Click on two faces, click add mate, click concentric.

Computer slows down.

The "view mates" window comes up and one of the parts is isolated.

Close view mates window.

Mate has not been added.

Repeat procedure for same mate.

No problem.

Repeat procedure for new mate.

Same problem.

This started today and happens practically every time. This is eating up an incredible amount of my time, as it takes about 30 seconds to add a mate. What horrific feature have I accidentally turned on?