OK, I figured out why this is happening. A web development project I’m working on is using Dinghy, which is a utility for working with Docker on macOS. Apparently, it starts NFS-sharing the home directory when it starts, which is why I’m getting these errors—it’s monitoring/touching every file under ~/user, so whenever I try to save a file, it complains about having been modified elsewhere.
This ‘note on NFS sharing’ explains how to limit the scope of the NFS sharing, which will probably fix this, but I’ve lost a lot of time trying to figure this out and I’m afraid to mess with the setup of this project, so I’ll just leave it for now. I’ll report back if I have anything else to report.