A stem must not pass through a beam it is not part of

When a voice's stem tip reaches toward the opposite voice's staff-half, it must not cross through that other voice's beam ink — a stem is a single thin line but a beam is a filled polygon with real height, so a stem tip that only just clears a note by the vs-notehead-stem-adjacent-voice margin can still pass straight through the wider beam band a few pixels further along the same x.

A voice-2 half-note's stem, forced upward by the SATB convention, reaches high enough to cross through voice-1's eighth-note beam two staff-spaces above the collision point vs-notehead-stem-adjacent-voice checks for.

General engraving practice extending Gould's part-combining discussion (Behind Bars) and the same principle behind vs-stem-beam-lattice-cross-voice, applied to the narrower case of a single stem (not a full lattice) crossing a foreign beam.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.