full-site-editing.php

<?php
/**
 * Full Site Editing Utils
 *
 * @package gutenberg
 */

/**
 * Returns whether the current theme is an FSE theme or not.
 *
 * @return boolean Whether the current theme is an FSE theme or not.
 */
function gutenberg_is_fse_theme() {
	return is_readable( get_theme_file_path( '/block-templates/index.html' ) ) ||
		is_readable( get_theme_file_path( '/templates/index.html' ) );
}

/**
 * Returns whether the current theme is FSE-enabled or not.
 *
 * @return boolean Whether the current theme is FSE-enabled or not.
 */
function gutenberg_supports_block_templates() {
	return gutenberg_is_fse_theme() || current_theme_supports( 'block-templates' );
}

/**
 * Removes legacy pages from FSE themes.
 */
function gutenberg_remove_legacy_pages() {
	if ( ! gutenberg_is_fse_theme() ) {
		return;
	}

	global $submenu;
	if ( isset( $submenu['themes.php'] ) ) {
		$indexes_to_remove = array();
		foreach ( $submenu['themes.php'] as $index => $menu_item ) {
			if ( false !== strpos( $menu_item[2], 'customize.php' ) && ! gutenberg_site_requires_customizer() ) {
				$indexes_to_remove[] = $index;
			}

			if ( false !== strpos( $menu_item[2], 'site-editor.php' ) ) {
				$indexes_to_remove[] = $index;
			}

			if ( false !== strpos( $menu_item[2], 'gutenberg-widgets' ) ) {
				$indexes_to_remove[] = $index;
			}
		}

		foreach ( $indexes_to_remove as $index ) {
			unset( $submenu['themes.php'][ $index ] );
		}
	}
}

add_action( 'admin_menu', 'gutenberg_remove_legacy_pages' );

/**
 * Removes legacy adminbar items from FSE themes.
 *
 * @param WP_Admin_Bar $wp_admin_bar The admin-bar instance.
 */
function gutenberg_adminbar_items( $wp_admin_bar ) {

	// Early exit if not an FSE theme.
	if ( ! gutenberg_is_fse_theme() ) {
		return;
	}

	// Remove customizer link, if this site does not rely on them for plugins or theme options.
	if ( ! gutenberg_site_requires_customizer() ) {
		$wp_admin_bar->remove_node( 'customize' );
		$wp_admin_bar->remove_node( 'customize-background' );
		$wp_admin_bar->remove_node( 'customize-header' );
		$wp_admin_bar->remove_node( 'widgets' );
	}

	// Add site-editor link.
	if ( ! is_admin() && current_user_can( 'edit_theme_options' ) ) {
		$wp_admin_bar->add_node(
			array(
				'id'    => 'site-editor',
				'title' => __( 'Edit site', 'gutenberg' ),
				'href'  => admin_url( 'themes.php?page=gutenberg-edit-site' ),
			)
		);
	}
}

add_action( 'admin_bar_menu', 'gutenberg_adminbar_items', 50 );

/**
 * Check if any plugin, or theme features, are using the Customizer.
 *
 * @return bool A boolean value indicating if Customizer support is needed.
 */
function gutenberg_site_requires_customizer() {
	if ( has_action( 'customize_register' ) ) {
		return true;
	}

	return false;
}

/**
 * Tells the script loader to load the scripts and styles of custom block on site editor screen.
 *
 * @param bool $is_block_editor_screen Current decision about loading block assets.
 * @return bool Filtered decision about loading block assets.
 */
function gutenberg_site_editor_load_block_editor_scripts_and_styles( $is_block_editor_screen ) {
	return ( is_callable( 'get_current_screen' ) && get_current_screen() && 'appearance_page_gutenberg-edit-site' === get_current_screen()->base )
		? true
		: $is_block_editor_screen;
}
add_filter( 'should_load_block_editor_scripts_and_styles', 'gutenberg_site_editor_load_block_editor_scripts_and_styles' );