js/leaflet.markercluster.js
function () {
    if (!this.options.removeOutsideVisibleBounds) {
      return this.getBounds();
    }

    var map = this._map,
      bounds = map.getBounds(),
      sw = bounds._southWest,
      ne = bounds._northEast,
      latDiff = L.Browser.mobile ? 0 : Math.abs(sw.lat - ne.lat),
      lngDiff = L.Browser.mobile ? 0 : Math.abs(sw.lng - ne.lng);

    return new L.LatLngBounds(
      new L.LatLng(sw.lat - latDiff, sw.lng - lngDiff, true),
      new L.LatLng(ne.lat + latDiff, ne.lng + lngDiff, true));
  }
function () {
    if (!this.options.removeOutsideVisibleBounds) {
      return this.getBounds();
    }

    var map = this._map,
      bounds = map.getBounds(),
      sw = bounds._southWest,
      ne = bounds._northEast,
      latDiff = L.Browser.mobile ? 0 : Math.abs(sw.lat - ne.lat),
      lngDiff = L.Browser.mobile ? 0 : Math.abs(sw.lng - ne.lng);

    return new L.LatLngBounds(
      new L.LatLng(sw.lat - latDiff, sw.lng - lngDiff, true),
      new L.LatLng(ne.lat + latDiff, ne.lng + lngDiff, true));
  }
Gets the maps visible bounds expanded in each direction by the size of the screen (so the user cannot see an area we do not cover in one pan)