q1

How do I add a Panel the Document sidebar?

This question has come up multiple times for me.

To be clear, this is the sidebar we are discussing:

TL;DR Solution

Use a standard meta box to add an item to the bottom of the panel.

Full Solution

There is no API available in Gutenberg to insert items into the sidebar. It has been discussed and may be implemented in the future.

The current solution is to use add_meta_box to register a meta box in PHP and set its context to side. This will add it to the bottom of the panel.

Code Example

/**
 * Register meta box(es).
 */
function register_meta_boxes() {
	add_meta_box(
		'meta-box-id',
		'Custom Meta Box',
		__NAMESPACE__ . '\render_meta_box',
		'post',
		'side' // this is important
	);
}
add_action( 'add_meta_boxes', __NAMESPACE__ . '\register_meta_boxes' );

/**
 * Meta box display callback.
 *
 * @param \WP_Post $post Current post object.
 */
function render_meta_box( $post ) {
	?>
	<p>Added to the Document Sidebar</p>
	<?php
}