code: function(immediately) {
          var box = new $.Rect(
              this._contentBounds.x,
              this._contentBounds.y + (this._contentBounds.height / 2),
              this._contentBounds.width,
              0);
          return this.fitBounds(box, immediately);
      }
docs: Zooms so the image just fills the viewer horizontally.
  @param {Boolean} immediately
  @return {OpenSeadragon.Viewport} Chainable.