fix mouseY not reaching 0 and general improvement on MouseEvent.EXIT

[clankill3r]

#4057

This is my first contribution ever. I was not sure where to place the comments.
I didn't want to place them in between the if and the else if cause it would make the else if harder to spot.

Please take a look at #4057 (comment) as well.

[benfry]

Does it do the right thing at the other edges of the window? (left, right, bottom?)

[clankill3r]

Yes it does: