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Test Info:

<!doctype html>
<title>CSS test: movement of relative-position ancestor correctly moves fixed-position descendant depending on the static position</title>
<link rel="match" href="hypothetical-dynamic-change-001-ref.html">
<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
<link rel="author" href="https://mozilla.org" title="Mozilla">
<style>
body { margin: 0 }
.ancestor, .child {
width: 100px;
height: 100px;
background-color: green;
}
.child {
position: fixed;
/* NOTE: child remains auto-positioned */
}
.ancestor {
position: relative;
left: 0;
top: 0;
background-color: red;
}
</style>
<div class="ancestor">
<div class="child"></div>
</div>
<script>
onload = function() {
let ancestor = document.querySelector(".ancestor");
window.unused = ancestor.getBoundingClientRect();
ancestor.style.left = "100px";
}
</script>