I'm running Plasma on Chakra Linux. I noticed this problem with Scribus and found it also affects Kate, although it doesn't seem to be affecting Gimp, Thunderbird, or LibreOffice. My menu bars have disappeared; i.e. File - Edit - etc. etc.

I have tried deleting the ~/.scribus directory in order to force recreation. Didn't work.
I have uninstalled and reinstalled Scribus. Didn't work.

On the Scribus forum, they suggest it could thus be a KDE problem, which is why I'm here. Anyone else have this kind of problem? It's driving me batsh*t! I just need to know where to look.

Kaz!

UPDATE: Included Dolphin

At least for Dolphin and Kate, does pressing Ctrl+M change the visibility of the menus?
Do you use the global menu?