I found two little issues.
I have a MacBook Pro 13 - 2017 and Mac OS X Catalina 10.15.6
Scrivener 3.1.5 for Mac
1/ When I use Composition mode, if I change of desktop with trakpad this removes the composition mode to the normal mode. I have the same issue when I use Antidote.
2/ When I leave the Composition mode normally or with a bug I cannot access to the menu. I need to go to another desktop, click, and go back to scrivener.
On the second issue, I’ve looked into this but haven’t been able to find a fix yet - I believe the switch between the two different types of full screen is triggering something odd in Apple’s full screen toolbar/menu code.
Fortunately it is very easy to work around: when you return to the main (but full screen window), just click onto an empty area of the toolbar at the top of the screen. After you do that, the menu will work again.