This hack was part of a solution to VSync judder in desktop OpenGL on
Windows. Rather than using blocking-SwapBuffers(), mpv could use
DwmFlush() to wait for the image to be presented by the compositor.
Since this would only work while the compositor was running, and the
compositor was silently disabled when OpenGL entered exclusive
fullscreen mode, mpv needed a way to detect exclusive fullscreen mode.
The code that is being removed could detect exclusive fullscreen mode by
checking the state of an undocumented mutex using undocumented native
API functions, but because of how fragile it was, it was always meant to
be removed when a better solution for accurate VSync in OpenGL was
found. Since then, mpv got the dxinterop backend, which uses desktop
OpenGL but has accurate VSync. It also got a native Direct3D 11 backend,
which is a viable alternative to OpenGL on Windows.
For people who are still using desktop OpenGL with WGL, there shouldn't
be much of a difference, since mpv can use other API functions to detect
exclusive fullscreen.
James Ross-Gowan