✏ EDIT: theme-templates.php

<?php

/**
 * Sets a custom slug when creating auto-draft template parts.
 *
 * This is only needed for auto-drafts created by the regular WP editor.
 * If this page is to be removed, this will not be necessary.
 *
 * @since 5.9.0
 *
 * @param int $post_id Post ID.
 */
function wp_set_unique_slug_on_create_template_part( $post_id ) {
	$post = get_post( $post_id );
	if ( 'auto-draft' !== $post->post_status ) {
		return;
	}

if ( ! $post->post_name ) {
		wp_update_post(
			array(
				'ID'        => $post_id,
				'post_name' => 'custom_slug_' . uniqid(),
			)
		);
	}

$terms = get_the_terms( $post_id, 'wp_theme' );
	if ( ! is_array( $terms ) || ! count( $terms ) ) {
		wp_set_post_terms( $post_id, get_stylesheet(), 'wp_theme' );
	}
}

/**
 * Generates a unique slug for templates.
 *
 * @access private
 * @since 5.8.0
 *
 * @param string $override_slug The filtered value of the slug (starts as `null` from apply_filter).
 * @param string $slug          The original/un-filtered slug (post_name).
 * @param int    $post_id       Post ID.
 * @param string $post_status   No uniqueness checks are made if the post is still draft or pending.
 * @param string $post_type     Post type.
 * @return string The original, desired slug.
 */
function wp_filter_wp_template_unique_post_slug( $override_slug, $slug, $post_id, $post_status, $post_type ) {
	if ( 'wp_template' !== $post_type && 'wp_template_part' !== $post_type ) {
		return $override_slug;
	}

if ( ! $override_slug ) {
		$override_slug = $slug;
	}

/*
	 * Template slugs must be unique within the same theme.
	 * TODO - Figure out how to update this to work for a multi-theme environment.
	 * Unfortunately using `get_the_terms()` for the 'wp-theme' term does not work
	 * in the case of new entities since is too early in the process to have been saved
	 * to the entity. So for now we use the currently activated theme for creation.
	 */
	$theme = get_stylesheet();
	$terms = get_the_terms( $post_id, 'wp_theme' );
	if ( $terms && ! is_wp_error( $terms ) ) {
		$theme = $terms[0]->name;
	}

$check_query_args = array(
		'post_name__in'  => array( $override_slug ),
		'post_type'      => $post_type,
		'posts_per_page' => 1,
		'no_found_rows'  => true,
		'post__not_in'   => array( $post_id ),
		'tax_query'      => array(
			array(
				'taxonomy' => 'wp_theme',
				'field'    => 'name',
				'terms'    => $theme,
			),
		),
	);
	$check_query      = new WP_Query( $check_query_args );
	$posts            = $check_query->posts;

if ( count( $posts ) > 0 ) {
		$suffix = 2;
		do {
			$query_args                  = $check_query_args;
			$alt_post_name               = _truncate_post_slug( $override_slug, 200 - ( strlen( $suffix ) + 1 ) ) . "-$suffix";
			$query_args['post_name__in'] = array( $alt_post_name );
			$query                       = new WP_Query( $query_args );
			++$suffix;
		} while ( count( $query->posts ) > 0 );
		$override_slug = $alt_post_name;
	}

return $override_slug;
}

/**
 * Enqueues the skip-link styles.
 *
 * @access private
 * @since 6.4.0
 * @since 7.0.0 A script is no longer printed in favor of being added via {@see _block_template_add_skip_link()}.
 *
 * @global string $_wp_current_template_content
 */
function wp_enqueue_block_template_skip_link() {
	global $_wp_current_template_content;

// Back-compat for plugins that disable functionality by unhooking this action.
	if ( ! has_action( 'wp_footer', 'the_block_template_skip_link' ) ) {
		return;
	}
	remove_action( 'wp_footer', 'the_block_template_skip_link' );

// Early exit if not a block theme.
	if ( ! current_theme_supports( 'block-templates' ) ) {
		return;
	}

// Early exit if not a block template.
	if ( ! $_wp_current_template_content ) {
		return;
	}

wp_enqueue_style( 'wp-block-template-skip-link' );
}

/**
 * Enables the block templates (editor mode) for themes with theme.json by default.
 *
 * @access private
 * @since 5.8.0
 */
function wp_enable_block_templates() {
	if ( wp_is_block_theme() || wp_theme_has_theme_json() ) {
		add_theme_support( 'block-templates' );
	}
}

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