Originally Posted by AaronP
Does the problem go away if you add a glFinish() or a SwapBuffers in your rendering loop? It's possible that your rendering requests are simply queuing up more work faster than the GPU can process it.
No, the problem does not go away if I add a glFinish.
The problem goes away if the entire back buffer is cleared by glClear.
But if the glClear is removed, or if the glClear doesn't clear the entire back buffer
(because of the scissoring), then the problem is present.