Unable to submit form via Ajax in a Canvas page

Overview

This popped up when testing a Mailchimp signup form, which uses Ajax to submit, but the same occurs with a Webform where it is configured to use Ajax.

On submit you get:
Drupal\Core\Form\FormAjaxException occurred during rendering of component bb97630b-f7e5-4d9e-b3f5-b3dc68ca0b7f in Page Home (8), field components:

Some easy steps to reproduce:

1. git clone git@git.drupal.org:project/drupal_cms.git
2. ddev launch
3. ddev install-byte
4. Go to /admin/structure/webform/manage/newsletter_signup/settings and enable 'Use Ajax'
5. Visit any page on the site and try to submit the signup form in the footer
6. See the browser error, then go to /admin/reports/dblog to get the actual error

Proposed resolution

TBC!

User interface changes

N/A