This is really cool - I love it, Kris! It will combine with the sidebar beautifully, allowing the core cats up top and a short personalised selection on the bar.
I’ve noticed two little UX issues that could really give it a lift if addressed:
UX: Category selection highlight
When on a category list, the correct category has a lovely subtle underline.
However, this doesn’t persist if a Topic or a subcategory is entered.
It would be lovely if it did persist in both of those situations.
UX: scroll / overflow on mobile
There is a nice little fade / subtle arrow visible when there is overflow of the categories on desktop.
Overflow is much more likely on mobile, but on my device (Chrome / Android) these subtle cues are not present - and it isn’t clear that scrolling is possible.
It might also help to have the categories bar a little taller on mobile to make scrolling easier.