js/theme.js
function isFixed(element) {
    var nodeName = element.nodeName;

    if (nodeName === 'BODY' || nodeName === 'HTML') {
      return false;
    }

    if (getStyleComputedProperty(element, 'position') === 'fixed') {
      return true;
    }

    var parentNode = getParentNode(element);

    if (!parentNode) {
      return false;
    }

    return isFixed(parentNode);
  }
function isFixed(element) {
    var nodeName = element.nodeName;

    if (nodeName === 'BODY' || nodeName === 'HTML') {
      return false;
    }

    if (getStyleComputedProperty(element, 'position') === 'fixed') {
      return true;
    }

    var parentNode = getParentNode(element);

    if (!parentNode) {
      return false;
    }

    return isFixed(parentNode);
  }
Check if the given element is fixed or is inside a fixed parent
@method
@memberof Popper.Utils
@argument {Element} element
@argument {Element} customContainer
@returns {Boolean} answer to "isFixed?"