rval = kPsychDontUseFlipperThread
It prevents conventional use of Screen’s internal background flipper thread
and thereby disables all use-cases and functionality that requires the thread,
e.g., some modes of fine-grained visual stimulus presentation timing via VRR,
our own homegrown frame-sequential stereo modes (stereoMode 11), and the async
flip function Screen(‘AsyncFlipBegin/Check/End’).
The flag will be often used for interoperation with external presentation
backends, e.g., Virtual Reality / Augmented Reality / Mixed Reality runtimes
like OpenXR for stimulus presentation by a VR compositor. Other applications
are conceivable, but OpenXR is the initial target application of this flag.