How to calculate if mouse pointer at bottom page using Prototype?

I want to create auto-hide menu, just the same thing like http://www.ringvemedia.com/shanghai-photos but the problem is that I also have content that is scrolling down so if I calculate pointerY it will return mouse absolute position so it could be 1000...999开发者_Go百科999 depends on how many content you have to scroll down, that why I cant use something like "if (pointerY+50) >= document.viewport.getDimensions().height then $("somethind").show();". What I want is to somehow calculate that mouse right now is on bottom of the screen (+- 50px) (not the content).

I think I have confused even myself :) hope someone will understand what I wrote.

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I think you have to use document.viewport.getScrollOffsets() and then substract the pointerY with the scroll offset to get the position in the viewport.