I am sitting in Panera right now - mask off when I sip my coffee, spaced about 15 feet from the next customer - but closer at times - when ordering, refilling etc.
Proper?
It comes down to level of risk.
And I don't know the level of transmission/risk in your community.
Personally I probably wouldn't be there, because you have to demask to drink & eat. And yeah, you're going to move around some and so are other customers.
And all of these relatively low-risk behaviors and situations add up and one time you're luck will run out. Probably a coffee shop or bar shouldn't be open except for take out. But again, I'm not familiar with the risk level in your area.
My strategy would be to err on the side of caution and avoid as many potential contracting situations as possible. And while I am a big mask booster, I do agree that it's fine to use discretion. Nobody anywhere near you, fine to put the mask down on your chin or even pocket it maybe. After the lockdown in China, I still wore my mask even while bicycling for another month. Then another two months I would mostly have it on my chin, and just slip it up when I was near other people, primarily just when I got stopped at traffic lights.
I think the best thing to recall is that you should wear a mask as a courtesy/responsibility to protect other people around you. It also affords you some protection, but mask wearing is mostly about keeping the virus out of the public realm, protecting everybody.
As for what constitutes public for the purposes of the virus response, I'd say anywhere outside your house where other people are nearby.