Why does the screen go dark in VirtualBox?

Question

linux

Put to play CentOS 7. Rolled guest additions. When you turn on the full screen mode, the screen darkens a little, as if the brightness was reduced, and the edges of the screen remain as bright as they should. Such nonsense both at Host + F and at Host + L. Where to dig?

Answer 1

это Линукс подвисает , бывает так