My knowledge comes from a few different parts:Ok. So if Rasmus is right, the white line is regarded as part of the circuit. Good to know! I couldn't find that on internet.
There are some sites that refer of keeping the tyres on the black stuff but I could find an official rule on that.
1. Most of the times, the white line is printed onto the tarmac, not the kerb/grass/gravel, so it basically IS still the "black stuff", just coloured white
2. While watching F1, there were some discussions, mostly with drivers crossing the pit exit line. The analytics always talked about the tyre having to cross AND leave the white line. The latest discussion was Verstappen at Monaco I think. There, Palmer's F1TV analysis is about "On the line, but not crossing the line". And Verstappen didn't get a penalty so as long as you touch the line, it's okay.
3. Also from F1: some close up shots at Red Bull Ring showed the cars going beyond the lines when they got the lap invalidated.
So I'm pretty sure that as long as you touch the line, it's fine! Also the cut detections in our simracing titles work like that. Mostly because they don't see the line but only the surface type. So they work like my first point