Ticket #25439: 25439.diff

src/wp-admin/js/customize-controls.js

@@ -1128 +1128 @@
                         });
                 });
 
+                // Prompt user with AYS dialog if leaving the Customizer with unsaved changes
+                $( window ).on( 'beforeunload', function () {
+                        if ( ! api.state( 'saved' )() ) {
+                                return api.l10n.saveAlert;
+                        }
+                } );
+
                 // Pass events through to the parent.
                 api.bind( 'saved', function() {
                         parent.send( 'saved' );

src/wp-includes/script-loader.php

@@ -381 +381 @@
         did_action( 'init' ) && $scripts->localize( 'customize-controls', '_wpCustomizeControlsL10n', array(
                 'activate'  => __( 'Save & Activate' ),
                 'save'      => __( 'Save & Publish' ),
+                'saveAlert' => __( 'The changes you made will be lost if you navigate away from this page.' ),
                 'saved'     => __( 'Saved' ),
                 'cancel'    => __( 'Cancel' ),
                 'close'     => __( 'Close' ),