fix: prevent false positive warnings for fetch in Firefox and Safari (#9680)

* change fetch warning stack trace cutoff

* changeset

* add test

Author: eltigerchino

.changeset/flat-trainers-fold.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: prevent false positive warnings for fetch in Firefox and Safari

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

@@ -29,14 +29,12 @@ if (DEV) {
 		// We use just the filename as the method name sometimes does not appear on the CI.
 		const url = input instanceof Request ? input.url : input.toString();
 		const stack_array = /** @type {string} */ (new Error().stack).split('\n');
-		// We need to do some Firefox-specific cutoff because it (impressively) maintains the stack
-		// across events and for example traces a `fetch` call triggered from a button back
-		// to the creation of the event listener and the element creation itself,
+		// We need to do a cutoff because Safari and Firefox maintain the stack
+		// across events and for example traces a `fetch` call triggered from a button
+		// back to the creation of the event listener and the element creation itself,
 		// where at some point client.js will show up, leading to false positives.
-		const firefox_cutoff = stack_array.findIndex((a) => a.includes('*listen@'));
-		const stack = stack_array
-			.slice(0, firefox_cutoff !== -1 ? firefox_cutoff : undefined)
-			.join('\n');
+		const cutoff = stack_array.findIndex((a) => a.includes('load@') || a.includes('at load'));
+		const stack = stack_array.slice(0, cutoff + 2).join('\n');
 
 		const heuristic = can_inspect_stack_trace
 			? stack.includes('src/runtime/client/client.js')

packages/kit/test/apps/basics/src/routes/load/window-fetch/outside-load/+page.svelte

@@ -0,0 +1,13 @@
+<script>
+	import { page } from '$app/stores';
+	import { onMount } from 'svelte';
+
+	let answer = 0;
+
+	onMount(async () => {
+		const res = await fetch(`${$page.url.origin}/load/window-fetch/data.json`);
+		({ answer } = await res.json());
+	});
+</script>
+
+<h1>{answer}</h1>

packages/kit/test/apps/basics/test/cross-platform/client.test.js

@@ -754,3 +754,24 @@ test.describe('Interactivity', () => {
 		expect(errored).toBe(false);
 	});
 });
+
+test.describe('Load', () => {
+	if (process.env.DEV) {
+		test('using window.fetch does not cause false-positive warning', async ({ page, baseURL }) => {
+			/** @type {string[]} */
+			const warnings = [];
+			page.on('console', (msg) => {
+				if (msg.type() === 'warning') {
+					warnings.push(msg.text());
+				}
+			});
+
+			await page.goto('/load/window-fetch/outside-load');
+			expect(await page.textContent('h1')).toBe('42');
+
+			expect(warnings).not.toContain(
+				`Loading ${baseURL}/load/window-fetch/data.json using \`window.fetch\`. For best results, use the \`fetch\` that is passed to your \`load\` function: https://kit.svelte.dev/docs/load#making-fetch-requests`
+			);
+		});
+	}
+});