diff -r 490d5cc509ed -r cf61fcea0001 wp/wp-includes/js/customize-loader.js
--- a/wp/wp-includes/js/customize-loader.js	Tue Jun 09 11:14:17 2015 +0000
+++ b/wp/wp-includes/js/customize-loader.js	Mon Oct 14 17:39:30 2019 +0200
@@ -1,4 +1,10 @@
-/* global _wpCustomizeLoaderSettings, confirm */
+/* global _wpCustomizeLoaderSettings */
+/**
+ * Expose a public API that allows the customizer to be
+ * loaded on any page.
+ *
+ * @namespace wp
+ */
 window.wp = window.wp || {};
 
 (function( exports, $ ){
@@ -18,9 +24,12 @@
 	 *
 	 *     e.g. <a class="load-customize" href="<?php echo wp_customize_url(); ?>">Open Customizer</a>
 	 *
+	 * @memberOf wp.customize
+	 *
+	 * @class
 	 * @augments wp.customize.Events
 	 */
-	Loader = $.extend( {}, api.Events, {
+	Loader = $.extend( {}, api.Events,/** @lends wp.customize.Loader.prototype */{
 		/**
 		 * Setup the Loader; triggered on document#ready.
 		 */
@@ -111,7 +120,11 @@
 			this.active = true;
 			this.body.addClass('customize-loading');
 
-			// Dirty state of Customizer in iframe
+			/*
+			 * Track the dirtiness state (whether the drafted changes have been published)
+			 * of the Customizer in the iframe. This is used to decide whether to display
+			 * an AYS alert if the user tries to close the window before saving changes.
+			 */
 			this.saved = new api.Value( true );
 
 			this.iframe = $( '<iframe />', { 'src': src, 'title': Loader.settings.l10n.mainIframeTitle } ).appendTo( this.element );
@@ -124,6 +137,19 @@
 				targetWindow: this.iframe[0].contentWindow
 			});
 
+			// Expose the changeset UUID on the parent window's URL so that the customized state can survive a refresh.
+			if ( history.replaceState ) {
+				this.messenger.bind( 'changeset-uuid', function( changesetUuid ) {
+					var urlParser = document.createElement( 'a' );
+					urlParser.href = location.href;
+					urlParser.search = $.param( _.extend(
+						api.utils.parseQueryString( urlParser.search.substr( 1 ) ),
+						{ changeset_uuid: changesetUuid }
+					) );
+					history.replaceState( { customize: urlParser.href }, '', urlParser.href );
+				} );
+			}
+
 			// Wait for the connection from the iframe before sending any postMessage events.
 			this.messenger.bind( 'ready', function() {
 				Loader.messenger.send( 'back' );
@@ -142,12 +168,6 @@
 			// Prompt AYS dialog when navigating away
 			$( window ).on( 'beforeunload', this.beforeunload );
 
-			this.messenger.bind( 'activated', function( location ) {
-				if ( location ) {
-					window.location = location;
-				}
-			});
-
 			this.messenger.bind( 'saved', function () {
 				Loader.saved( true );
 			} );
@@ -181,37 +201,37 @@
 		 * Callback after the Customizer has been opened.
 		 */
 		opened: function() {
-			Loader.body.addClass( 'customize-active full-overlay-active' );
+			Loader.body.addClass( 'customize-active full-overlay-active' ).attr( 'aria-busy', 'true' );
 		},
 
 		/**
-		 * Close the Customizer overlay and return focus to the link that opened it.
+		 * Close the Customizer overlay.
 		 */
 		close: function() {
-			if ( ! this.active ) {
+			var self = this, onConfirmClose;
+			if ( ! self.active ) {
 				return;
 			}
 
-			// Display AYS dialog if Customizer is dirty
-			if ( ! this.saved() && ! confirm( Loader.settings.l10n.saveAlert ) ) {
-				// Go forward since Customizer is exited by history.back()
-				history.forward();
-				return;
-			}
-
-			this.active = false;
+			onConfirmClose = function( confirmed ) {
+				if ( confirmed ) {
+					self.active = false;
+					self.trigger( 'close' );
 
-			this.trigger( 'close' );
+					// Restore document title prior to opening the Live Preview
+					if ( self.originalDocumentTitle ) {
+						document.title = self.originalDocumentTitle;
+					}
+				} else {
 
-			// Restore document title prior to opening the Live Preview
-			if ( this.originalDocumentTitle ) {
-				document.title = this.originalDocumentTitle;
-			}
+					// Go forward since Customizer is exited by history.back()
+					history.forward();
+				}
+				self.messenger.unbind( 'confirmed-close', onConfirmClose );
+			};
+			self.messenger.bind( 'confirmed-close', onConfirmClose );
 
-			// Return focus to link that was originally clicked.
-			if ( this.link ) {
-				this.link.focus();
-			}
+			Loader.messenger.send( 'confirm-close' );
 		},
 
 		/**
@@ -225,13 +245,20 @@
 			Loader.saved     = null;
 			Loader.body.removeClass( 'customize-active full-overlay-active' ).removeClass( 'customize-loading' );
 			$( window ).off( 'beforeunload', Loader.beforeunload );
+			/*
+			 * Return focus to the link that opened the Customizer overlay after
+			 * the body element visibility is restored.
+			 */
+			if ( Loader.link ) {
+				Loader.link.focus();
+			}
 		},
 
 		/**
 		 * Callback for the `load` event on the Customizer iframe.
 		 */
 		loaded: function() {
-			Loader.body.removeClass('customize-loading');
+			Loader.body.removeClass( 'customize-loading' ).attr( 'aria-busy', 'false' );
 		},
 
 		/**