diff -r be944660c56a -r 3d72ae0968f4 wp/wp-includes/js/wp-embed.js
--- a/wp/wp-includes/js/wp-embed.js	Wed Sep 21 18:19:35 2022 +0200
+++ b/wp/wp-includes/js/wp-embed.js	Tue Sep 27 16:37:53 2022 +0200
@@ -27,6 +27,11 @@
 		return;
 	}
 
+	/**
+	 * Receive embed message.
+	 *
+	 * @param {MessageEvent} e
+	 */
 	window.wp.receiveEmbedMessage = function( e ) {
 		var data = e.data;
 
@@ -102,9 +107,11 @@
 			iframeClone, i, source, secret;
 
 		for ( i = 0; i < iframes.length; i++ ) {
+			/** @var {IframeElement} */
 			source = iframes[ i ];
 
-			if ( ! source.getAttribute( 'data-secret' ) ) {
+			secret = source.getAttribute( 'data-secret' );
+			if ( ! secret ) {
 				/* Add secret to iframe */
 				secret = Math.random().toString( 36 ).substr( 2, 10 );
 				source.src += '#?secret=' + secret;
@@ -117,6 +124,16 @@
 				iframeClone.removeAttribute( 'security' );
 				source.parentNode.replaceChild( iframeClone, source );
 			}
+
+			/*
+			 * Let post embed window know that the parent is ready for receiving the height message, in case the iframe
+			 * loaded before wp-embed.js was loaded. When the ready message is received by the post embed window, the
+			 * window will then (re-)send the height message right away.
+			 */
+			source.contentWindow.postMessage( {
+				message: 'ready',
+				secret: secret
+			}, '*' );
 		}
 	}