Add check to ensure that window.navigation.activation.from is set.

src/wp-content/themes/twentyfifteen/functions.php

@@ -479,15 +479,15 @@ function twentyfifteen_scripts() {
 		'twentyfifteen-view-transitions',
 		get_template_directory_uri() . '/css/view-transitions.css',
 		array(),
-		'20241219'
+		'20250115'
 	);
 
 	// This script must be loaded prior to rendering, i.e. not in the footer and not deferred or async.
 	wp_enqueue_script(
 		'twentyfifteen-view-transitions',
 		get_template_directory_uri() . '/js/view-transitions.js',
 		array(),
-		'20241219'
+		'20250115'
 	);
 }
 add_action( 'wp_enqueue_scripts', 'twentyfifteen_scripts' );

src/wp-content/themes/twentyfifteen/js/view-transitions.js

@@ -47,6 +47,10 @@ if ( !! window.navigation && 'CSSViewTransitionRule' in window ) {
 	} );
 
 	window.addEventListener( 'pagereveal', ( e ) => {
+		if ( ! window.navigation.activation.from ) {
+			return;
+		}
+
 		if ( e.viewTransition ) {
 			const transitionType = determineTransitionType( window.navigation.activation.from, window.navigation.activation.entry );