Missing documentation on hooks for Gutenberg metaboxes

[matthewmcvickar]

I am trying to extend/alter the functionality of the 'Post Excerpt' metabox in Gutenberg and cannot find documentation or suggestions on how to do this or a clear status of whether it is possible. I have extensively searched issues and PRs here, checked the Gutenberg developer handbook, and searched Stack Overflow, but have not found any answers. Can anyone point me in the right direction?

Where I've looked, and what I've found:

• In the Block Editor Handbook under Developer Documentation > SlotFills Reference there is documentation for many of the sidebar controls, but not the excerpt metabox.
• In the Block Editor Handbook under Developer Documentation > Backward Compatibility > Meta Boxes, there is documentation for converting existing custom metaboxes but nothing about built-in/editor core metaboxes.
• New Block: Post Excerpt #19698 contains a new 'Post Excerpt' block (presumably changing the sidebar metabox into its own block in the editor?), but I can’t determine if this has been added to the Gutenberg plugin, if it has been added to the Gutenberg in WP core, or is planned for a future Gutenberg plugin release. What is the status of this block, and where can I see the status of blocks in general?
• Hooks and filters support and documentation #1316 and Document WordPress classic editor integration points and Gutenberg equivalents #4151 both refer to hook and filter documentation for Classic and Gutenberg editors, but the ‘Gutenberg Migration Guide’ information repo it points to is archived, presumably out of date (it's three years old), and includes nothing about the Excerpt metabox.
• Plugins: Let's discuss pluggable areas (w. updated blueprints) #1352 discusses 'pluggable areas' in the Gutenberg editor, but the thread is closed in favor of Native Gutenberg Extensibility Overview #3330, which ends with "Closing this as everything has an implementation by now. Improvements should come in individual tasks. Thanks all!" Unfortunately, it isn't clear what "everything" is in this context or where the documentation for that implementation is.
• Gutenberg Editor : Custom Shortcodes and Metaboxes #2002 asks about customizing/controlling the metaboxes in the Gutenberg sidebar, but the answer points to a now-missing piece of documentation.

What am I missing?

(Specifically, the changes I want to make to the 'Post Excerpt' metabox are: 1) add a brief instruction about keeping the word count around 20 for the excerpt, 2) add a live word counter to the excerpt editor, so that writers are reminded to keep things brief in their excerpts.)

[talldan]

@matthewmcvickar It doesn't look like this input can be extended, but it seems like it would be possible to replace it with something else.

A new panel can be added to the document settings using the appropriate slot:
https://developer.wordpress.org/block-editor/developers/slotfills/plugin-document-setting-panel/

And the existing panel can be removed with the following code snippet:

wp.data.dispatch( 'core/edit-post' ).removeEditorPanel( 'post-excerpt' );

[alkah3st]

Why is it possible to hook into and extend the Featured Image metabox in Gutenberg but not the Post Excerpt metabox?

For example, I can do this to change the instructions with the Featured Image:

var el = wp.element.createElement;

function wrapPostFeaturedImage( OriginalComponent ) { 
	return function( props ) {
			return (
					el(
							wp.element.Fragment,
							{}, 
							'',
							el(
									OriginalComponent,
									props
							),
							'The featured image is used throughout the site to show a thumbnail representing the post.'
					)
			);
	} 
} 

wp.hooks.addFilter( 
	'editor.PostFeaturedImage', 
	'wpx/wrap-post-featured-image', 
	wrapPostFeaturedImage
);

Why do we have to replace the entire Post Excerpt metabox to achieve the same with it?

[ibes]

Any wise person who can help there?

This is also a limitation that bothers me with Gutenberg.
And I don't know if it is an architectural limitattion or just a knowledge limitiation that I can learn to overcome.