<!DOCTYPE html>
<title>CSS Anchor Positioning: last successful updated on resizeObserver timing</title>
<link rel="help" href="https://drafts.csswg.org/css-anchor-position/#last-successful-position-fallback">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="support/test-common.js"></script>
<style>
#container {
position: relative;
width: 400px;
height: 400px;
background: teal;
}
#anchor {
position: relative;
top: 100px;
left: 100px;
width: 100px;
height: 100px;
background: red;
anchor-name: --a;
}
#anchored {
position-anchor: --a;
position-try-fallbacks: flip-block;
position: absolute;
width: 100px;
height: 200px;
position-area: top center;
background: lime;
}
</style>
<div id="container">
<div id="anchor"></div>
<div id="anchored"></div>
</div>
<script>
promise_test(async () => {
await waitUntilNextAnimationFrame();
await waitUntilNextAnimationFrame();
assert_equals(anchored.offsetTop, 200);
}, "Starts rendering with flip-block");
promise_test(async () => {
anchor.style.top = "200px";
// Temporarily successful position for base style, but last successful is
// updated resize observer time.
assert_equals(anchored.offsetTop, 0);
anchor.style.top = "150px";
await waitUntilNextAnimationFrame();
await waitUntilNextAnimationFrame();
assert_equals(anchored.offsetTop, 250);
}, "No successful position (with intermediate successful), keep flip-block");
</script>
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