[fix] set status to 303 when POST provides a location

.changeset/gentle-news-shout.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Return a 303 response when a `POST` handler provides a `location`

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

@@ -77,7 +77,7 @@ export async function render_page(event, route, options, state, resolve_opts) {
 					}
 
 					if (event.request.method === 'POST' && result?.location) {
-						return redirect_response(status, result.location);
+						return redirect_response(303, result.location);
 					}
 				} else {
 					event.setHeaders({

packages/kit/test/apps/basics/src/routes/shadowed/post-success-redirect/+page.server.js

@@ -0,0 +1,5 @@
+export function POST() {
+	return {
+		location: '/shadowed/post-success-redirect/redirected'
+	};
+}

packages/kit/test/apps/basics/src/routes/shadowed/post-success-redirect/+page.svelte

@@ -0,0 +1,3 @@
+<form method="POST">
+	<button>Click me</button>
+</form>

packages/kit/test/apps/basics/src/routes/shadowed/post-success-redirect/redirected/+page.svelte

@@ -0,0 +1 @@
+<h1>POST was successful</h1>

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

@@ -188,6 +188,12 @@ test.describe('Shadowed pages', () => {
 		);
 	});
 
+	test('Handles POST success with returned location', async ({ page }) => {
+		await page.goto('/shadowed/post-success-redirect');
+		await Promise.all([page.waitForNavigation(), page.click('button')]);
+		expect(await page.textContent('h1')).toBe('POST was successful');
+	});
+
 	test('Renders error page for 4xx and 5xx responses from GET', async ({ page, clicknav }) => {
 		await page.goto('/shadowed');
 		await clicknav('[href="/shadowed/error-get"]');