[fix] Reset navigating store upon return to site with a bfcache hit

Fixes #5415

The alternative fix would have been to do this in beforeunload, but this might result in more possibilities of flickering than the other way around.

Author: dummdidumm

.changeset/tough-dolls-relate.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Reset navigating store upon return to site with a bfcache hit

packages/kit/src/runtime/client/client.js

@@ -1219,6 +1219,16 @@ export function create_client({ target, session, base, trailing_slash }) {
 					);
 				}
 			});
+
+			addEventListener('pageshow', (event) => {
+				// If the user navigates to another site and then uses the back button and
+				// bfcache hits, we need to set navigating to null, the site doesn't know
+				// the navigation away from it was successful.
+				// Info about bfcache here: https://web.dev/bfcache
+				if (event.persisted) {
+					stores.navigating.set(null);
+				}
+			});
 		},
 
 		_hydrate: async ({ status, error, nodes, params, routeId }) => {