Try to restrict user to drag to outside of maxBounds on Leaflet 0.6.4, nicely. (It access private objects, so it might not work properly on another version)

restrict-to-maxBounds.js

L.Map.addInitHook(function() {
  var map = this;
  this.dragging._draggable.on("predrag", function() {
    var bounds_sw = map.latLngToLayerPoint(map.options.maxBounds.getSouthWest()).add(this._newPos);
    var bounds_ne = map.latLngToLayerPoint(map.options.maxBounds.getNorthEast()).add(this._newPos);
    var size = map.getSize();

    if (bounds_ne.y > 0) {
      this._newPos.y -= bounds_ne.y / 1.2;
    }
    if (bounds_sw.x > 0) {
      this._newPos.x -= bounds_sw.x / 1.2;
    }
    if (bounds_ne.x < size.x) {
      this._newPos.x -= (bounds_ne.x - size.x) / 1.2;
    }
    if (bounds_sw.y < size.y) {
      this._newPos.y -= (bounds_sw.y - size.y) / 1.2;
    }
  });
});