Editor: Metaboxes fail to save after heartbeat reauthentication in block editor

[mslinnea]

This pull request contains the previously submitted patch. It also adds a unit test.

Fixes https://core.trac.wordpress.org/ticket/52584

For testing this, you will need to add a metabox to a post. Here is some example code:

<?php

function wporg_add_custom_box() {
   add_meta_box(
         'example_metabox',
         'Meta Box Test',
         function( $post ) {
            $value = get_post_meta( $post->ID, '_wporg_meta_key', true );
            ?>
            <label for="wporg_field">Test metabox</label>
            <textarea id="wporg_field" name="wporg_field" rows="4" cols="50"><?php echo esc_html( $value ); ?></textarea>
            <?php
         },
         'post'
   );
}
add_action( 'add_meta_boxes', 'wporg_add_custom_box' );
function wporg_save_postdata( $post_id ) {
   if ( array_key_exists( 'wporg_field', $_POST ) ) {
      update_post_meta(
            $post_id,
            '_wporg_meta_key',
            $_POST['wporg_field']
      );
   }
}
add_action( 'save_post', 'wporg_save_postdata' );

[github-actions]

Hi @mslinnea! 👋

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[mslinnea]

Please see this recording of this issue, which demos the data loss in the metabox.

[mslinnea]

Merged in 3fbc7e5