/var/db/.com.apple.iokit.graphics is the configuration file for displays on macos. The entries are not at all labeled, but in my experience a new monitor will be on the bottom of the list. Each entry is structured as follows:
pcsn key and following integer are overscan. My first monitor had some value besides 10000, and changing the value to 10000 fixed it. I tried this with my second monitor and it didn't work, so I just deleted the last few entries. Either method requires a restart.