<!DOCTYPE HTML>
<title>Testcase, bug 1299736</title>
<div id="A" style="transform: translateX(50px)">
<div id="B">
<div id="C" style="position: fixed">
</div>
</div>
</div>
<script>
document.getElementById("C").offsetLeft; // flush
document.getElementById("B").style.transform = "translateX(50px)";
document.getElementById("A").style.transform = "";
</script>