I have the same problem on Linux Mint.
Menus (right click + Opera menu) are not showing.
After restarting Opera it works for few hours and then it stops.
It works like Opera thinks that menu is shown because after right click it does not allow to do anything.
71.0.3770.228 (newest version at the time of writing).
Linux Mint 19.3 (x86_64; X-Cinnamon)