Scala traits are interesting because they can be used for inclusion polymorphism and to mixin behaviour. I’ve found tension here though, as the former uses inheritance and the later is more about code re-use. So when a Scala class extends a trait with behaviour, it seems to go against the generally accepted view that using inheritance as a mechanism for code re-use is a bad idea.
It can be tricky not break the inheritance vs. composition principle when using traits with behaviour. Is it clear to you when you might be?