OpenGL on Apple GPUs is implemented on top of Metal with some private extensions to be closer to OpenGL behaviour. Making things work performantly on top of regular Metal is quite a bit harder.

Not sure why this is downvoted, it's true: https://rosenzweig.io/blog/asahi-gpu-part-6.html

> Comparing traces, the render pass "OpenGL mode" sets an unknown bit in this kernel-processed block. If we set the same bit in our OpenGL driver, we find the clip space changes to -1/1.