Block Editor: Prevent the meta box form from being accidentally submitted.

Meta boxes are loaded inside a wrapper form, that the block editor submits as part of the saving process. It shouldn't be submitted manually.

Props pento.
Merges [43880] to trunk.
Fixes #45284.

git-svn-id: https://develop.svn.wordpress.org/trunk@44240 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/post.php

@@ -2210,7 +2210,7 @@ function the_block_editor_meta_boxes() {
 	<?php the_block_editor_meta_box_post_form_hidden_fields( $post ); ?>
 	</form>
 	<?php foreach ( $locations as $location ) : ?>
-		<form class="metabox-location-<?php echo esc_attr( $location ); ?>">
+		<form class="metabox-location-<?php echo esc_attr( $location ); ?>" onsubmit="return false;">
 			<div id="poststuff" class="sidebar-open">
 				<div id="postbox-container-2" class="postbox-container">
 					<?php