Remove white "menu space" when scrolling Twenty fourteen

I’ve created this site using a child theme of Twenty fourteen: https://awkwardsilen.se/raa/rallylydnad-med-siv/

What bothers me is that scrolling creates a white space between the header and in this case the featured image. (I’m looking to turn the featured image into the header in another post, so the problem lies in the content getting pushed down, rather than the image.)

I think the top menu used to “pop down there” when scrolling, before I decided to have it stay above the header at all times… So that’s probably where the space comes from. But how do I get rid of it?

Many thanks,
Anna