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?
If you don’t mind losing your settings, perhaps you may need to reset GIMP’s configuration?
rm -r ~/.gimp-2.8
I use GIMP quite often and haven’t ran into this problem. My system is under Compiz with Ubuntu MATE panel layouts (not the Mutiny layout which could be responsible for removing the menu bar). Unity is a Compiz plugin after all.
I have just re-set mate panel and the gimp menu flashed on for 1/2 a second, then went off. I don’t think this is a Gimp issue. I also uninstalled gimp and purged all configuration files before reinstalling it. Nevertheless, I will do what you have suggested and report back.
Nope, just the same. No menu. I have checked in preferences and the men u bar is checked. As I said, this problem, has been reported on the main Ubuntu forums and has been acknowledged as a xenial bug. Their suggestion is to reset unity. I have tried reset Caja with no effect. I have reset mate panels and the Gimp menubar flashes on for a second then disappears.
I currently use compton compositor. I will turn that off and try again.
Nope, no difference. However, something very weird has just happened:
I tried resetting mate panel again. When I did, the gimp menu bar appeared…on the left hand side of the Mate top panel like as if it was bleeding Unity…! I shut gimp down by clicking on the close button on the right hand side, at which point, the Gimp menu bar disappeared from the top mate panel
Which panel layout are you using? Are you using the TopMenu Panel Applet?
If so, temporarially “killing” it should restore the menu bars back onto the program:
killall topmenu-mate-panel-applet
I would suspect it’s a bug in Ubuntu with how their menu bars are being “taken apart” from GTK2 programs, like GIMP. That would be strange since it’s working fine here. 
I am using just the standard default mate panel with indicators switched on
I can report it’s not just Gimp. It’s Inkscape as well
I am going to check through all my apps and report if any other have this problem
I don’t know if eye of mate is supposed to have a menu, but there isn’t one. So impossible to open a picture from it
So, that’s:
Gimp
Inkscape
Eye of Mate
Shotwell
Synfig Studio
None of these have menubars
Okay, fixed it, at some cost…
Went to Mate Tweak, switched off indicators, then switched them back on. Don’t ask me why. Just a hunch, I guess.
Opened up Gimp and the others. Menu bars are now back.
However, disabling and re-enabling indicators seems to have completely reset my main panels such that I have lost all of my personalized launchers on the top menu bar…which is a pain in the backside
Strange bug. Looks like it just affected GTK2 applications.
For the launchers, you might be able to pick up the scraps from here:
~/.config/mate/panel2.d/default/launchers/
These copies may ease the pain. 
No worries lah7. I am being melodramatic about them really. It only takes me about 5 minutes to recreate them.
All they consist of are:
off the shelf;
firefox
chromium
chrome
tor
system monitor
cpu monitor
simple screen recorder
weather report
self built;
link to BBC weather for my town
reset sound script
restart panel script
stop compton
start compton
HI Steve,
just a suggestion for you, I use the following weather app which can also be added to the panel!: