js/leaflet.markercluster.js
function (fn) {
    this._queue.push(fn);
    if (!this._queueTimeout) {
      this._queueTimeout = setTimeout(L.bind(this._processQueue, this), 300);
    }
  }
function (fn) {
    this._queue.push(fn);
    if (!this._queueTimeout) {
      this._queueTimeout = setTimeout(L.bind(this._processQueue, this), 300);
    }
  }
Enqueue code to fire after the marker expand/contract has happened