Right, unfortunately there’s not a simple way to do this using CSS without also restructuring the layout.
The Horizon theme currently makes the entire .list-controls container sticky, which includes the category header. We can’t make the child .navigation-container sticky on its own, because sticky elements are stuck within their parent scrolling context.
We could restructure this area, but it’s one of those changes that would impact all the themes that style using the existing structure… so there’s a good chance of regression for other themes and customizations, even if they don’t have a sticky nav like this.