Minor inaccuracy

[olleicua]

Looking at the code I noticed that in the case where we are checking for partial visibility it uses <= and >= which is technically incorrect because if the top edge of the element lines up exactly with the bottom edge of the container then it would be inaccurate to consider it visible. It's the sort of thing that rarely comes up but it's also the sort of thing that makes things like this possible:

http://www.youtube.com/watch?v=W1U5RUVENNE