Method snapToGraphemeBoundary

src/utils/Cursor.ts:1516–1529

* Snap an arbitrary code-unit offset to the start of the containing grapheme. * If offset is already on a boundary, returns it unchanged.

(offset: number)

   * If offset is already on a boundary, returns it unchanged.
   */
  snapToGraphemeBoundary(offset: number): number {
    if (offset <= 0) return 0
    if (offset >= this.text.length) return this.text.length
    const boundaries = this.getGraphemeBoundaries()
    // Binary search for largest boundary <= offset
    let lo = 0
    let hi = boundaries.length - 1
    while (lo < hi) {
      const mid = (lo + hi + 1) >> 1
      if (boundaries[mid]! <= offset) lo = mid
      else hi = mid - 1
    }
    return boundaries[lo]!
  }
}