fix: yield main thread before navigating

.changeset/pink-tigers-whisper.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: yield main thread before navigating

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

@@ -2027,7 +2027,7 @@ function _start_router() {
 	}
 
 	/** @param {MouseEvent} event */
-	container.addEventListener('click', (event) => {
+	container.addEventListener('click', async (event) => {
 		// Adapted from https://github.com/visionmedia/page.js
 		// MIT license https://github.com/visionmedia/page.js#license
 		if (event.button || event.which !== 1) return;
@@ -2120,6 +2120,16 @@ function _start_router() {
 
 		event.preventDefault();
 
+		// allow the browser to repaint before navigating —
+		// this prevents INP scores being penalised
+		await new Promise((fulfil) => {
+			requestAnimationFrame(() => {
+				setTimeout(fulfil, 0);
+			});
+
+			setTimeout(fulfil, 100); // fallback for edge case where rAF doesn't fire because e.g. tab was backgrounded
+		});
+
 		navigate({
 			type: 'link',
 			url,

packages/kit/test/apps/basics/src/routes/routing/+page.svelte

@@ -6,6 +6,7 @@
 
 <a href="/routing/a">a</a>
 <a href="/routing/ambiguous/ok.json" rel="external">ok</a>
+<a href="/routing/next-paint">next-paint</a>
 <a href="/routing/symlink-from">symlinked</a>
 <a href="http://localhost:{$page.url.searchParams.get('port')}">elsewhere</a>
 <a href="/static.json">static.json</a>

packages/kit/test/apps/basics/src/routes/routing/next-paint/+page.svelte

@@ -0,0 +1 @@
+<p>next-paint</p>

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

@@ -1163,3 +1163,32 @@ test.describe('reroute', () => {
 		expect(await page.textContent('h1')).toContain('Full Navigation');
 	});
 });
+
+test.describe('INP', () => {
+	test('does not block next paint', async ({ page }) => {
+		// Thanks to https://publishing-project.rivendellweb.net/measuring-performance-tasks-with-playwright/#interaction-to-next-paint-inp
+		async function measureInteractionToPaint(selector) {
+			return page.evaluate(async (selector) => {
+				return new Promise((resolve) => {
+					const startTime = performance.now();
+					document.querySelector(selector).click();
+					requestAnimationFrame(() => {
+						const endTime = performance.now();
+						resolve(endTime - startTime);
+					});
+				});
+			}, selector);
+		}
+
+		await page.goto('/routing');
+
+		const client = await page.context().newCDPSession(page);
+		await client.send('Emulation.setCPUThrottlingRate', { rate: 100 });
+
+		const time = await measureInteractionToPaint('a[href="/routing/next-paint"]');
+
+		// we may need to tweak this number, and the `rate` above,
+		// depending on if this proves flaky
+		expect(time).toBeLessThan(400);
+	});
+});