If you view the page in Chrome, everything works as expected (scroll past the landing and the nav bar comes into view). However, it stays above everything in Safari, in both OS X and iOS. You can pull the page down and see the nav bar up there.
I rarely view the site on mobile or Safari, but I do know that this worked in the past and that no changes to the CSS have been made on our end. That said, I'm wondering if it was a Safari change that caused this.
One last note - I'm not a web dev by trade, but I know enough to be dangerous. I'm happy to provide any other details needed.
Thanks!