Use both the ID and class for the "poststuff" wrapper.

This adds support for past and future versions of WordPress, and may be removed or changed at a later date.

See: https://core.trac.wordpress.org/ticket/46964#comment:42

Author: JJJ

wp-multi-network/includes/classes/class-wp-ms-networks-admin.php

@@ -391,9 +391,9 @@ public function page_edit_network() {
 			</h1>
 
 			<form method="post" id="edit-network-form" action="">
-				<div id="poststuff">
+				<div id="poststuff" class="poststuff">
 					<div id="post-body" class="metabox-holder columns-2">
-						<div id="post-body-content" style="position: relative;">
+						<div id="post-body-content">
 							<div id="titlediv">
 								<div id="titlewrap">
 									<label class="screen-reader-text" id="title-prompt-text" for="title"><?php echo esc_html_e( 'Enter network title here', 'wp-multi-network' ); ?></label>