Fix safari transparency bug. Fix modal scrolling

There are currently 2 bugs:

In safari, the background is much more opaque than it should be.
When scrolling down the page, some of the page is not covered by the overlay (position absolute instead of fixed). Instead, the modal becomes fixed, and we let the modal scroll, but disable scrolling on the body by setting overflow hidden.

This now supports large pages and also modals larger than the page.