Hi!
It’s probably due to Revolution slider settings – it’s responsive, so when you resize it resizes too.
We fixed the style, the menu should scroll up with the header. We are not sure we
updated it, we will update soon.
In the meantime we’ll give you responsive.css and style.css, so you can check the differences of our style and yours – look at the menu on the lowest media query. If there are differences, just paste our code in place of yours, this should work.