fix(sveltekit): Read navigation untracked in the Kit 3 fetch wrapper

dev-packages/e2e-tests/test-applications/sveltekit-3/src/routes/effect-fetch/+page.svelte

@@ -0,0 +1,13 @@
+<script lang="ts">
+  let responses = $state(0);
+
+  $effect(() => {
+    fetch('/api/users').then(() => {
+      responses++;
+    });
+  });
+</script>
+
+<p id="effect-fetch-responses">responses: {responses}</p>
+<a href="/effect-fetch?q=1">Same route, new query</a>
+<a href="/nav1">Leave page</a>

dev-packages/e2e-tests/test-applications/sveltekit-3/tests/tracing.client.test.ts

@@ -77,3 +77,23 @@ test.describe('client-specific performance events', () => {
     );
   });
 });
+
+test('fetch instrumentation does not make effects re-run on navigation', async ({ page }) => {
+  const effectFetches: string[] = [];
+  page.on('request', request => {
+    if (new URL(request.url()).pathname === '/api/users') {
+      effectFetches.push(request.url());
+    }
+  });
+
+  await waitForInitialPageload(page, { route: '/effect-fetch' });
+  await expect(page.locator('#effect-fetch-responses')).toHaveText('responses: 1');
+
+  await page.getByText('Same route, new query').click();
+  await page.waitForURL('**/effect-fetch?q=1');
+
+  await page.getByText('Leave page').click();
+  await expect(page.getByText('Navigation 1')).toBeVisible();
+
+  expect(effectFetches).toHaveLength(1);
+});

packages/sveltekit/rollup.npm.config.mjs

@@ -20,7 +20,13 @@ export default makeNPMConfigVariants(
     packageSpecificConfig: {
       // Keep the variant subpath external so the transpiled output preserves the import for the
       // consumer to resolve (via `exports` or the `sentrySvelteKit()` plugin).
-      external: ['$app/state', '$app/stores', '@sentry/sveltekit/browser-tracing-variant', 'cloudflare:workers'],
+      external: [
+        'svelte',
+        '$app/state',
+        '$app/stores',
+        '@sentry/sveltekit/browser-tracing-variant',
+        'cloudflare:workers',
+      ],
       output: {
         dynamicImportInCjs: true,
       },

packages/sveltekit/src/client/navigationState.svelte.ts

@@ -1,5 +1,6 @@
 import type { Navigation } from '@sveltejs/kit';
 import { navigating, page } from '$app/state';
+import { untrack } from 'svelte';
 
 declare const $effect: {
   (callback: () => void): void;
@@ -31,9 +32,11 @@ export function onNavigationChange(callback: (navigation: Navigation | null) =>
  * Reads the current navigation synchronously. `$effect`s are always batched to a microtask, which is
  * too late to catch SvelteKit's synchronous data-request `fetch` at navigation start; a synchronous
  * rune read lets us start the navigation span before that request so it propagates the right trace.
+ * The read is untracked: the `fetch` wrapper calls this from user code, often inside a `$effect`,
+ * and a tracked read would make that effect re-run on every navigation.
  */
 export function getCurrentNavigation(): Navigation | null {
-  return _snapshot();
+  return untrack(_snapshot);
 }
 
 function _snapshot(): Navigation | null {

packages/sveltekit/src/client/svelte5.d.ts

@@ -0,0 +1,7 @@
+// The package is type-checked against Svelte 4, but the Kit 3 variant only ever runs on Svelte 5.
+// Declares the Svelte 5 export it uses; build-time only and never emitted.
+export {};
+
+declare module 'svelte' {
+  export function untrack<T>(fn: () => T): T;
+}