The vertical coves where two outside walls meet open up every winter and close every summer, apparently in response to temperature change in the framing lumber (2x6 studs, which move a lot more than 2x4); I say temperature because the motion shows up long before the humidity in the house starts to drop. Doubtless if I had been more attentive to letting one wall float in the corner this would have been minimized, and using paper tape instead of mesh would probably have been better too. But the corners are pretty well screwed down and have mesh tape.
So.... Is there a fix for this? We are talking a crack which goes from more or less 0 to 1/8" or more, and then back to 0. I almost think that even if my drywall was floating this would be a lot of motion to cope with. I would think some kind of paintable stretchy tape could be used to cover the crack and move with it, but I don't know if stuff like that exists.