js/leaflet.markercluster.js
function () {
    var bounds = new L.LatLngBounds();

    if (this._topClusterLevel) {
      bounds.extend(this._topClusterLevel._bounds);
    }

    for (var i = this._needsClustering.length - 1; i >= 0; i--) {
      bounds.extend(this._needsClustering[i].getLatLng());
    }

    bounds.extend(this._nonPointGroup.getBounds());

    return bounds;
  }
function () {
    var bounds = new L.LatLngBounds();

    if (this._topClusterLevel) {
      bounds.extend(this._topClusterLevel._bounds);
    }

    for (var i = this._needsClustering.length - 1; i >= 0; i--) {
      bounds.extend(this._needsClustering[i].getLatLng());
    }

    bounds.extend(this._nonPointGroup.getBounds());

    return bounds;
  }
Override FeatureGroup.getBounds as it doesn't work