Fix app dir page detection (#38475)

## Bug

- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

packages/next/server/app-render.tsx

@@ -479,8 +479,7 @@ export async function renderToHTML(
   }): { Component: React.ComponentType } => {
     const Loading = loading ? interopDefault(loading()) : undefined
     const layoutOrPageMod = layout ? layout() : page ? page() : undefined
-    // TODO: improve detection
-    const isPage = !firstItem && segment === ''
+    const isPage = typeof page !== undefined
 
     const isClientComponentModule =
       layoutOrPageMod && !layoutOrPageMod.hasOwnProperty('__next_rsc__')

test/e2e/app-dir/rsc-basic.test.ts

@@ -155,10 +155,10 @@ describe('app dir - react server components', () => {
 
     await browser.waitForElementByCss('#goto-next-link').click()
     await new Promise((res) => setTimeout(res, 1000))
-    expect(await browser.url()).toBe(`${next.url}/next-api/link`)
+    await check(() => browser.url(), `${next.url}/next-api/link`)
     await browser.waitForElementByCss('#goto-home').click()
     await new Promise((res) => setTimeout(res, 1000))
-    expect(await browser.url()).toBe(`${next.url}/root`)
+    await check(() => browser.url(), `${next.url}/root`)
     const content = await browser.elementByCss('body').text()
     expect(content).toContain('component:root.server')
 
@@ -180,7 +180,7 @@ describe('app dir - react server components', () => {
     expect(content).toMatchInlineSnapshot('"next_streaming_data"')
   })
 
-  it.skip('should support next/link in server components', async () => {
+  it('should support next/link in server components', async () => {
     const linkHTML = await renderViaHTTP(next.url, '/next-api/link')
     const linkText = getNodeBySelector(
       linkHTML,