1 files changed, 52 insertions, 0 deletions
diff --git a/testing/web-platform/tests/layout-instability/composited-element-movement.html b/testing/web-platform/tests/layout-instability/composited-element-movement.html
new file mode 100644
index 0000000000..c990690335
--- /dev/null
+++ b/testing/web-platform/tests/layout-instability/composited-element-movement.html
@@ -0,0 +1,52 @@
+<!DOCTYPE html>
+<title>Layout Instability: element with compositing layer hint</title>
+<link rel="help" href="https://wicg.github.io/layout-instability/" />
+<style>
+
+#shift {
+  position: relative;
+  width: 500px;
+  height: 200px;
+  background: yellow;
+}
+#container {
+  position: absolute;
+  width: 350px;
+  height: 400px;
+  right: 50px;
+  top: 100px;
+  background: #ccc;
+}
+.promote { will-change: transform; }
+
+</style>
+<div id="container" class="promote">
+  <div id="space"></div>
+  <div id="shift" class="promote"></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);
+
+  // Induce a shift.
+  document.querySelector("#space").style = "height: 100px";
+
+  // #shift is 400x200 after viewport intersection with correct application of
+  // composited #container offset, and 100px lower after shifting, so impact
+  // region is 400 * (200 + 100).
+  const expectedScore = computeExpectedScore(400 * (200 + 100), 100);
+
+  // Observer fires after the frame is painted.
+  assert_equals(watcher.score, 0);
+  await watcher.promise;
+  assert_equals(watcher.score, expectedScore);
+}, "Element with compositing layer hint.");
+
+</script>