<!DOCTYPE html>
<title>Tests position fallback with bottom-up scroller</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>
#containing-block {
  width: 400px;
  height: 400px;
  transform: scale(1);
  border: 1px solid black;
}

#scroller {
  display: flex;
  flex-direction: column-reverse;
  width: 400px;
  height: 400px;
  overflow-y: scroll;
}

.box {
  min-height: 100px;
  width: 100px;
}

#anchor {
  anchor-name: --a;
  background: orange;
}

#anchored {
  position: absolute;
  width: 100px;
  height: 100px;
  background: green;
  anchor-scroll: --a;
  position-fallback: --pf;
}

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

<div id="containing-block">
  <div id="scroller">
    <div class="box"></div>
    <div class="box" id="anchor"></div>
    <div class="box"></div>
    <div class="box"></div>
    <div class="box"></div>
  </div>
  <div id="anchored"></div>
</div>

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

promise_test(async () => {
  scroller.scrollTop = -1;
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'top');
}, 'Should use the second fallback position after scrolling up');
</script>
