Unable to save posts/pages with specific shortcode

[GlennMartin1]

Describe the bug
A post cannot save/autosave when inserting a shortcode containing the [rpwe] shortcode. This shortcode is from the plugin https://wordpress.org/plugins/recent-posts-widget-extended/

To Reproduce

1. Install and activate Recent Posts Widget Extended https://wordpress.org/plugins/recent-posts-widget-extended/
2. Create/modify a post/page containing a shortcode with [rpwe]
3. Save or wait for autosave
4. Note that save/autosave fails.

[designsimply]

Any issues with a release candidate should be added to the still open milestone. Just note in the issue body the version you are testing such as 3.9.0-rc.1. (ref)

[GlennMartin1]

@designsimply Thanks - can you please add this to the 3.9 milestone?

[mcsf]

OK, a few notes here:

• I was able to repro in 3.8, so this isn't 3.9-specific, thus a priori invalidating the need to rush it into this milestone.
• Saving doesn't actually fail, as confirmable via inspection of server log and saved page (or reloading the editor).
• The issue is that the response to the POST request to save the page, while of status code 200, is malformed due to injection of markup and PHP warnings caused by operation of the plugin:

The warnings are of the following type:

Warning: Cannot modify header information - headers already sent by (output started at /var/www/html/wp-content/plugins/recent-posts-widget-extended/includes/functions.php:309) in /var/www/html/wp-includes/rest-api/class-wp-rest-server.php on line 1248

I'd consider this an issue to bring to the plugin developers. In any case, I'll remove it from the 3.9 milestone.

[chrisvanpatten]

Sounds like this relates to #4936.

[designsimply]

Awesome sleuthing! I learned to look deeper at API responses because of the replies here! 🤓 I might not have tried that before because I usually only scan for something other than a 200 response.

I noticed the issue was reported to the plugin developers at https://wordpress.org/support/topic/gutenberg-compatibility-15/ which is awesome, and I will close the issue here.