Originally Posted by AaronP
Yes, agreed. However, there is something different about SDLMAME (or perhaps it's X or Twinview), when SDLMAME is run in "fullscreen", the game window shows up on the primary monitor as expected, but the second monitor goes black. This indicates to me that for some reason SDLMAME "fullscreen" mode is spanning both (all) screens, even though the game image is only shown on the primary display. This is exactly why __GL_SYNC_DISPLAY_DEVICE does not work when SDLMAME is in fullscreen mode. You are forced to run windowed in order to make SDLMAME stay on the primary screen, and thus sync to it.