Force the display of hidden meta boxes

[danielbachhuber]

Description

Forces the display of hidden meta boxes in Gutenberg, because Gutenberg doesn't have UI for managing meta boxes.

It remains to be seen whether Gutenberg will have the concept of hidden meta boxes, so this is an interim solution to the problem that may become the permanent solution.

Fixes #6495
Fixes #5841
To be documented in #4186

How has this been tested?

1. Register a custom meta box:

add_action( 'add_meta_boxes', function(){
	add_meta_box( 'gutenberg-test', 'Gutenberg Test', function(){
		echo 'Hello World';
	}, get_post_type(), 'side' );
});

2. View the meta box as present in the Classic Editor right column:

3. Hide the meta box using Screen Options:

4. Notice that the meta box displays correctly in Extended Settings:

Types of changes

Bug fix.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[youknowriad]

Agreed with this change, let's wait for other opinions though

[danielbachhuber]

@youknowriad Do you want to flag whomever else you'd like an opinion from?

[youknowriad]

cc @mtias @karmatosed

[jasmussen]

I think this is a good solution for back compat. You can also collapse metaboxes if you like.

[karmatosed]

Looks like a good idea to me, thanks.