No top menu bars when running under KDE

The Problem

When using the top menu bar with KDE, the Mailspring menu stays inside the window, instead of moving to the top.

Proposed Solution

On flatpak I can do something like:
sudo flatpak override --talk-name=com.canonical.AppMenu.Registrar com.appdomain.appname

Which forces the window menu to the top bar menu.

It works correctly for me, I don’t remember if I had to touch the settings (bit for sure I did not use any specific command line arguments). I’m using the rpm version, not a flatpack

Similarly, it worked as expected for me without having to tweak anything: my KDE panel has a Global Menu and Mailspring puts the application menu into it.
Kubuntu 20.10, Mailspring 1.9.1 snap version installed from Software Centre