Function shift

src/ink/selection.ts:540–544

(p: Point, vRow: number)

  // means a top-clamped point can stay top-clamped during a dRow>0 reverse
  // shift — dRow-based clampCol would give it the bottom col.
  const shift = (p: Point, vRow: number): Point => {
    if (vRow < minRow) return { col: 0, row: minRow }
    if (vRow > maxRow) return { col: width - 1, row: maxRow }
    return { col: p.col, row: vRow }
  }
  s.anchor = shift(s.anchor, vAnchor)
  s.focus = shift(s.focus, vFocus)
  s.virtualAnchorRow =