Get real element offset with an offsetParent referencing element

getClientOffset.js

/**
 * Get real element offset with an offsetParent referencing element
 *
 * `getBoundingClientRect` return values is not correct under CSS multi-column
 * layout, so we recursively get `offsetLeft`/`offsetTop` instead.
 * @param {HTMLElement} el - element of interest
 * @param {HTMLElement} stopEl - one of the offsetParent as a reference
 * @return {object} object wth top and left offset
 */
export function getClientOffset (el, stopEl) {
  /// tail-called optimize
  function getOffset(el, stopEl, left = 0, top = 0) {
    if (el === stopEl) {
      return { left, top }
    }
    return getOffset(
      el.offsetParent,
      stopEl,
      left + el.offsetLeft,
      top + el.offsetTop
    )
  }
  return getOffset(el, stopEl)
}