I found some answers online, but I don't really understand the nuance.
People seem to say that つまらない is "boring" in the sense that it is uninteresting to you. And 退屈 is "boring" in the sense that you don't have anything else to do. As such, it can often be translated as "bored" or a する verb to "feel bored"
But several examples I see seem to go against that. 退屈な人 isn't uncommon, and seems to often refer to a boring person, not a bored person. And things like 退屈な講義 and 退屈な話 also don't seem to fit.
So what is the actual distinction here?