Well, you’ve probably also figured it out already, but I’m also posting this for future searchers – I just found this page in a Google search while trying to fix it, and it guided me to the right line at least!
It might be a bit of a hack, but hard coding a subtraction into the relevant line posted above sorted it out for me. Worked in Firefox and IE8 so far.
this.lightbox.setStyle({ top: (lightboxTop-50) + ‘px’, left: lightboxLeft + ‘px’ }).show();
