<!DOCTYPE html>
<title>Tests position fallback with initially out-of-viewport anchor in vertial-lr</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;
  width: 200vw;
  height: 200vh;
}

html {
  writing-mode: vertical-lr;
}

#anchor {
  anchor-name: --a;
  width: 100px;
  height: 100px;
  margin-block-start: 100vb;
  margin-inline-start: 100vi;
  background: orange;
}

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

@position-fallback --pf {
  @try {
    inset-block-start: anchor(--a end);
    inset-inline-start: anchor(--a end);
  }
  @try {
    inset-block-end: anchor(--a start);
    inset-inline-start: anchor(--a end);
  }
  @try {
    inset-block-start: anchor(--a end);
    inset-inline-end: anchor(--a start);
  }
  @try {
    inset-block-end: anchor(--a start);
    inset-inline-end: anchor(--a start);
  }
}
</style>

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

<script>
promise_test(async () => {
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'top');
  assert_fallback_position(anchored, anchor, 'left');
}, 'Should use the last fallback position initially');

promise_test(async () => {
  // Scroll left to have enough space right to the anchor, but not enough below.
  document.documentElement.scrollLeft = 250;
  document.documentElement.scrollTop = 150;
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'right');
  assert_fallback_position(anchored, anchor, 'top');
}, 'Should use the third fallback position with enough space right');

promise_test(async () => {
  // Scroll down to have enough space below the anchor, but not enough right.
  document.documentElement.scrollLeft = 150;
  document.documentElement.scrollTop = 250;
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'left');
  assert_fallback_position(anchored, anchor, 'bottom');
}, 'Should use the second fallback position with enough space below');

promise_test(async () => {
  // Scroll down and right to have enough space on both axes.
  document.documentElement.scrollLeft = 250;
  document.documentElement.scrollTop = 250;
  await waitUntilNextAnimationFrame();
  assert_fallback_position(anchored, anchor, 'right');
  assert_fallback_position(anchored, anchor, 'bottom');
}, 'Should use the first fallback position with enough space right and below');
</script>
