Not working on scrollingElement

[alex7071]

This works fine on any container in the page that has width/height and overflow scroll, but the obvious case in which the scroll is on the document.scrollingElement, it doesn't. Computationally it should be easier, and for older browsers https://github.com/mathiasbynens/document.scrollingElement can be used to have document.scrollingElement available.

LE: Since there are calculation differences between versions of browsers (i.e. Chrome 59 scrollingElement differs from Chrome 61 calculations) using scrollingElement is done only to get the DOM element, viewPort calculations has been changed to use older more reliable measurements which won't change in the predictable future.