Plugin: Remove lingering references to integrated classic editor

[aduth]

Closes #12888

This pull request seeks to remove all remaining references to the classic editor option which had been supported in Gutenberg versions preceding the release to WordPress 5.0. Since WordPress 5.0, this functionality has been broken in the Gutenberg plugin. Users should use the Classic Editor plugin instead for the classic editor experience.

Primarily impacted:

• There were a handful of filters affecting the behavior of revisions to ensure that the user would be directed back to either the classic editor or the Gutenberg editor, depending on from they had arrived at the revisions screen. Since this can be assumed to be the block editor, it is no longer necessary (including the ?gutenberg query argument).
• The "JavaScript disabled" messaging would direct the user to the (broken) classic editor experience. It now directs to the Classic Editor plugin instead.
• While not entirely related to "classic editor", the Meta Box documentation was left outdated from the related pull request Plugin: Deprecate metabox supports, fall back to core #13449 . (Aside: This serves as testament to the fact that user-facing documentation shouldn't be written as so tied to a specific implementation, as it was already quite out of date before these changes)

Testing instructions:

Verify there are no regressions in the behavior of:

• Navigating to and from the revisions screen
• Disabled JavaScript messaging directing to the Classic Editor plugin

[oandregal]

Tested this with WordPress 5.0.3.

Disabling JS

• using Gutenberg master: I'm shown a link with &classic-editor that doesn't work.
• using this PR: correctly points to the plugin.

Revisions

• using Gutenberg master: when I try to navigate from a revision to the editor, I face a WSOD.
• using this PR: I'm correctly redirected to the block editor. I've also checked that after installing and activating the classic plugin I'm redirected properly to the classic editor.

Load a post with &classic-editor

• using Gutenberg master: WSOD.
• using this PR: the block editor loads fine. After installing and activating the classic plugin, the classic editor is loaded instead.

I'm not very knowledgeable about meta boxes, but in the "Testing, Converting, and Maintaining Existing Meta Boxes" section, the document says:

WordPress will fall back to the Classic editor, where the meta box will continue working as before.

Explicitly setting __block_editor_compatible_meta_box to true will cause WordPress to stay in Gutenberg (assuming another meta box doesn't cause a fallback).

If we're using this PR to update the docs, perhaps that's something that we also want to tackle. Happy to address it in a follow-up PR if necessary, so that's why I approve this.

[aduth]

Thanks for the thorough testing @nosolosw !

If we're using this PR to update the docs, perhaps that's something that we also want to tackle.

I really didn't want to tackle documentation here, but a keyword search for classic-editor had me stumble upon the document in a state which was, to me, offensively inaccurate. My hope was to get it to a point of at least moderately representative, and not much more than that.

I'm not entirely clear what from your quoted text would be considered an issue. There is still a classic editor fallback for incompatible meta boxes present in core, and this is expected to continue to work today. All the same, I think further improvements should be done separately.

[oandregal]

There is still a classic editor fallback for incompatible meta boxes present in core

That's exactly what confuses me. If core doesn't provide the classic editor any more, how do we fallback to it when a meta box is incompatible?

[aduth]

There is still a classic editor fallback for incompatible meta boxes present in core

That's exactly what confuses me. If core doesn't provide the classic editor any more, how do we fallback to it when a meta box is incompatible?

I was operating on an assumption that the classic editor was still "secretly" bundled with core, and that either the presence of the Classic Editor plugin or a meta box marked with __block_editor_compatible_meta_box would just activate it.

This must have changed at some point though. Adding the meta box flag does show a message, but it's still the block editor.

I guess the documentation should be updated accordingly then.

---

Demo Code:

<?php
/**
 * Plugin name: Demo Meta
 */

function demo_meta_box_add() {
	add_meta_box(
		'demo-meta-box',
		'Demo Meta Box',
		'__return_empty_string',
		'post',
		'normal',
		'high',
		array(
			'__block_editor_compatible_meta_box' => false,
		)
	);
}
add_action( 'add_meta_boxes', 'demo_meta_box_add' );

[oandregal]

Follow up at #13733