Offset rbox by window scroll, because getBoundingClientRect changes when window is scrolled

As per https://developer.mozilla.org/en-US/docs/Web/API/Element.getBoundingClientRect "If this is not the desired behaviour just add the current scrolling position to the top and left property (via window.scrollX and window.scrollY) to get constant values independent from the current scrolling position."
Merge pull request #207 from bryhoyt/patch-1

Offset rbox by window scroll, because getBoundingClientRect changes when...