Oops.

First, put the two utility functions with the other utility functions and name them Calendar.getViewportScrollX and Calendar.getViewportScrollY.

Second, capitali[z|s]ation is important.

Replace the two lines setting br.x and br.y with the following:

br.x += Calendar.getViewportScrollX();
br.y += Calendar.getViewportScrollY();

Hopefully this should all now just work.

Now all you need to do is grep through Calendar.js for document.body.ScrollTop and ScrollLeft and make everything use these two new utility functions instead.