Two voices sharing a stave need a minimum vertical clearance between their combined note-plus-stem extents, independent of any single collision check

Beyond the specific notehead/rest/stem collision rules above, engraving practice maintains an overall minimum vertical buffer between the LOWEST extent of the upper voice's material (notehead, ledger line, or downward-extending element at that beat) and the HIGHEST extent of the lower voice's material at the same beat — roughly one staff space of daylight in normal engraving, tightening only when the specific offset rules above have already resolved a direct collision.

A passage that passes every individual per-onset collision check above but still reads as visually cramped because the voices sit consistently close together with no onset ever triggering a specific rule's threshold, yet the cumulative effect is a hard-to-read texture.

Gould, Behind Bars, general spacing principle for contrapuntal lines sharing a stave.

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