Issue Overview

A lot of existing meta boxes, especially in enterprise environments, rely on server side validation / sanitation, including meaningful feedback to the user.

This fails in Gutenberg currently.

Steps to Reproduce (for bugs)

1. Setup validation as in https://tommcfarlin.com/post-meta-data-error-messages/ , this requires a CPT 'Location' and a meta box with a single text field.

2. Create a new location, fill in data and save it.

3. API Call will fail with an error "Call to undefined function add_settings_error()", which is used in the example

4. Replace add_settings_error() with a simple string to continue testing.
   https://gist.github.com/wsydney76/0dd717fd061908d8037d29ce12569b86

5. Enter a value and save.

6. Then leave the text field blank and save. "Post saved" indicates ok, no error messages shown, the field is still blank.

7. Reload. The old field value is still present.

8. Enter content like 'with tags and save. Tags will be stripped server side, but that is not reflected in the editor. Only a reload shows the altered content.

Gutenberg 1.9

Expected Behavior

The user has to get information about the data being rejected or changed server side. After a "successful" update the content in the editor must not differ from the content in the database.

Current Behavior

If it works at all, the users workflow is broken, potentially resulting in invalid data.

Possible Solution

Documentation is needed:

• Which kind of implementations of server side validation breaks in Gutenberg?
• How can it be changed to work in Gutenberg?