This rather important but terribly-written piece of software has no Close button, and it's not unique in that. With the menu button removed too, the only way a user can even close something like this is via the terminal or System Monitor: not something the average user is going to be able to deal with when presented with a defective app like this.
Is the MenuIcon-eating code smart enough to leave the icon alone if the window style is missing the standard min/max/close icons? If not, it should probably get patched prior to the 20.04 release.
If you want the titlebar menu, you should know that you can get it back with dconf Editor. Org>mate>marco>general>button-layout. Add "menu" before the colon. Bingo!
Thankfully it's also in MATE-Tweak, IIRC, so I don't have to try to walk people through dconf over the phone.
My problem with this isn't that the option EXISTS - it's that it's on by default, for no good reason and lots of pain for people who are used to the menu being there.
Random breakage of existing desktop functionality on a whim is exactly the sort of thing many of us use / recommend MATE to get AWAY from in the first place.
Have you filed a bug report about this program being stupid enough to render action buttons outside visible screen area? Obviously there's a lot of screen estate available so this seems like a bug in layout.
As usual, it's sat there for 18 months with no activity. (Not blaming anyone, that's just how Ubuntu is). I think I've seen a dup of it somewhere that DID update sometime in the last few months, so it may actually be fixed now and they just haven't closed all the dups. But it's also possible that I'm misremembering it or the fox was for something similar but not actually this bug, so...