A slope's seal can sit in a cell its neighbour leaves open (a slope's
open corner, or under a grate, screw or screen), but removal only
cleared it when the neighbour was a sloped wall, so cutting a sloped
belt next to a sloped filter or grate could leave a one-cell belt that
nothing owned and nothing could remove. Removal now asks whether the
neighbour fills that cell itself, and loading a world clears belt and
screen cells that no block or seal owns.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>