Updated to make the modal display and behave properly on pages with scroll bars

I have a web app that has too much content to fit on one page, so I have scroll bars.  I had to make the change in this Pull Request for my application, so I thought I'd give back.  It doesn't matter where you are scrolled on the page; the modal dialog will appear in the center of the page and disable the scroll bars.  

Thank you for sharing your outstanding work.  This helped me tremendously get a jump start.  

Thanks,
Joe