Merge branch 't/6504'

Author: oleq

core/editor.js

@@ -250,7 +250,9 @@
 				editor.fireOnce( 'customConfigLoaded' );
 		} else {
 			// Load the custom configuration file.
-			CKEDITOR.scriptLoader.load( customConfig, function() {
+			// To resolve customConfig race conflicts, use scriptLoader#queue
+			// instead of scriptLoader#load (#6504).
+			CKEDITOR.scriptLoader.queue( customConfig, function() {
 				// If the CKEDITOR.editorConfig function has been properly
 				// defined in the custom configuration file, cache it.
 				if ( CKEDITOR.editorConfig )

core/scriptloader.js

@@ -152,6 +152,51 @@ CKEDITOR.scriptLoader = (function() {
 			for ( var i = 0; i < scriptCount; i++ ) {
 				loadScript( scriptUrl[ i ] );
 			}
-		}
+		},
+
+		/**
+		 * Loads a script in a queue, so only one is loaded at the same time.
+		 *
+		 * @since 4.1.2
+		 * @param {String} scriptUrl URL pointing to the script to be loaded.
+		 * @param {Function} [callback] A function to be called when the script
+		 * is loaded and executed. A boolean parameter is passed to the callback,
+		 * indicating the success of the load.
+		 *
+		 * @see CKEDITOR.scriptLoader#load
+		 */
+		queue: ( function() {
+			var pending = [];
+
+			// Loads the very first script from queue and removes it.
+			function loadNext() {
+				var script;
+
+				if ( ( script = pending[ 0 ] ) )
+					this.load( script.scriptUrl, script.callback, CKEDITOR, 0 );
+			}
+
+			return function( scriptUrl, callback ) {
+				var that = this;
+
+				// This callback calls the standard callback for the script
+				// and loads the very next script from pending list.
+				function callbackWrapper() {
+					callback && callback.apply( this, arguments );
+
+					// Removed the just loaded script from the queue.
+					pending.shift();
+
+					loadNext.call( that );
+				};
+
+				// Let's add this script to the queue
+				pending.push( { 'scriptUrl': scriptUrl, 'callback': callbackWrapper } );
+
+				// If the queue was empty, then start loading.
+				if ( pending.length == 1 )
+					loadNext.call( this );
+			};
+		})()
 	};
 })();