Respect forms with enctype set for view transitions (#9466)

* Respect forms with enctype set for view transitions

* Add changeset

* Revert "Respect forms with enctype set for view transitions"

This reverts commit 6d3e04a.

* Review feedback

* Handle submitter case

* Move comment

* Update .changeset/rude-geckos-rush.md

* Add tests

---------

Co-authored-by: Nate Moore <natemoo-re@users.noreply.github.com>

.changeset/rude-geckos-rush.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Updates view transitions `form` handling with logic for the [`enctype`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLFormElement/enctype) attribute

packages/astro/e2e/fixtures/view-transitions/src/pages/form-one.astro

@@ -1,13 +1,15 @@
 ---
 import Layout from '../components/Layout.astro';
 const method = Astro.url.searchParams.get('method') ?? 'POST';
+const enctype = Astro.url.searchParams.get('enctype');
 const postShowThrow = Astro.url.searchParams.has('throw') ?? false;
 ---
+
 <Layout>
 	<h2>Contact Form</h2>
-	<form action="/contact" method={method}>
-		<input type="hidden" name="name" value="Testing">
-		{postShowThrow ? <input type="hidden" name="throw" value="true"> : ''}
-		<input type="submit" value="Submit" id="submit">
+	<form action="/contact" method={method} {...enctype ? { enctype } : {}}>
+		<input type="hidden" name="name" value="Testing" />
+		{postShowThrow ? <input type="hidden" name="throw" value="true" /> : ''}
+		<input type="submit" value="Submit" id="submit" />
 	</form>
 </Layout>

packages/astro/e2e/view-transitions.test.js

@@ -976,6 +976,83 @@ test.describe('View Transitions', () => {
 		).toEqual(1);
 	});
 
+	test('form POST defaults to multipart/form-data (Astro 4.x compatibility)', async ({
+		page,
+		astro,
+	}) => {
+		const loads = [];
+
+		page.addListener('load', async (p) => {
+			loads.push(p);
+		});
+
+		const postedEncodings = [];
+
+		await page.route('**/contact', async (route) => {
+			const request = route.request();
+
+			if (request.method() === 'POST') {
+				postedEncodings.push(request.headers()['content-type'].split(';')[0]);
+			}
+
+			await route.continue();
+		});
+
+		await page.goto(astro.resolveUrl('/form-one'));
+
+		// Submit the form
+		await page.click('#submit');
+
+		expect(
+			loads.length,
+			'There should be only 1 page load. No additional loads for the form submission'
+		).toEqual(1);
+
+		expect(
+			postedEncodings,
+			'There should be 1 POST, with encoding set to `multipart/form-data`'
+		).toEqual(['multipart/form-data']);
+	});
+
+	test('form POST respects enctype attribute', async ({ page, astro }) => {
+		const loads = [];
+
+		page.addListener('load', async (p) => {
+			loads.push(p);
+		});
+
+		const postedEncodings = [];
+
+		await page.route('**/contact', async (route) => {
+			const request = route.request();
+
+			if (request.method() === 'POST') {
+				postedEncodings.push(request.headers()['content-type'].split(';')[0]);
+			}
+
+			await route.continue();
+		});
+
+		await page.goto(
+			astro.resolveUrl(
+				`/form-one?${new URLSearchParams({ enctype: 'application/x-www-form-urlencoded' })}`
+			)
+		);
+
+		// Submit the form
+		await page.click('#submit');
+
+		expect(
+			loads.length,
+			'There should be only 1 page load. No additional loads for the form submission'
+		).toEqual(1);
+
+		expect(
+			postedEncodings,
+			'There should be 1 POST, with encoding set to `multipart/form-data`'
+		).toEqual(['application/x-www-form-urlencoded']);
+	});
+
 	test('Route announcer is invisible on page transition', async ({ page, astro }) => {
 		await page.goto(astro.resolveUrl('/no-directive-one'));
 

packages/astro/src/transitions/router.ts

@@ -463,7 +463,25 @@ async function transition(
 		const init: RequestInit = {};
 		if (preparationEvent.formData) {
 			init.method = 'POST';
-			init.body = preparationEvent.formData;
+			const form =
+				preparationEvent.sourceElement instanceof HTMLFormElement
+					? preparationEvent.sourceElement
+					: preparationEvent.sourceElement instanceof HTMLElement &&
+					    'form' in preparationEvent.sourceElement
+					  ? (preparationEvent.sourceElement.form as HTMLFormElement)
+					  : preparationEvent.sourceElement?.closest('form');
+			// Form elements without enctype explicitly set default to application/x-www-form-urlencoded.
+			// In order to maintain compatibility with Astro 4.x, we need to check the value of enctype
+			// on the attributes property rather than accessing .enctype directly. Astro 5.x may
+			// introduce defaulting to application/x-www-form-urlencoded as a breaking change, and then
+			// we can access .enctype directly.
+			//
+			// Note: getNamedItem can return null in real life, even if TypeScript doesn't think so, hence
+			// the ?.
+			init.body =
+				form?.attributes.getNamedItem('enctype')?.value === 'application/x-www-form-urlencoded'
+					? new URLSearchParams(preparationEvent.formData as any)
+					: preparationEvent.formData;
 		}
 		const response = await fetchHTML(href, init);
 		// If there is a problem fetching the new page, just do an MPA navigation to it.