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<!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;
top: anchor(bottom);
width: 100px;
height: 100px;
background: green;
position-anchor: --a;
position-try-fallbacks: --pf;
}
@position-try --pf {
top: auto;
bottom: anchor(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>
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