.");

</script>