I guess it varies from person to person. If somebody fixed something I wrote without asking I would be thrilled! I recognize that I am imperfect, limited by time, and striving to move to more impactful projects so I would be more than happy if somebody wants to improve a system I was inexperienced with or didn’t have the time to fix. I would probably give them the low down on the system and try to transfer maintainership of the system to them.
Reading the GP comment (going skunk), I wondered if the factors behind that turning out positively was the position of the fixer relative to the code/systems in question. If they're not stepping on anyone else's toes it works great.
This comment echoes that sentiment a bit, I think; there's an implicit dependency in there on the project not having a collective structural investment, whether that's explicit (overt, with explicitly delineated boundaries) or implicit (implied, fuzzy, part of collective culture), on the thing being fixed.
