Function shiftAnchor

src/ink/selection.ts:573–602

(
  s: SelectionState,
  dRow: number,
  minRow: number,
  maxRow: number,
)

 * must follow it. Focus is left unchanged (it stays at the mouse position).
 */
export function shiftAnchor(
  s: SelectionState,
  dRow: number,
  minRow: number,
  maxRow: number,
): void {
  if (!s.anchor) return
  // Same virtual-row tracking as shiftSelection/shiftSelectionForFollow: the
  // drag→follow transition hands off to shiftSelectionForFollow, which reads
  // (virtualAnchorRow ?? anchor.row). Without this, drag-phase clamping
  // leaves virtual undefined → follow initializes from the already-clamped
  // row, under-counting total drift → shiftSelection's invariant-restore
  // prematurely clears valid drag-phase accumulator entries.
  const raw = (s.virtualAnchorRow ?? s.anchor.row) + dRow
  s.anchor = { col: s.anchor.col, row: clamp(raw, minRow, maxRow) }
  s.virtualAnchorRow = raw < minRow || raw > maxRow ? raw : undefined
  // anchorSpan not virtual-tracked (word/line extend, irrelevant to
  // keyboard-scroll round-trip) — plain clamp from current row.
  if (s.anchorSpan) {
    const shift = (p: Point): Point => ({
      col: p.col,
      row: clamp(p.row + dRow, minRow, maxRow),
    })
    s.anchorSpan = {
      lo: shift(s.anchorSpan.lo),
      hi: shift(s.anchorSpan.hi),
      kind: s.anchorSpan.kind,
    }
  }
}

/**
 * Shift the whole selection (anchor + focus + anchorSpan) by dRow, clamped