Oh wow, thank you very much. With this JS both headroom and navsticky work.
Navsticky is even smoother than headroom when scrolling over the header (the thing between naviation and intro in g5_helium theme). It does not make such a hop during scroll, as with headroom.
But there is a little issue left. The issue is the same with headroom and navsticky: The menu is only sticky or scroll-sticky for screen widths of at least 640px. This seems to be defined in file ./user/data/gantry5/themes/g5_helium/css/jluikit.min.css constant named jl-breakpoint-s. I can’t see any config param for this in Gantry settings.
For screens narrower than 640px the site behaves as if there were no headroom or navsticky enabled at all.
Can you please give a hint or explaination for this, if you know?
Sorry, but the site behind the link https://hoemei.frank-sieber.de/ is not updated any more. So it can not be used as base for discussion about this.