Workaround for Gutenberg Issue 39423

workaround_39423.php

<?php

/**
 * Plugin Name: Workaround for Gutenberg Issue #39423
 * Description: Override core until header block colors are fixed. <a href="https://github.com/WordPress/gutenberg/issues/39423">https://github.com/WordPress/gutenberg/issues/39423</a>
 * Author:      Tony Gravagno (<a href="https://profiles.wordpress.org/starbuck/">wordpress.org @starbuck</a>)
 * License:     MIT
 */

/**
 * The problem : Headers over a cover block (and perhaps elsewhere) are not styled.
 * Cause: wp-includes/blocks/heading.php is not adding class 'has-text-color'
 * This workaround cancels the default action from heading.php, and replaces
 * it with the exact same functionality, but adding class 'has-text-color' to
 * _all_ header tags.
 * That causes this CSS to not activate in the page:
 *    .wp-block-cover h1:not(.has-text-color) ... h2, h3...  { color:inherit; }
 * So headers DO have that class, they do NOT inherit the color from the container,
 * and they are rendered with the color assigned to them in the theme or style variation.
 * To ensure headings have the right color, set colors for the default Heading block,
 * and all H1-6 tags as well, so that an explicit color will be used. Otherwise
 * the color that will be used is (at this time) undefined.
 * 
 * To use:
 * 1) Create folder wp-content/mu-plugins. Set owner/group, and 755 perms.
 * 2) Save this file into that folder as workaround_39423.php. Set owner/group, and 664 perms.
 * 
 * You should now see this in your Must-Use Plugins list.
 * Just refresh your pages and you should see the correctly styled colors on headings.
 * 
 * Notes:
 * - Unfortunately a 'doing_it_wrong_trigger_error' is generated when this is used and
 * WP_DEBUG is active. For this specific issue, a filter here returns false.
 * - A local_log function is included here if you want to see when the events trigger.
 */


// Basic security, prevents file from being loaded directly.
defined('ABSPATH') or die('Cheatin&#8217; uh?');

/**
 *
 * @param array  $attributes Attributes of the block being rendered.
 * @param string $content Content of the block being rendered.
 *
 * @return string The content of the block being rendered.
 */
function block_core_heading_render_39423($attributes, $content)
{
    local_log('Processing header block');
    if (!$content) {
    local_log('Returning, no content');
        return $content;
    }

    $p = new WP_HTML_Tag_Processor($content);
    local_log('Processing headers');
    $header_tags = array('H1', 'H2', 'H3', 'H4', 'H5', 'H6');
    while ($p->next_tag()) {
        if (in_array($p->get_tag(), $header_tags, true)) {
            $p->add_class('wp-block-heading');
            $p->add_class('has-text-color'); // This is the only override line
            break;
        }
    }

    local_log('Returning');
    return $p->get_updated_html();
}

/**
 * Registers the `core/heading` block on server.
 */
function register_block_core_heading_39423()
{
    local_log('Registering block type');
    register_block_type_from_metadata(
        WPINC . '/blocks/heading',
        array(
            'render_callback' => 'block_core_heading_render_39423',
        )
    );
}

add_action('init', function () {
    local_log('init action');
    remove_action('init', 'register_block_core_heading');
    add_action('init', 'register_block_core_heading_39423');
    add_filter('doing_it_wrong_trigger_error', function ($value, $function_name, $message, $version) {
       if ('WP_Block_Type_Registry::register' === $function_name)
            if ('Block type "core/heading" is already registered.' === $message) {
                local_log('Expected error for core/heading');        
                return false;
            }
    },10,4);
}, 1);

/**
 *  Temporary Logging
 */
function local_log($message) {
    if(!false) return; // toggle true/False to enable/disable
    $log_path = WP_CONTENT_DIR  . DIRECTORY_SEPARATOR . 'uploads' . DIRECTORY_SEPARATOR . 'local.log';
    file_put_contents(
        $log_path,
        $message . PHP_EOL,
        FILE_APPEND
    );
}