Try Shift+Ctrl+J to toggle Full Screen. If that doesn't work (non-English UI, other key bindings, ...) try Shift+Esc that presents a search command input field, enter full and it should list "View -> Full Screen" (in English UI); hit that.

It's not really "Full Screen", but it is missing the window decoration and "File Edit ..." menu I think if I can put it into tabbed menu mode that will help. I am not finding the icon bar that provides the "tabbed menu" button.

So - mark this one [solved]

So I finally decided to just re-install Libre Office. This worked, at least I had my whole window after that...

but when I removed Libre Office it took way too much of the system with it - Discover (The app store) was even gone.

After trying to install the missing stuff again, I gave up and availed myself of the opportunity to 'test my backups'. I nuked and re-paved Debian then restored my home directory and went about re-setting up my environment in KDE the way I like it.

Press random buttons. Start with the F keys. Maybe research how to hide things in libre so you know the keys to unhide them.