A beam group must never span a system (line) break

When music wraps onto a new engraved system, every beamed group must be complete within one system; a beam cannot visually continue across the line break.

A beam spans from the last note of one system's stave to the first note of the next.

Gould, Behind Bars, ch. Beaming; universal engraving practice (Ross; Read) — a note whose beam would cross a system break is simply re-beamed to start fresh on the new system.

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