docs: function getFixedParent (el) {
      let currentParent = el.offsetParent,
          parent;
  
      while (parent === undefined) {
  
          if (currentParent === null) {
              parent = null;
              break;
          }
  
          if (css(currentParent, 'position') !== 'fixed') {
              currentParent = currentParent.offsetParent;
          } else {
              parent = currentParent;
          }
      }
  
      return parent;
  }