I got the same thing, I made my own login handler and the mouse Y position seems to be hooking the wrong value or something. Getting y locations in the millions and neg numbers etc.
For example, if I hold the mouse in the middle of the screen, this is the x, y reported.
tempY += 40;g.drawString("Pos: " + getMouse().getPosition().x + ", " + getMouse().getPosition().y, paintX + 10, tempY);
I tested on latest + dev build (2.5.60) with the same results. Hopefully this is helpful/useful for the devs.