The FIRST app that gains control of the screen does not allow other apps do the same. Try it. Open Windows Media Player, play a video, then open VLC and play a video. You'll see that VLC output is pixelated - that's because it's rendering using RGB and not via hardware overlay.
In such a case an IT admin remote controlling a PC, or an employee goofing off with Windows Player will prevent my app from gaining exclusivity. That does not solve the problem.
Also, exclusive access to the screen does not guarantee your program will go uninterrupted. Retaining exclusivity is impossible, because you just might get an ALT+TAB, CTRL+ALT+DEL or a WM_KILLFOCUS from who knows where. There are custom techniques for getting around each of those, but they're all along the lines of what's described in the article. Attaining and retaining exclusive access to the screen is next to impossible.
I'm not sure you get it. This is hardware overlay. It's not running in user-space, it's being moved from memory to graphic card directly. A DirectX drawn window is not affected by focus or anything else. You can stop it by killing the process.
If someone came ahead of time to prevent it from ever gaining screen access, then that would be a problem, but since the order of app launches are known and can be controlled, it would still work.
In DirectX you will lose exclusive control of the screen and all rendering surfaces if your app loses focus. Retaining focus is not possible, unless by way of trickery. Game forums are riddled with developers trying to overcome this problem.
