A system's left margin must widen when the FIRST note of that system carries an accidental

Optical/left-edge alignment: the visual 'start' of a system is the leftmost notehead, but if that first note has an accidental, the accidental — not the notehead — becomes the true left edge, and the system's furniture must leave room for it. Ross calls this the distinction between optical and mathematical spacing.

`LEFT_MARGIN = 25` (line 29) is a single constant applied uniformly to every system regardless of what the first note of that system needs — there is no check for 'does this system's first note carry an accidental' anywhere in the width math (lines 748-831). On a wrapped block, any system (not just the first) whose leading note happens to need an accidental is exposed to this, since `LEFT_MARGIN` is identical for every system.

Ross, The Art of Music Engraving (optical vs. mathematical spacing); Gould, Behind Bars (left-edge/optical alignment discussion).

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