When a user makes a change using the off-canvas tray and saves the message that the change was successful appears on the site page. This is a bit odd since the expected behavior of submitting a form in Drupal is that the messages appear at the top of the form.
Additionally form validation messages do appear in the off-canvas tray which strengthens the expectation that the 'success' message will appear there as well.
Present the success message inside the off-canvas tray above the form and do not automatically close the tray on save, but let the user close it when the
Move the messages to the tray.
User interface changes
Messages appear in the tray
Data model changes