Comment 2 for bug 160636

Ahh, yes, you're right. Disabling compositing does give the correct behaviour (screen is black at unlock dialog).

The option was at WIndow Manager Tweaks. I'm not sure what the 'previous tab' is that you're referring to (I have Placement, and that's not relevant). I did find that unchecking 'Display full screen overlay windows directly' on the same tab made the problem go away.