There is some issue with the Menus rendering. I installed the latest Unity version and it is working better but still some issue.
When I click on any menu, it only shows first the frame with no content (transparent) and after some fraction of second or a second it will show the content.
I am running Linux Mint. I tested on other Linux Mint machine and the menus work fine. So there is some little differences somewhere…
EDIT: I experiment with XFCE on same machine and it works ok. The issue was while using Cinnamon.