A full-width section divider modeled on Goodyear-welted shoe construction: a needle locks one welt stitch at a time at a single working point while, a few stitches behind it, the channel flap that exposed the last batch of stitches folds shut on a real CSS 3D hinge and hides them for good. The lockstitch is never drawn; only a plain flush seam is ever visible once a flap closes.