Index: src/wp-includes/class-wp-editor.php
===================================================================
--- src/wp-includes/class-wp-editor.php	(revision 51078)
+++ src/wp-includes/class-wp-editor.php	(working copy)
@@ -1663,19 +1663,27 @@
 		?>
 
 		( function() {
-			var init, id, $wrap;
+			var initialize = function() {
+				var init, id, inPostbox, $wrap;
+				var readyState = document.readyState;
 
-			if ( typeof tinymce !== 'undefined' ) {
-				if ( tinymce.Env.ie && tinymce.Env.ie < 11 ) {
-					tinymce.$( '.wp-editor-wrap ' ).removeClass( 'tmce-active' ).addClass( 'html-active' );
+				if ( readyState !== 'complete' && readyState !== 'interactive' ) {
 					return;
 				}
 
 				for ( id in tinyMCEPreInit.mceInit ) {
-					init = tinyMCEPreInit.mceInit[id];
-					$wrap = tinymce.$( '#wp-' + id + '-wrap' );
+					init      = tinyMCEPreInit.mceInit[id];
+					$wrap     = tinymce.$( '#wp-' + id + '-wrap' );
+					inPostbox = $wrap.parents( '.postbox' ).length > 0;
 
-					if ( ( $wrap.hasClass( 'tmce-active' ) || ! tinyMCEPreInit.qtInit.hasOwnProperty( id ) ) && ! init.wp_skip_init ) {
+					if (
+						! init.wp_skip_init &&
+						( $wrap.hasClass( 'tmce-active' ) || ! tinyMCEPreInit.qtInit.hasOwnProperty( id ) ) &&
+						(
+							( ! inPostbox && ( readyState === 'interactive' || readyState === 'complete' ) ) ||
+							( inPostbox && readyState === 'complete' )
+						)
+					) {
 						tinymce.init( init );
 
 						if ( ! window.wpActiveEditor ) {
@@ -1685,6 +1693,18 @@
 				}
 			}
 
+			if ( typeof tinymce !== 'undefined' ) {
+				if ( tinymce.Env.ie && tinymce.Env.ie < 11 ) {
+					tinymce.$( '.wp-editor-wrap ' ).removeClass( 'tmce-active' ).addClass( 'html-active' );
+				} else {
+					if ( document.readyState === 'complete' ) {
+						initialize();
+					} else {
+						document.addEventListener( 'readystatechange', initialize );
+					}
+				}
+			}
+
 			if ( typeof quicktags !== 'undefined' ) {
 				for ( id in tinyMCEPreInit.qtInit ) {
 					quicktags( tinyMCEPreInit.qtInit[id] );