I think this happened a few weeks ago after updating the system. Before that VirtualBox fullscreen menus worked fine.
The problem is, when I switched VirtualBox to fullscreen, the menu is totally not visible anymore, although the menus and buttons can still be clicked and they work as expected.
This is not an issue, just an incovenience as now whenever I want to minimize/close the VBox instance I need to blindly look for the location where the button is supposed to be and if I’m lucky I should be able to find it with only a few clicks.
I don’t know where I should look for as VBox is a Qt program probably affected by Kvantum, but it doesn’t seem to make any difference after fiddling Kvantum settings a bit.
An update. Rebooted the host system after disabling composite effects kind of fixed it. I can see the menu now. Will see if the issue would resurface in the future.