Fluorescent light fixture--can't seem to get it open. It doesn't seem to want to unscrew at all, or squeeze in as if it had tabs that held it in. No visible screws on the metal either.
It might be that the entire fixture is supposed to rotate a little and unlock from a base plate, but someone was lazy when painting the ceiling last time and didn't remove the fixture first, they just painted around it and got paint in the joint. So now it is being prevented from turning by adhesion to the painted surface. I would run a razor blade /Exacto knife around the rim where it touches the ceiling paint to break that bond.