<!DOCTYPE html>
<html dir="rtl">
<title>Tests position fallback with rtl scroller and vertical-rl OOF</title>
<link rel="author" href="mailto:xiaochengh@chromium.org">
<link rel="help" href="https://drafts.csswg.org/css-anchor-1/#scroll">
<link rel="help" href="https://drafts.csswg.org/css-anchor-1/#fallback-apply">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="support/test-common.js"></script>

<style>
body {
  margin: 0;
}

#anchor {
  anchor-name: --a;
  width: 100px;
  height: 100px;
  margin-right: 200px;
  background: orange;
}

#spacer {
  width: 1000vw;
  height: 1px;
}

#anchored {
  writing-mode: vertical-rl;
  position: fixed;
  width: 100px;
  height: 100px;
  background: green;
  anchor-scroll: --a;
  top: anchor(--a top);
  position-fallback: --pf;
}

@position-fallback --pf {
  @try { left: anchor(--a right); }
  @try { right: anchor(--a left); }
}
</style>

<div id="anchor"></div>
<div id="spacer"></div>
<div id="anchored"></div>

<script>
promise_test(async () => {
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'right');
}, 'Should use the first fallback position at the initial scroll offset');

promise_test(async () => {
  document.documentElement.scrollLeft = -101;
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'left');
}, 'Should use the second fallback position after scrolling left');
</script>

</html>
