> Hmm, I'd like it if that always made sense, but sometimes the default state, with nothing active, is for Foo to be on.

Then foo will be checked on by default. That's not a problem.

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Is perfectly unambiguous.

What I'd like to see with every option like that is something showing whether it is in default state or not so user can easily glance what was changed from default. "reset to defaults" per page of settings is also appreciated.