It seems that the OS creates some kind of obj-C notification subclass for our window and view classes, and is just a warning at shutdown that when we clean up and release our old window classes, the OS-derived versions are still hanging around. I have no idea how we’d work around that, except to just leak our obj-C classes… but if anyone has some guru-level knowledge about what on earth those NSKVONotifying_* things are, or how to make them go away, let us know!
But TL;DR: Don’t worry about it, it’s not harmful, just a warning.