As per title
The main menu is missing including “file”, “edit” etc.
Uninstalled, purged and reinstalled. Still the same. Been on the main Ubuntu forum and, apparently, this is a problem with Gimp and a number of other apps and is due to a bug in Xenial and they are working on fixing it. Until then, the advice on the main Ubuntu forum is to reset Unity.
However, this not being Unity, how to I temporarily fix this on Ubuntu Mate?