Yes, this is what I (he) wants for now, because it does solve the problem but only because I have only one user at the moment (my wife).
A more general solution would be to let the user decide by an app-specific preference setting which would work on all platforms (as richmond62 rightly points out) and the default of which would be, in my case, that it shows up in all spaces.
Begin of rant:
Also as richmond62 points out, Linux distros are indeed a minefield (to which I would add that the "forking" in open-source software is probably the major reason why it does not take off). There is too much of it, and it changes too often. That's the main reason why I've still not gone Linux: I cannot afford the time to keep up. "Nightly builds" are a nightmare, not a solution, those who are proud of it don't live in any real world. Already Apple's yearly tweaks are too often. Once every two years is more than enough. We now have one machine on "Mojave" (which I insist on pronouncing "Moh-Jave") and there is nothing whatever useful in that release compared to the previous releases. Fix bugs, patch security holes, but do not force me to change everything. Plus that in each new release Apple purposely renders some apps inoperative. We just had to spend 800€ for a new version of FileMaker because our version no longer runs, whereas all my old LiveCode apps still happily function. This is pure marketing and milking of the user.
End of rant.
Good night everyone, and thanks for all the fish. I mean thanks for all the help. It was not as easy as 42 after all.