This happened a few years ago in a bus form Paris to Cologne. I leaned the seat backwards as far as possible and the person behind me heavily complained about not having enough space. I saw that she leaned her seat backwards too (to be fair, there were no people behind her), but I said we all don't have a lot of place and I'm trying to sleep, so I leaned the seat backwards anyway. The person in front of me did it too and I didn't complain about it, because obviously I'm going to have less space, but it's a lot easier to sleep if you lean your seat backwards.
I'm not looking for interpersonal advice. I'm more interested in knowing if there are any guidelines or rules about this. Am I allowed to lean the seat backwards or does the person behind me need to approve this?
If it matters, the bus, me and the person behind me were all from Germany. The bus company is "Mango Tours", but I'm hoping for an answer which doesn't depend on the company.
This answer provides information about flights, so I guess it's the same in a bus. But I can't say for sure.