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Removing the frame of a panel in a strip is a classic thing to do to lessen monotony (I guess), and it's very often done in Calvin & Hobbes for instance (systematically done, I think, actually). I never do it, because I'm old school like that, but when I do do it, it's for a reason. Here, it's supposed to look like how Mr. Everyman feels: that there's nothing holding you up anymore, no boundaries left to your world, and you might just fall into an eternal abyss.
breakup, break-up, break up, relationship.