Duplicate meta box content on non-Gutenberg screens

[swissspidy]

Issue Overview

We're still using CMB on some projects and are working on migrating everything to Gutenberg.

Now we noticed that the meta boxes contain duplicate content on some post types Gutenberg isn't enabled for (mainly because they don't show up in the REST API). Every field is shown twice.

It has to do with #2804 and how Gutenberg tricks plugins into registering meta boxes to be used in the new editor.

However, this should not run when Gutenberg can't even be used for the current post or post type.

Steps to Reproduce (for bugs)

1. Add some meta boxes to a
2. Notice how meta box content is duplicated.

Expected Behavior

Gutenberg doesn't change anything when using the classic editor.

Current Behavior

The meta box content is duplicated.

Possible Solution

Add some gutenberg_can_edit_post() and gutenberg_can_edit_post_type() checks. PR imminent.

Screenshots / Video

Without Gutenberg:

With Gutenberg:

Related Issues and/or PRs

#2804