All the information I have suggests MTKView's rendering, whether via its own drawRect function or its delegate's drawInMTKView function, occurs on the main thread. Everything I've read indicates this, and it appears to be the case empirically as well, at least for drawInMTKView (as determined by logging the current thread in that function).
However, I can't find this documented anywhere, and it seems important to have this guaranteed for proper interoperation with other functions such as notification and touch callbacks.
It may be that this is documented and I've just missed it, in which case I apologize. That said, is it documented anywhere on what thread MTKView renders? Is it guaranteed to be the main thread?
