A fermata over a barline (rather than over a note) marks an ensemble grand pause, not extra length on one note

A fermata drawn directly above a barline (not above any specific notehead) is a distinct, standard marking indicating a brief silent pause affecting the whole ensemble at that point, independent of any single note's duration — different from a fermata over a note or rest, which extends that specific sound. This is also the convention used in printed Bach chorale collections at internal phrase endings, where the fermata over the final note of a phrase does NOT lengthen the note — it marks the phrase boundary while the underlying pulse (tactus) continues, a convention specific to the chorale-harmonization genre Gradus teaches directly.

Gould, Behind Bars, ch. Fermatas, distinguishes a fermata-over-barline (grand pause) from a fermata-over-note/rest; the Bach-chorale phrase-marking convention (fermata does not lengthen, marks a breath/phrase point only) is standard performance-practice knowledge for chorale harmonization repertoire and is documented in period chorale collections (e.g. the Riemenschneider edition's editorial notes on Bach's fermata usage), directly relevant since Gradus's curriculum includes Bach chorale harmony as a named unit.

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