A whole/half notehead a second apart from the opposite voice needs a WIDER horizontal offset than a quarter notehead, because the open notehead glyph is wider and has no stem to visually break up the collision

Because whole and half noteheads (open, no stem for whole notes) are physically wider glyphs than filled (quarter/eighth) noteheads, and a whole note additionally has no stem line to help the eye separate two close voices, the horizontal clearance needed between two seconds-apart noteheads scales with notehead width, not a single fixed offset tuned for the more common filled-notehead case.

A whole note and a step-apart voice's note rendered with the SAME horizontal offset VexFlow would use for two quarter noteheads a second apart, under-clearing the wider whole-note glyph so it visually touches or overlaps the other voice's note.

Read, Music Notation, and Ross, The Art of Music Engraving, on notehead-collision clearance scaling with glyph width; SMuFL's `noteheadWhole` glyph bounding box is measurably wider than `noteheadBlack` — the numeric basis this rule needs is not yet verified against real SMuFL/Bravura metadata in Gradus's codebase.

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