Function offset

src/utils/dom.ts:27–33

(elem: HTMLElement)

 * @param elem  DOM 元素
 */
export function offset(elem: HTMLElement): { left: number; top: number } {
  const bounding = elem.getBoundingClientRect()
  return {
    left: window.pageXOffset + bounding.left,
    top: window.pageYOffset + bounding.top,
  }
}

/**
 * 不管是 MutationObserver 还是 DOMNodeRemoved，