tableheader.js throws error when running Drupal inside an iframe

We have an end-2-end test-suite running with Cypress.io (https://www.cypress.io/)
All pages that have tableheader.js included fail. One of those page is /admin/content so obviously this makes it hard to test the Drupal admin backend.

The error is Uncaught TypeError: Cannot read property 'displace' of undefined
on last line in tableheader.js

(function ($, Drupal, displace) {

   ...

})(jQuery, Drupal, window.parent.Drupal.displace);

Actually window.parent is the first undefined object here.

The reason is:
1. Cypress.io is running the test-suite in a browser.
2. It includes the Drupal website inside an iframe and starts interacting with it (that how Cypress.io works).
3. Now window.parent.Drupal.displace tries to interact with 1. the parent of the iframe. But this is forbidden due to XSS-prevention on browser level.

When reading javascript / browser documentation I read that window.parent refers to the enclosing page if the script is running in an iframe. Otherwise window.parent refers to the page itself. See https://developer.mozilla.org/en-US/docs/Web/API/Window/parent

When I replace window.parent.Drupal.displace with window.Drupal.displace my problems are fixed.

I cannot think about a use case where Drupal renders an iframe with a tableheader inside it.
So I wonder why we have window.parent in place now?