fix(vite): skip full reload for server only modules scanned by client css (#19745)

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## Summary

- Closes #19744
- Closes vitejs/vite-plugin-react#1118
- Closes wakujs/waku#1963

The change in #19670
didn't take account for server only modules managed by SSR framework.
Forcing full reload for this path breaks server HMR. This PR added a
check to determine whether the same modified file has associated modules
in a different environment module graph to avoid this.

## Test plan

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Explain how you tested your changes. Include the exact commands that you
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Added an integration test for React router HDR (server loader hmr). This
test fails on main.

Also the local build is tested on `@vitejs/plugin-rsc` CI and confirmed
the fix vitejs/vite-plugin-react#1132

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

Author: hi-ogawa

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Guard object lookups against inherited prototype properties ([#19725](https://github.com/tailwindlabs/tailwindcss/pull/19725))
 - Canonicalize `calc(var(--spacing)*…)` expressions into `--spacing(…)` ([#19769](https://github.com/tailwindlabs/tailwindcss/pull/19769))
 - Fix crash in canonicalization step when handling utilities with empty property maps ([#19727](https://github.com/tailwindlabs/tailwindcss/pull/19727))
+- Skip full reload for server only modules when using `@tailwindcss/vite` ([#19745](https://github.com/tailwindlabs/tailwindcss/pull/19745))
 
 ## [4.2.1] - 2026-02-23
 

integrations/vite/react-router.test.ts

@@ -102,6 +102,94 @@ test('dev mode', { fs: WORKSPACE }, async ({ fs, spawn, expect }) => {
   })
 })
 
+test(
+  // cf. https://github.com/remix-run/react-router/blob/00cb4d7b310663b2e84152700c05d3b503005e83/integration/vite-hmr-hdr-test.ts#L311-L318
+  'dev mode, editing a server-only loader dependency triggers HDR instead of a full reload',
+  {
+    fs: {
+      ...WORKSPACE,
+      'package.json': json`
+        {
+          "type": "module",
+          "dependencies": {
+            "@react-router/dev": "^7",
+            "@react-router/node": "^7",
+            "@react-router/serve": "^7",
+            "@tailwindcss/vite": "workspace:^",
+            "@types/node": "^20",
+            "@types/react-dom": "^19",
+            "@types/react": "^19",
+            "isbot": "^5",
+            "react-dom": "^19",
+            "react-router": "^7",
+            "react": "^19",
+            "tailwindcss": "workspace:^",
+            "vite": "^7"
+          }
+        }
+      `,
+      'app/routes/home.tsx': ts`
+        import type { Route } from './+types/home'
+        import { direct } from '../direct-hdr-dep'
+
+        export async function loader() {
+          return { message: direct }
+        }
+
+        export default function Home({ loaderData }: Route.ComponentProps) {
+          return (
+            <div>
+              <h1 className="font-bold">{loaderData.message}</h1>
+              <input data-testinput />
+            </div>
+          )
+        }
+      `,
+      'app/direct-hdr-dep.ts': ts` export const direct = 'HDR: 0' `,
+    },
+  },
+  async ({ fs, spawn, expect }) => {
+    let process = await spawn('pnpm react-router dev')
+
+    let url = ''
+    await process.onStdout((m) => {
+      let match = /Local:\s*(http.*)\//.exec(m)
+      if (match) url = match[1]
+      return Boolean(url)
+    })
+
+    // check initial state
+    await retryAssertion(async () => {
+      let html = await (await fetch(url)).text()
+      expect(html).toContain('HDR: 0')
+
+      let css = await fetchStyles(url)
+      expect(css).toContain(candidate`font-bold`)
+    })
+
+    // Flush stdout so we only see messages triggered by the edit below.
+    process.flush()
+
+    // Edit the server-only module. The client environment watches this file
+    // but it only exists in the server module graph. Without the fix, the
+    // Tailwind CSS plugin would trigger a full page reload on the client
+    // instead of letting react-router handle HDR.
+    await fs.write('app/direct-hdr-dep.ts', ts` export const direct = 'HDR: 1' `)
+
+    // check update
+    await retryAssertion(async () => {
+      let html = await (await fetch(url)).text()
+      expect(html).toContain('HDR: 1')
+
+      let css = await fetchStyles(url)
+      expect(css).toContain(candidate`font-bold`)
+    })
+
+    // Assert the client receives an HMR update (not a full page reload).
+    await process.onStdout((m) => m.includes('(client) hmr update'))
+  },
+)
+
 test('build mode', { fs: WORKSPACE }, async ({ spawn, exec, expect }) => {
   await exec('pnpm react-router build')
   let process = await spawn('pnpm react-router-serve ./build/server/index.js')

packages/@tailwindcss-vite/src/index.ts

@@ -208,9 +208,30 @@ export default function tailwindcss(opts: PluginOptions = {}): Plugin[] {
           // Note: in Vite v7.0.6 the modules here will have a type of `js`, not
           // 'asset'. But it will also have a `HARD_INVALIDATED` state and will
           // do a full page reload already.
-          let isExternalFile = modules.every((mod) => mod.type === 'asset' || mod.id === undefined)
+          //
+          // Empty modules can be skipped since it means it's not `addWatchFile`d and thus irrelevant to Tailwind.
+          let isExternalFile =
+            modules.length > 0 &&
+            modules.every((mod) => mod.type === 'asset' || mod.id === undefined)
           if (!isExternalFile) return
 
+          // Skip if the module exists in other environments. SSR framework has
+          // its own server side hmr/reload mechanism when handling server
+          // only modules. See https://v6.vite.dev/guide/migration.html
+          // > Updates to an SSR-only module no longer triggers a full page reload in the client. ...
+          for (let environment of Object.values(server.environments)) {
+            if (environment.name === this.environment.name) continue
+
+            let modules = environment.moduleGraph.getModulesByFile(file)
+            if (modules) {
+              for (let module of modules) {
+                if (module.type !== 'asset') {
+                  return
+                }
+              }
+            }
+          }
+
           for (let env of new Set([this.environment.name, 'client'])) {
             let roots = rootsByEnv.get(env)
             if (roots.size === 0) continue