A single note value must not obscure a primary beat it starts before and ends after (tie instead of a longer value)

In simple meter, a note that begins off a primary beat and sustains across the next beat boundary must be written as two tied notes, never as one value spanning the boundary, because the single value hides where the beat falls. The canonical example: in 4/4, a quarter note starting on the 'and' of beat 2 and lasting through beat 3 must be tied at the beat, never written as one plain quarter note off the grid. Because every barline is itself a primary-beat multiple by construction (beatsPerBar is always an integer multiple of primaryBeatBeats — verified: simple meter gives ratio=num, an integer; compound meter, e.g. 6/8, gives ratio=2), this same rule automatically subsumes the most severe case, a single note spanning two BARS with no tie.

notes = [{dur:'e'},{dur:'q'},{dur:'e'}] in 4/4 starting at beat 2 — the middle quarter note's onset (beat 2.5) plus duration (1 beat) ends at beat 3.5, straddling beat 3 with no tie.

Gould, Behind Bars, ch. 'Notes and note relationships', ties across the beat; Read, Music Notation, ch. on rhythmic notation, ties used to preserve visual beat structure.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.