About fourth_wall, I think that it's a grave misunderstanding to use it for characters bleeding outside the panels. The fourth wall has nothing to do with the panels in a comic. Unless the characters are acknowledging the existence of the panels it's simply an artistic choice. The fourth wall is the imaginary wall between the audience and the characters on a stage. The fourth wall is broken when the characters acknowledge either the audience or the fact that they are in a piece of fiction.
Having the characters bleed outside the panels is not a narrative choice, unlike breaking the fourth wall which is entirely narrative.
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Ahem
With that out of the way, what I really wanted was the outside_border tag, which I still think is being misused for images where the background happens to be a rectangle (maybe rectangle_background would be a better tag for those). What I want with this tag is to make a derivative tag specifically for when a character's breasts are hanging outside the panel borders. Because I'm sure there's more people than just me that would be interested in such a tag.
Does breasts_outside_border work for you guys?
Or perhaps breasts_over_border or panel_breaking_breasts?
I think the first one best signals the implication to the outside_border tag, while the second tag better describes the phenomenon as the breasts often look to be resting on the border. The last one feels most impactful.