.active for nav elements not working in page route with children

[shalberd]

Hello

What determines whether a list item in the navigation bar is marked as active or non-active?
I assume one point is to look for inclusion of the singlepagenav.js, but what does singlepagenav.js bind to and how does it determine if a routing point is active or not active?
I have this problem since the last update of the antimatter theme. Can someone help clarify?
This may also have to do with recent url hardening changes in Grav.