Top of the window has a negative value when register form has multiple fields

I have used this module before and works all right, but now I had to use it on a website where the register form had multiple fields grouped, then when the window tries to show it generates a negative top value and half of the window dissapears outside of the window and even the close button cannot be reached.

If you resize the window if recalculates the ajax window top and then it shows ok.

The simple solution is to add a validation, so that if the calculated top value is negative replace it with the default window top. I'm attaching a simple patch for that.