I think the best way to go (if you're the "Good Samaritan Who Wants to Do His Civic Duty but Also Doesn't Want to End Up Spending the Week in Nerd Court") is to fork it, fix it, and leave a brief comment in the issue thread that says "hey, I fixed this in my fork".

That way, people with the same issue who come in to the issue thread through a search aren't reduced to combing through all the forks to see if one happens to fix the issue. Then instead of spamming up the thread with "me too, this is soo frustrating, fix asap pls", they can spam it up with "merge this fork into the main project asap pls" :-)