A minor point not really related to my questions: It’s certainly the case that for most apps it makes no sense to update the window if nothing has changed. However, in an OpenGL animation, it is often very important that the amount of time per second devoted to computation and the amount of time per second devoted to rendering the scene change very little from one second to the next, in order to ensure a smooth animation. This is the reason why, counterintuitively, such animation-oriented apps deliberately render every N milliseconds, whether there has been a change in the 3D objects (or the camera position) or not.