Interestingly enough, passing any uint32_t (0 or 1) to CGDisplayRotation is fine so long as CGMainDisplayID is called before. A bit bizarre, but why is that?

Yes interesting. It does run when replaced with a call to CGDisplayRotation. Re - CGDisplayRotation. I only used it as an example. Actually I'm trying to port this package's code https://github.com/BoboTiG/python-mss/blob/main/src/mss/darwin.py to Julia but ran into issues similar blocking behavior with CGDisplayBounds. I transferred to C directly only to find out I was suffering from the same issue. One question (partly from a lack of C knowledge) - isn't CGDirectDisplayID equivalent to uint32_t? I tried passing in 1 directly and it stalls...