fix(next): use correct hmr url if assetPrefix is set in next config (#10859)

The next.js assetPrefix needs to be included in the websocket URL.

Previously, we were appending both assetPrefix and basePath, which is incorrect. `assetPrefix` overrides `basePath` if both are set. This PR mimics the way Next.js connects to the HMR server.

Sources:
- https://github.com/AlessioGr/next.js/blob/canary/packages/next/src/server/lib/router-server.ts#L688
- https://github.com/AlessioGr/next.js/blob/canary/packages/next/src/server/config.ts#L322
- https://github.com/AlessioGr/next.js/blob/canary/packages/next/src/client/components/react-dev-overlay/app/client-entry.tsx

Author: AlessioGr

packages/next/src/withPayload.js

@@ -147,10 +147,6 @@ export const withPayload = (nextConfig = {}) => {
     toReturn.env.NEXT_BASE_PATH = nextConfig.basePath
   }
 
-  if (nextConfig.assetPrefix) {
-    toReturn.env.NEXT_ASSET_PREFIX = nextConfig.assetPrefix
-  }
-
   return toReturn
 }
 

packages/payload/src/index.ts

@@ -922,11 +922,13 @@ export const getPayload = async (
     ) {
       try {
         const port = process.env.PORT || '3000'
-        const basePath = process.env.NEXT_BASE_PATH || ''
-        const assetPrefix = process.env.NEXT_ASSET_PREFIX || ''
+
+        const path = '/_next/webpack-hmr'
+        // The __NEXT_ASSET_PREFIX env variable is set for both assetPrefix and basePath (tested in Next.js 15.1.6)
+        const prefix = process.env.__NEXT_ASSET_PREFIX ?? ''
 
         cached.ws = new WebSocket(
-          `ws://localhost:${port}${basePath}${assetPrefix}/_next/webpack-hmr`,
+          process.env.PAYLOAD_HMR_URL_OVERRIDE ?? `ws://localhost:${port}${prefix}${path}`,
         )
 
         cached.ws.onmessage = (event) => {