fix: properly purge server load cache (#8893)

fixes #8272

When going from a page with server load to a page without server load, the cache wasn't purged properly

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/fast-rules-exercise.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: don't reuse previous server load cache when there's no server load function

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

@@ -853,7 +853,7 @@ export function create_client({ target }) {
 					// server_data_node is undefined if it wasn't reloaded from the server;
 					// and if current loader uses server data, we want to reuse previous data.
 					server_data_node === undefined && loader[0] ? { type: 'skip' } : server_data_node ?? null,
-					previous?.server
+					loader[0] ? previous?.server : undefined
 				)
 			});
 		});

packages/kit/test/apps/basics/src/routes/load/server-data-reuse/with-changing-parent/+layout.server.js

@@ -0,0 +1,6 @@
+/** @type {import('./$types').LayoutServerLoad} */
+export function load(input) {
+	return {
+		title: input.url.pathname
+	};
+}

packages/kit/test/apps/basics/src/routes/load/server-data-reuse/with-changing-parent/+layout.svelte

@@ -0,0 +1 @@
+<slot />

packages/kit/test/apps/basics/src/routes/load/server-data-reuse/with-changing-parent/no-load/+page.svelte

@@ -0,0 +1,7 @@
+<script>
+	export let data;
+</script>
+
+<p>Page without load</p>
+
+<pre>{JSON.stringify(data)}</pre>

packages/kit/test/apps/basics/src/routes/load/server-data-reuse/with-changing-parent/with-server-load/+page.server.js

@@ -0,0 +1,4 @@
+/** @type {import('./$types').PageServerLoad} */
+export function load() {
+	return { server: true };
+}

packages/kit/test/apps/basics/src/routes/load/server-data-reuse/with-changing-parent/with-server-load/+page.svelte

@@ -0,0 +1,8 @@
+<script>
+	/** @type {import('./$types').PageData} */
+	export let data;
+</script>
+
+<p>Page with server load</p>
+
+<pre>{JSON.stringify(data)}</pre>

packages/kit/test/apps/basics/test/client.test.js

@@ -102,6 +102,22 @@ test.describe('Load', () => {
 		);
 		await app.goto('/load/server-data-reuse/no-load');
 		expect(await page.textContent('pre')).toBe(JSON.stringify({ foo: { bar: 'Custom layout' } }));
+
+		await page.goto('/load/server-data-reuse/with-changing-parent/with-server-load');
+		expect(await page.textContent('pre')).toBe(
+			JSON.stringify({
+				foo: { bar: 'Custom layout' },
+				title: '/load/server-data-reuse/with-changing-parent/with-server-load',
+				server: true
+			})
+		);
+		await app.goto('/load/server-data-reuse/with-changing-parent/no-load');
+		expect(await page.textContent('pre')).toBe(
+			JSON.stringify({
+				foo: { bar: 'Custom layout' },
+				title: '/load/server-data-reuse/with-changing-parent/no-load'
+			})
+		);
 	});
 
 	test('keeps server data when valid while not reusing client load data', async ({