AFAIK mpv does prevent Windows from going to sleep / turning off the screen while running. Maybe check how mpv does it?

Just to mention that most systems running Windows nowadays are using "modern standby" (and in many cases it cannot be disabled), and it seems that it's not possible to prevent sleeping while the screen is turned off. For example, see:

https://answers.microsoft.com/en-us/windows/forum/all/stop-modern-standby-from-activating-on-sp4/466f2392-6f54-42c0-a1f8-ee3eb1135f15

and

https://www.asus.com/support/FAQ/1043537/

There's an api called ShutdownBlockReasonCreate provided by microsoft: https://docs.microsoft.com/zh-cn/windows/win32/api/winuser/nf-winuser-shutdownblockreasoncreate

Maybe we should give it a shot.

Also, the application is expected to react with WM_QUERYENDSESSION to prevent shutdown either?

Another reference about Shutdown Changes for Windows Vista should be helpful.

I'm not familiar with the Windows stuff, but this should be work somehow

As of now OKEGUI is the only program that is able to block shutdown and being killed by the system. The other major processes(e.g. x26x, vspipe, EAC, etc.) are still killed by Windows.

Is there any way to run these stuff as a subthread of the OKEGUI?