When I use claws-mail under KDE Plasma in Wayland, no global menu is shown and also no alternative "regular" menu (like Firefox for example does it). When I use: $ GDK_BACKEND=x11 claws-mail The global menu works as expected.
Sry made a mistake here, my problem is not about the global menu.
I am not sure what you mean by '[global] menu' and '"regular" menu', but all menus are working correctly under KDE PLasma with wayland and X11 for me. The main menu is toggled on and off by the F12 by default. If that does not help you then it very much seems that the problem is solved ourtside of Claws Mail, maybe by the window manager.
As I said in my second comment, this is not about the global menu. I did a mistake in thinking that the button I mean is referred to as "global menu". What I mean is the top menu that usually seems to look like this: File - Edit - View- Message - Tools - Configuration... In my KDE Plasma, programs either show a button right on the top left that opens the main menu or they show the main menu itself. It is clearly missing in Wayland in Claws Mail only (AFAIK), and it is there in X11, so I think it is either a Claws-Mail issue or a GTK issue.
In my experience it is clearly working in wayland in kde plasma.