Problem
The Add group button of CKEditor toolbar configuration interface is dysfunctional when Bootstrap modals are active.
When you click the Add group button, the whole form is submitted instead of displaying the expected modal dialog.
It seems that dialog.showModal(); from core/modules/ckeditor/js/ckeditor.admin.js:355 is not correctly fired.
Workaround
Turn off Bootstrap modals from the theme settings form.
Comments
Comment #2
markhalliwellThis is likely just another symptom of dialog/modal JS issues.