The "just use the Mono framework" solution for the Metadata Editor on Mac was terrible to begin with, until it stopped working completely a couple versions of macOS ago.

Worse, I'd been promising a better Mac experience for the editor longer than I've been promising better sheet music. Despite my attempts to find a Xamarin contractor to do the development (so I could keep focusing all my energy on Synthesia 11), in the end I wasn't able to find a great fit. So, I just did it myself. I suppose the good news is that it took less than a week.
Now you get a native Cocoa UI that feels as good (or better) than the Windows version. Mac users are finally first-class citizens.

Download the latest Synthesia Metadata Editor release at GitHub.