This is something randomly and infrequently observed on Windows.
Unfortunately we haven't determined a sure way to reproduce it.
After awhile some or all the menus in the menubar aren't drawn or are transparent when clicked.
I am talking about the drop-down portion.
If you click on the menu item in the menubar, and then slide down the cursor to the invisible drop-down menu and click, it will launch the appropriate submenu action that exists at the invisible position.
Usually a program restart fixes the problem.
Last known working version is Qt 5.8.0.
This has been confirmed by users under Windows 7, 8.1 and 10. All in OS are x64 and QT is x64.
In fact a user of ours has made a gif showing the problem. Click here.
Relevant issue in our bug tracker is here.
Unfortunately the bug is so random and infrequent that we haven't any more info.