I recently developed a need for an off-screen buffer so I could render images via a command line tool that would not be able to produce a window for rendering. I looked up the needed code changes (PFD_DRAW_TO_BITMAP in Windows, for example), but it didn't seem to be working right. I eventually tripped over glFlush() and looked up why that made things work. I had no idea that OpenGL commands are actually buffered and processed via means unknown at appropriate times like when the system's SwapBuffers or equivalent are called. I simply assumed that if I send a command, it gets executed and I have my pixels in my memory buffer. Well, that's not the case. So to prevent someone else from losing a portion of their hair when working with single-buffered, non-windowed OpenGL contexts: remember to use glFlush() to make sure everything's taken care of and all pixels are rendered!

Even better! You've saved my future hair from forceful removal.

If you're planning on doing some fairly CPU intensive work while rendering stuff, you can make all of the rendering calls, then call glFlush() to start the rendering, and then begin the other work, and once that's done, you can then swap the buffers, or call glFinish() (depending on what you're doing...). That's the way I do things anyway... It works in theory, and I haven't had any problems with it, so I assume it'd be the best way to go about things...