[fix] avoid unnecessary $page store updates (#8457)

Clicking a page link and staying on the same page with nothing changing still changed the $page.store, because
- form was always !== undefined (it's null)
- $page.error was set to undefined after hydration, but it should be null

.changeset/wise-otters-swim.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[fix] avoid unnecessary $page store updates

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

@@ -467,7 +467,7 @@ export function create_client({ target, base }) {
 			!current.url ||
 			url.href !== current.url.href ||
 			current.error !== error ||
-			form !== undefined ||
+			(form !== undefined && form !== page.form) ||
 			data_changed;
 
 		if (page_changed) {

packages/kit/src/runtime/client/types.d.ts

@@ -26,7 +26,7 @@ export interface Client {
 	// private API
 	_hydrate(opts: {
 		status: number;
-		error: App.Error;
+		error: App.Error | null;
 		node_ids: number[];
 		params: Record<string, string>;
 		route: { id: string | null };

packages/kit/src/runtime/server/page/render.js

@@ -158,7 +158,7 @@ export async function render_response({
 	/** @param {string} path */
 	const prefixed = (path) => (path.startsWith('/') ? path : `${assets}/${path}`);
 
-	const serialized = { data: '', form: 'null' };
+	const serialized = { data: '', form: 'null', error: 'null' };
 
 	try {
 		serialized.data = `[${branch
@@ -196,6 +196,10 @@ export async function render_response({
 		serialized.form = uneval_action_response(form_value, /** @type {string} */ (event.route.id));
 	}
 
+	if (error) {
+		serialized.error = devalue.uneval(error);
+	}
+
 	if (inline_styles.size > 0) {
 		const content = Array.from(inline_styles.values()).join('\n');
 
@@ -256,17 +260,14 @@ export async function render_response({
 			const hydrate = [
 				`node_ids: [${branch.map(({ node }) => node.index).join(', ')}]`,
 				`data: ${serialized.data}`,
-				`form: ${serialized.form}`
+				`form: ${serialized.form}`,
+				`error: ${serialized.error}`
 			];
 
 			if (status !== 200) {
 				hydrate.push(`status: ${status}`);
 			}
 
-			if (error) {
-				hydrate.push(`error: ${devalue.uneval(error)}`);
-			}
-
 			if (options.embedded) {
 				hydrate.push(`params: ${devalue.uneval(event.params)}`, `route: ${s(event.route)}`);
 			}