fix: useId mismatches when deferring props.

packages/react/src/runtime/deferred-shell.ts

@@ -0,0 +1,2 @@
+export const DEFERRED_SHELL_END_ID: string = '__VOID_DEFERRED_SHELL_END__';
+export const DEFERRED_SHELL_END_MARKER: string = `<template id="${DEFERRED_SHELL_END_ID}"></template>`;

packages/react/src/runtime/pages-client.ts

@@ -35,6 +35,7 @@ import {
   RouterContext,
   SharedContext,
 } from './context.ts';
+import { DEFERRED_SHELL_END_ID } from './deferred-shell.ts';
 
 // oxlint-disable-next-line no-explicit-any -- component props are narrowed by generated route types
 type ComponentModule = { default: ComponentType<any> };
@@ -625,13 +626,24 @@ export async function startReactPages(config: ReactPagesConfig): Promise<ReactPa
     throw new Error('pages: Missing #app element for Void React pages hydration.');
   }
 
-  const app = React.createElement(App, {
+  const appRoot = React.createElement(App, {
     engine,
     router,
     viewTransitions: config.viewTransitions,
   });
+  const isSsr = appEl.getAttribute('data-void-ssr') !== 'false';
+  const hasDeferred = Boolean((engine.initialPageData as ReactPageData).deferredKeys);
+  const app =
+    isSsr && hasDeferred
+      ? React.createElement(
+          React.Fragment,
+          null,
+          appRoot,
+          React.createElement('template', { id: DEFERRED_SHELL_END_ID }),
+        )
+      : appRoot;
   startTransition(() => {
-    if (appEl.getAttribute('data-void-ssr') === 'false') {
+    if (!isSsr) {
       createRoot(appEl).render(app);
     } else {
       hydrateRoot(appEl, app);

packages/react/src/runtime/pages-server.ts

@@ -11,12 +11,11 @@ import {
   RouterContext,
   SharedContext,
 } from './context.ts';
+import { DEFERRED_SHELL_END_ID, DEFERRED_SHELL_END_MARKER } from './deferred-shell.ts';
 
 // oxlint-disable-next-line no-explicit-any -- component props are narrowed by generated route types
 type ComponentModule = { default: ComponentType<any> };
 const DEFERRED_SHELL_CANCEL_REASON = 'Void deferred shell complete';
-const DEFERRED_SHELL_END_ID = '__VOID_DEFERRED_SHELL_END__';
-const DEFERRED_SHELL_END_MARKER = `<template id="${DEFERRED_SHELL_END_ID}"></template>`;
 
 export type ReactPagesServerConfig = Readonly<{
   pageObj: PageObject;
@@ -102,7 +101,7 @@ function readDeferredShell(stream: ReadableStream<Uint8Array>): ReadableStream<U
           buffer += decoder.decode(value, { stream: true });
           const markerIndex = buffer.indexOf(DEFERRED_SHELL_END_MARKER);
           if (markerIndex !== -1) {
-            flush(buffer.slice(0, markerIndex));
+            flush(buffer.slice(0, markerIndex + DEFERRED_SHELL_END_MARKER.length));
             await reader.cancel(DEFERRED_SHELL_CANCEL_REASON);
             break;
           }

packages/react/test/integration/react-pages-client.test.ts

@@ -571,18 +571,46 @@ function defineClientTests(getBase: () => string) {
       await page.goto(BASE + '/');
       await waitForHydration(page);
 
-      // SPA navigate to deferred page
-      await page.click('a[href="/deferred"]');
+      await page.evaluate(() => {
+        window.__SPA_MARKER = true;
+      });
+
+      // Avoid hover prefetch so this exercises the deferred NDJSON navigation path.
+      await page.evaluate(() => {
+        document.querySelector<HTMLAnchorElement>('a[href="/deferred"]')?.click();
+      });
       await page.waitForFunction(() => document.querySelector('[data-testid="deferred-resolved"]'));
 
       const text = await page.textContent('[data-testid="deferred-resolved"]');
       expect(text).toBe('Count: 42');
+      expect(await page.evaluate(() => window.__SPA_MARKER)).toBe(true);
       expect(page.url()).toBe(BASE + '/deferred');
     } finally {
       await page.context().close();
     }
   });
 
+  it('deferred props: useId hydrates without mismatches', async () => {
+    const page = await newPage();
+    try {
+      await page.goto(BASE + '/deferred');
+      await waitForHydration(page);
+      await page.waitForFunction(() => document.querySelector('[data-testid="deferred-resolved"]'));
+      await page.waitForFunction(
+        () =>
+          document.querySelector<HTMLElement>('[data-testid="deferred-title"]')?.dataset.clientId,
+      );
+
+      const ids = await page.evaluate(() => {
+        const title = document.querySelector<HTMLElement>('[data-testid="deferred-title"]')!;
+        return { server: title.id, client: title.dataset.clientId };
+      });
+      expect(ids.server).toBe(ids.client);
+    } finally {
+      await page.context().close();
+    }
+  });
+
   it('SPA navigation updates document.title from head()', async () => {
     const page = await newPage();
     try {

playground/react-pages/pages/deferred.tsx

@@ -1,4 +1,4 @@
-import { Suspense, use } from 'react';
+import { Suspense, use, useId } from 'react';
 import type { Props } from './deferred.server.ts';
 
 function Stats({ stats }: Pick<Props, 'stats'>) {
@@ -8,9 +8,21 @@ function Stats({ stats }: Pick<Props, 'stats'>) {
 }
 
 export default function DeferredPage({ title, stats }: Props) {
+  const titleId = useId();
+
   return (
     <div>
-      <h1>{title}</h1>
+      <h1
+        id={titleId}
+        data-testid="deferred-title"
+        ref={(element) => {
+          if (element) {
+            element.dataset.clientId = titleId;
+          }
+        }}
+      >
+        {title}
+      </h1>
       <Suspense fallback={<div data-testid="deferred-loading">Loading stats...</div>}>
         <Stats stats={stats} />
       </Suspense>