always-load-block-styles-on-demand.php

<?php
/**
 * Plugin Name: Always Load Block Styles on Demand
 * Plugin URI: https://gist.github.com/westonruter/99af37489334927aaf04a46eaf2a11eb
 * Description: In classic themes a lot more CSS is added to a page than is needed because when the HEAD is rendered before the rest of the page, so it is not yet known what blocks will be used. This can be fixed with output buffering. This is a demonstration plugin for Core Trac ticket #43258 which adds output buffering for the rendered template.
 * Requires at least: 6.8-beta1
 * Version: 0.1.0
 * Author: Weston Ruter
 * Author URI: https://weston.ruter.net/
 * License: GPLv2 or later
 * License URI: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 * Update URI: https://gist.github.com/westonruter/99af37489334927aaf04a46eaf2a11eb
 * GitHub Plugin URI: https://gist.github.com/westonruter/99af37489334927aaf04a46eaf2a11eb
 *
 * @package always-load-block-styles-on-demand
 */

add_action(
	'after_setup_theme',
	static function () {
		if ( wp_is_block_theme() ) {
			return;
		}

		// Load separate block styles so that the large block-library stylesheet is not enqueued unconditionally,
		// and so that block-specific styles will only be enqueued when they are used on the page.
		add_filter( 'should_load_separate_core_block_assets', '__return_true' );

		// Also ensure that block assets are loaded on demand (although the default value is should_load_separate_core_block_assets).
		add_filter( 'should_load_block_assets_on_demand', '__return_true' );

		// While normally late styles are printed, there is a filter to disable late styles, so this makes sure they are printed.
		add_filter( 'print_late_styles', '__return_true', 100 );

		// Print a placeholder comment to inject late styles right after the head styles are printed.
		$placeholder = sprintf( '<!--%s:%s-->', 'late_styles', wp_generate_uuid4() );
		remove_action( 'wp_head', 'wp_print_styles', 8 );
		add_action(
			'wp_head',
			static function () use ( $placeholder ) {
				wp_print_styles();
				echo $placeholder;
			},
			8
		);

		// Replace logic that prints scripts and styles in the footer,
		$late_styles = '';
		remove_action( 'wp_print_footer_scripts', '_wp_footer_scripts' );
		add_action(
			'wp_print_footer_scripts',
			static function () use ( &$late_styles ) {
				ob_start();
				print_late_styles();
				$late_styles = ob_get_clean();

				print_footer_scripts();
			}
		);

		// Replace placeholder with the captured late styles.
		add_filter(
			'wp_template_output_buffer',
			static function ( $buffer ) use ( $placeholder, &$late_styles ) {
				return str_replace( $placeholder, $late_styles, $buffer );
			}
		);
	}
);