Method constructor

src/ink/log-update.ts:758–763

(
    origin: Point,
    readonly viewportWidth: number,
  )

  diff: Diff = []

  constructor(
    origin: Point,
    readonly viewportWidth: number,
  ) {
    this.cursor = { ...origin }
  }

  txn(fn: (prev: Point) => [patches: Diff, next: Delta]): void {
    const [patches, next] = fn(this.cursor)