Function shiftSelectionForFollow

src/ink/selection.ts:625–674

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

 * notifySelectionChange (recursion), must fire listeners directly.
 */
export function shiftSelectionForFollow(
  s: SelectionState,
  dRow: number,
  minRow: number,
  maxRow: number,
): boolean {
  if (!s.anchor) return false
  // Mirror shiftSelection: compute raw (unclamped) positions from virtual
  // if set, else current. This handles BOTH the update path (virtual already
  // set from a prior keyboard scroll) AND the initialize path (first clamp
  // happens HERE via follow-scroll, no prior keyboard scroll). Without the
  // initialize path, follow-scroll-first leaves virtual undefined even
  // though the clamp below occurred → a later PgUp computes debt from the
  // clamped row instead of the true pre-clamp row and never pops the
  // accumulator — getSelectedText double-counts the off-screen rows.
  const rawAnchor = (s.virtualAnchorRow ?? s.anchor.row) + dRow
  const rawFocus = s.focus
    ? (s.virtualFocusRow ?? s.focus.row) + dRow
    : undefined
  if (rawAnchor < minRow && rawFocus !== undefined && rawFocus < minRow) {
    clearSelection(s)
    return true
  }
  // Clamp from raw, not p.row+dRow — so a virtual position coming back
  // in-bounds lands at the TRUE position, not the stale clamped one.
  s.anchor = { col: s.anchor.col, row: clamp(rawAnchor, minRow, maxRow) }
  if (s.focus && rawFocus !== undefined) {
    s.focus = { col: s.focus.col, row: clamp(rawFocus, minRow, maxRow) }
  }
  s.virtualAnchorRow =
    rawAnchor < minRow || rawAnchor > maxRow ? rawAnchor : undefined
  s.virtualFocusRow =
    rawFocus !== undefined && (rawFocus < minRow || rawFocus > maxRow)
      ? rawFocus
      : 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,
    }
  }
  return false
}

export function hasSelection(s: SelectionState): boolean {
  return s.anchor !== null && s.focus !== null