I had to download the nightly build for another problem so ended up testing this on my own application.
I have a problem that might not be easily fixed. When I hold down the mouse and move it to a negative X or Y (outside the applet below or to the left of it), the Mouse.getX() and Mouse.getY() never goes below 0 so I don't know if it is just outside the corner or a kilometer outside.
Any ideas?