Issue #3351600 by andrew.farquharson, Ranjit1032002, Anybody, sanduhrs,...

    // CKEditor 5 in modals can work as expected.

    // @see https://api.jqueryui.com/dialog/#method-_allowInteraction

    _allowInteraction(event) {

      // Fixes "Uncaught TypeError: event.target.classList is undefined"

      // in Firefox (only).

      // @see https://www.drupal.org/project/drupal/issues/3351600

      if (event.target.classList === undefined) {

        return this._super(event);

      }

      return event.target.classList.contains('ck') || this._super(event);

    },

  });