fix(html): leave URLs rewritten through the History API alone

`history.pushState()` and `replaceState()` fire a `navigate` event as
well, and the router intercepted it whenever the new URL was a page of
the site. A script rewriting the URL in place (dropping tracking
parameters, say) had the page fetched again and swapped in over itself,
remounting every island.

Those are same-document pushes and replaces, which the router now
leaves alone. Back and forward between pages navigated to client-side
are same-document too, but traversals, so they are still intercepted.

Assisted-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: ovflowd

e2e/client-side-navigation.spec.js

@@ -149,6 +149,21 @@ test.describe('Client-side navigation', () => {
     expect(await page.evaluate(() => window.__document)).toBeUndefined();
   });
 
+  test('leaves the URL to scripts that rewrite it', async ({ page }) => {
+    // As a script dropping tracking parameters does
+    const swapped = await page.evaluate(async () => {
+      const { body } = document;
+
+      history.replaceState(null, '', `${location.pathname}?utm_source=docs`);
+      await navigation.transition?.finished;
+
+      return document.body !== body;
+    });
+
+    await expect(page).toHaveURL(/\/assert\?utm_source=docs$/);
+    expect(swapped).toBe(false);
+  });
+
   test('loads the page in full when it cannot swap it in', async ({ page }) => {
     // The router's copy is the page as another build writes it, with router
     // data this one cannot read; the full load gets the real page

packages/react/src/html/ui/__tests__/router.test.mjs

@@ -63,6 +63,7 @@ describe('shouldIntercept', () => {
    */
   const event = overrides => ({
     canIntercept: true,
+    destination: { sameDocument: false },
     hashChange: false,
     downloadRequest: null,
     formData: null,
@@ -75,9 +76,24 @@ describe('shouldIntercept', () => {
   });
 
   it('intercepts going back and forward between pages', () => {
-    assert.ok(
-      shouldIntercept(event({ navigationType: 'traverse' }), url, root)
-    );
+    // Pages navigated to client-side share the document
+    const traverse = event({
+      navigationType: 'traverse',
+      destination: { sameDocument: true },
+    });
+
+    assert.ok(shouldIntercept(traverse, url, root));
+  });
+
+  it('leaves history.pushState() and replaceState() alone', () => {
+    for (const navigationType of ['push', 'replace']) {
+      const update = event({
+        navigationType,
+        destination: { sameDocument: true },
+      });
+
+      assert.ok(!shouldIntercept(update, url, root), navigationType);
+    }
   });
 
   it('never intercepts a reload, which is always a full load', () => {

packages/react/src/html/ui/router.mjs

@@ -65,6 +65,8 @@ export const shouldFollowLink = link =>
  */
 export const shouldIntercept = (event, url, root) =>
   event.canIntercept &&
+  // `history.pushState()` and `replaceState()` change the URL in place
+  (event.navigationType === 'traverse' || !event.destination.sameDocument) &&
   !event.hashChange &&
   event.downloadRequest === null &&
   !event.formData &&