B2G/gecko/toolkit/content/tests/fennec-tile-testapp/chrome/content/WidgetStack.js
// null or undefined, only move the other axis
null or undefined, only move the other axis
null or undefined only move the other axis
function panTo(x, y) {
    if (x == undefined || x == null)
      x = this._viewingRect.x;
    if (y == undefined || y == null)
      y = this._viewingRect.y;
    this.panBy(x - this._viewingRect.x, y - this._viewingRect.y, true);
  }
function panTo ( x , y ) { if ( x == undefined || x == null ) x = this . _viewingRect . x ; if ( y == undefined || y == null ) y = this . _viewingRect . y ; this . panBy ( x - this . _viewingRect . x , y - this . _viewingRect . y , true ) ; }
null or undefined, only move the other axis
null or undefined only move the other axis