<!DOCTYPE html>
<title>Tests that position fallback responds to scrolling</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>
#cb {
  width: 400px;
  height: 400px;
  margin: 100px;
  transform: scale(1);
  outline: 1px solid black;
}

#scroller {
  width: 400px;
  height: 400px;
  overflow: scroll;
}

#anchor {
  width: 100px;
  height: 100px;
  margin-left: 150px;
  margin-right: 275px;
  margin-top: 300px;
  margin-bottom: 300px;
  background: orange;
  anchor-name: --a;
}

#anchored {
  position: absolute;
  background: green;
  anchor-scroll: --a;
  position-fallback: --fallback;
}

@position-fallback --fallback {
  /* Above the anchor */
  @try {
    width: 100px; height: 100px;
    left: anchor(--a left);
    bottom: anchor(--a top);
  }
  /* Left of the anchor */
  @try {
    width: 100px; height: 100px;
    right: anchor(--a left);
    top: anchor(--a top);
  }
  /* Right of the anchor */
  @try {
    width: 100px; height: 100px;
    left: anchor(--a right);
    top: anchor(--a top);
  }
}
</style>

<div id="cb">
  <div id="scroller">
    <div id="anchor"></div>
  </div>
  <div id="anchored"></div>
</div>

<script>
promise_test(async () => {
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'top');
}, 'Should be above the anchor when at initial scroll position');

promise_test(async () => {
  scroller.scrollTop = 200;
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'top');
}, 'Scroll down until the top edge of #anchor touches container but not overflowing');

promise_test(async () => {
  scroller.scrollTop = 201;
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'left');
}, 'Scroll further down, making the first fallback position overflow by 1px');

promise_test(async () => {
  scroller.scrollTop = 200;
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'top');
}, 'Scroll back up to reuse the first fallback position');

promise_test(async () => {
  scroller.scrollTop = 249;
  scroller.scrollLeft = 51;
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'right');
}, 'Scroll bottom-right to make the first three fallback positions overflow');
</script>
