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diff --git a/testing/web-platform/tests/layout-instability/inline-flow-shift-one-line.html b/testing/web-platform/tests/layout-instability/inline-flow-shift-one-line.html
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index 0000000000..0c477e0e98
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+++ b/testing/web-platform/tests/layout-instability/inline-flow-shift-one-line.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<title>Layout Instability: inline/text movement is detected</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/util.js"></script>
+<div style="width: 200px; font-size: 20px; line-height: 25px">
+  1AAAAAAA<br>
+  2AAAAAAA<br>
+  3AAAAAAA<br>
+  <div id="inline-block" style="display: inline-block">4AAAAAAA</div><br>
+  <div id="shift" style="display: inline-block"></div>5AAAAAAA<br>
+  6AAAAAAA<br>
+  7AAAAAAA<br>
+</div>
+<script>
+
+promise_test(async () => {
+  const watcher = new ScoreWatcher;
+
+  // Wait for the initial render to complete.
+  await waitForAnimationFrames(2);
+
+  // Modify the width of |shift|.
+  document.querySelector("#shift").style.width = '50px';
+
+  // Only the line after |shift| is shifted right by 50px.
+  // The implementation may measure the real width of the shifted text
+  // or use the available width (i.e. width of the containing block).
+  // Also tolerate extra 10% error.
+  const text_width = document.querySelector("#inline-block").clientWidth;
+  const expectedScoreMin = computeExpectedScore((text_width + 50) * 20, 50) * 0.9;
+  const expectedScoreMax = computeExpectedScore(200 * 25, 50) * 1.1;
+
+  // Observer fires after the frame is painted.
+  assert_equals(watcher.score, 0);
+  await watcher.promise;
+  assert_between_exclusive(watcher.score, expectedScoreMin, expectedScoreMax);
+}, 'Inline flow movement.');
+
+</script>