Doesn't work with an iframe's window as scroller

[WickyNilliams]

Given the following html:

<header>foobar</header>
<iframe srcdoc='[lots of content]'></iframe>

This will not work:

const header = document.querySelector("header");
const iframe = document.querySelector("iframe");

const hr = new Headroom(header, { scroller: iframe.contentWindow });
hr.init();