diff -r 490d5cc509ed -r cf61fcea0001 wp/wp-includes/js/wp-auth-check.js
--- a/wp/wp-includes/js/wp-auth-check.js	Tue Jun 09 11:14:17 2015 +0000
+++ b/wp/wp-includes/js/wp-auth-check.js	Mon Oct 14 17:39:30 2019 +0200
@@ -16,10 +16,12 @@
 			});
 
 			frame = $('<iframe id="wp-auth-check-frame" frameborder="0">').attr( 'title', noframe.text() );
-			frame.load( function() {
+			frame.on( 'load', function() {
 				var height, body;
 
 				loaded = true;
+				// Remove the spinner to avoid unnecessary CPU/GPU usage.
+				form.removeClass( 'loading' );
 
 				try {
 					body = $(this).contents().find('body');
@@ -46,7 +48,7 @@
 				}
 			}).attr( 'src', form.data('src') );
 
-			$('#wp-auth-check-form').append( frame );
+			form.append( frame );
 		}
 
 		$( 'body' ).addClass( 'modal-open' );
@@ -75,6 +77,7 @@
 		if ( typeof adminpage !== 'undefined' && ( adminpage === 'post-php' || adminpage === 'post-new-php' ) &&
 			typeof wp !== 'undefined' && wp.heartbeat ) {
 
+			$(document).off( 'heartbeat-tick.wp-auth-check' );
 			wp.heartbeat.connectNow();
 		}