Stems on ledgered notes must lengthen enough to visibly reach the staff

A note's stem is normally about 3.5 staff-spaces long (one octave). When a notehead sits on or beyond a ledger line, the stem must extend further so that it visibly bridges the gap between the ledgered notehead and the staff proper — the stem should reach at least to the staff's near edge, and conventionally continues to (or toward) the middle staff line for notes with several ledger lines, so the eye can track the note's relationship to the staff via the stem.

A note 3 ledger lines above the treble staff rendered with the ordinary un-extended 3.5-space stem, leaving a visible gap between the bottom of the stem and the top of the staff — the notehead reads as floating, disconnected from the staff it belongs to.

Gould, Behind Bars, Stems chapter, states the extension rule; this exact formula is already extracted from VexFlow's own source and documented in the Stems and Flags chapter of this rulebook (`stem-direction-clef-middle-line-pivot`'s engineNeeds): `Tables.STEM_HEIGHT = 35`, `Tables.STAVE_LINE_DISTANCE = 10` (giving the un-extended 3.5-space default), and the extension formula `extension = (mid_line_distance - 3.5) * spacing_between_lines`. Ross, The Art of Music Engraving, independently states the same 'stem reaches to the staff' convention.

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.