Add a window selector

[fabiendv]

Scrolling works perfectly when using a single window.

However, if there is a <div> element scrolling in the middle of the page (a list with messages, max-height 200px, scrolling on y), the scrolling button won't be shown while you scroll inside the list of messages.

Is there a way to implement a window selector? So it can refer to the parent y-scroll instead of the window one?