<script>
if (window.testRunner)
testRunner.dumpAsText();
onload = ()=> {
// Force layout first.
document.documentElement.offsetTop;
// Then expand the root frameset grid from 2x1 to 2x2.
document.getElementById("rootSet").setAttribute("rows", "*, 0");
}
</script>
<frameset id="rootSet" cols="0, *">
<frame id="firstChild"></frame>
<frame src="data:text/html,PASS if no crash or assertion failure."></frame>
<frameset>
<frame src="data:text/html,3"></frame>
</frameset>
</frameset>