Comment by kllrnohj

In both the GNOME and Windows "Fullscreen Optimizations" it's the compositor doing an internal optimzation to avoid a copy when it's not necessary. In neither scenario is the system nor applications "overriding" or bypassing the compositor. The compositor still has exclusive ownership of the display. And the application's swapchain is still configured as if it was going through a composition pass (eg, it's probably not double-buffered)