DP has some windows related bugs for a while now, such as when you alt-tab out of fullscreen and are in a game and return , the fire button comes back permanently on. I think that is still a bug....and what you describe sounds like it could be related if you alt-tab from full screen, or if you have other applications open that are older apps - I have found these older apps, though they run in windows, will mess up DP because probably the newer windows versions use slightly different toolbar code that the older apps dont recognize. Many times I click on the minimized full screen DP to get back into it, and it pops me through other apps on the toolbar in sequence util it reaches DP.

You might want to make sure you have vertical sync disabled in the dp video options, that way you get max fps out of your card - also do showfps 1 in the console to see your fps, and I would start with a lower res like 1024-768 , then work your way up to see the best results.

Thanks for the reply, Cobalt.

I wasn't trying to switch between full and windowed screens, so I don't think it's an alt-tab error — nor was I running any other applications at the time.

I was able to reproduce the same error with QuakeSpasm. Additionally, QuakeSpasm doesn't support vsync (something DarkPlaces did support — crucially important to me).

I copied the following information from the QuakeSpasm console (I don't know if any of it is helpful):

Warning: vertical sync not supported
(SDL_GL_GetAttribute failed)
Intel display adapter detected, initializing GL_Clear

I also ran glView to check the functionality of my GPU and drivers. Everything was 100% supported through OpenGL version 4.2 (there were some incompatibilities with 4.3 and 4.4). Again — I don't know if any of this is helpful, but I'm trying to gather as much information as I can.

If anyone was watching this post, a user at the Steam forums correctly determined that DPI scaling was the culprit. Case closed!