A slur or tie spanning a system break is normal and must be redrawn as two segments, not silently dropped

Unlike beams and tuplets, slurs and ties crossing a system break are completely standard — the curve is drawn open-ended at the right edge of the first system and resumes open-ended at the left edge of the next. This is NOT a fault to prevent by re-breaking; it is a rendering case that must be HANDLED.

Gradus's own code comment states the actual behavior plainly: 'A tie into the next system is dropped: VexFlow needs both noteheads and they are on different lines... a stray arc across a line break reads worse than a missing one' (lines 1227-1230) — and slurs are handled the same way via the shared `inSlice` bounds check (lines 1176-1185, `if (!inSlice(first) || !inSlice(last)) return;`, line 1178). This is a deliberate, acknowledged design tradeoff rather than an oversight, but it means Gradus CANNOT currently notate a tie or slur that legitimately crosses a system break — content that needs one (a suspension resolving across a line break in chorale writing, entirely normal) silently loses it with no warning.

Gould, Behind Bars (slurs/ties across a system break, explicit two-segment treatment); Ross, The Art of Music Engraving (tie-across-break rendering).

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