repaintColumns cleared and redrew only [start,end], but a neighbouring cell's
glyph can overhang into that span. The clearRect erased the overhang and the
neighbour was never redrawn, leaving black notches through the line 1-2 cells
before the cursor that survived until a full rerender.
Redraw text for a couple of extra cells on each side, clipped to the cleared
span, so overhang from just-outside cells is restored without touching their
own cell areas. Keeps the per-column repaint efficiency (vs the full-row
repaint debug toggle, which fixed the bars but repainted every dirty cell).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
gregorandClaude Opus 4.8