Comment 15 for bug 1740365

okay, so you don't need a fancy replacement widget for the menu bar and can simply hide it.

I had a PR which does the hiding in a rather simple manner (some may consider it a hack :) just set the menu bar height to 0. This keeps the keyboard shortcuts working.
https://github.com/mixxxdj/mixxx/pull/3184
I closed the because there was a concurrent solution, thus I considered it a waste of time to polish mine (in some scenarios keyboard shortcuts stopped working in fullscreen mode).

I'm using this solution myself ever since on xUbuntu 20.04 and didn't experience any issues.
Just opened an extended PR against my fork. Since you are able to build Mixxx yourself I suggest you take a look:
https://github.com/ronso0/mixxx/pull/25
For a true 'locked' mode see the comments in the last commit.