Remove extra suspense boundary for default next/dynamic (#64716)

### What

Removing the Suspense boundary on top of `next/dynamic` by default, make
it as `React.lazy` component with preloading CSS feature.

### Why

Extra Suspense boundary is causing extra useless rendering. For SSR, it
shouldn't render `loading` by default

Related: #64060
Related: #64687
Closes
[NEXT-3074](https://linear.app/vercel/issue/NEXT-3074/app-router-content-flickering-with-reactcreatecontext-and-nextdynamic)

This is sort of a breaking change, since removing the Suspense boundary
on top of `next/dynamic` by default. If there's error happening in side
the dynamic component you need to wrap an extra Suspense boundary on top
of it

packages/next/src/shared/lib/dynamic.tsx

@@ -41,6 +41,7 @@ export type DynamicOptions<P = {}> = LoadableGeneratedOptions & {
   loadableGenerated?: LoadableGeneratedOptions
   ssr?: boolean
   /**
+   * TODO: remove this in next major version v15
    * @deprecated `suspense` prop is not required anymore
    */
   suspense?: boolean

packages/next/src/shared/lib/lazy-dynamic/loadable.tsx

@@ -1,4 +1,4 @@
-import { Suspense, lazy } from 'react'
+import { Suspense, Fragment, lazy } from 'react'
 import { BailoutToCSR } from './dynamic-bailout-to-csr'
 import type { ComponentModule } from './types'
 import { PreloadCss } from './preload-css'
@@ -48,7 +48,10 @@ function Loadable(options: LoadableOptions) {
       <Loading isLoading={true} pastDelay={true} error={null} />
     ) : null
 
-    const children = opts.ssr ? (
+    const isSSR = opts.ssr
+    const Wrap = isSSR ? Fragment : Suspense
+    const wrapProps = isSSR ? {} : { fallback: fallbackElement }
+    const children = isSSR ? (
       <>
         {/* During SSR, we need to preload the CSS from the dynamic component to avoid flash of unstyled content */}
         {typeof window === 'undefined' ? (
@@ -62,7 +65,7 @@ function Loadable(options: LoadableOptions) {
       </BailoutToCSR>
     )
 
-    return <Suspense fallback={fallbackElement}>{children}</Suspense>
+    return <Wrap {...wrapProps}>{children}</Wrap>
   }
 
   LoadableComponent.displayName = 'LoadableComponent'

test/e2e/app-dir/dynamic/app/default/dynamic-component.js

@@ -0,0 +1,12 @@
+const isDevTest = false
+
+const DynamicImportComponent = () => {
+  if (isDevTest && typeof window === 'undefined') {
+    throw new Error('This component should only be rendered on the client side')
+  }
+  return (
+    <div id="dynamic-component">This is a dynamically imported component</div>
+  )
+}
+
+export default DynamicImportComponent

test/e2e/app-dir/dynamic/app/default/page.js

@@ -0,0 +1,17 @@
+'use client'
+
+import dynamic from 'next/dynamic'
+
+const DynamicHeader = dynamic(() => import('./dynamic-component'), {
+  loading: () => <p>Loading...</p>,
+})
+
+const ClientWrapper = () => {
+  return (
+    <div>
+      <DynamicHeader />
+    </div>
+  )
+}
+
+export default ClientWrapper

test/e2e/app-dir/dynamic/dynamic.test.ts

@@ -1,7 +1,8 @@
 import { nextTestSetup } from 'e2e-utils'
+import { retry } from 'next-test-utils'
 
 describe('app dir - next/dynamic', () => {
-  const { next, isNextStart, skipped } = nextTestSetup({
+  const { next, isNextStart, isNextDev, skipped } = nextTestSetup({
     files: __dirname,
     skipDeployment: true,
   })
@@ -57,6 +58,26 @@ describe('app dir - next/dynamic', () => {
     expect($('h1').text()).toBe('hello')
   })
 
+  it('should not render loading by default', async () => {
+    const $ = await next.render$('/default')
+    expect($('#dynamic-component').text()).not.toContain('loading')
+  })
+
+  if (isNextDev) {
+    it('should directly raise error when dynamic component error on server', async () => {
+      const pagePath = 'app/default/dynamic-component.js'
+      const page = await next.readFile(pagePath)
+      await next.patchFile(
+        pagePath,
+        page.replace('const isDevTest = false', 'const isDevTest = true')
+      )
+      await retry(async () => {
+        const { status } = await next.fetch('/default')
+        expect(status).toBe(500)
+      })
+    })
+  }
+
   describe('no SSR', () => {
     it('should not render client component imported through ssr: false in client components in edge runtime', async () => {
       // noSSR should not show up in html