[devtools] Allow opting out of environment feature flags (#80897)

Author: eps1lon

packages/next/src/build/define-env.ts

@@ -107,9 +107,7 @@ export function getDefineEnv({
   const isDynamicIOEnabled = !!config.experimental.dynamicIO
   const isUseCacheEnabled = !!config.experimental.useCache
 
-  const isDevToolPanelUIEnabled =
-    !!process.env.__NEXT_DEVTOOL_NEW_PANEL_UI ||
-    !!config.experimental.devtoolNewPanelUI
+  const isDevToolPanelUIEnabled = Boolean(config.experimental.devtoolNewPanelUI)
 
   const defineEnv: DefineEnv = {
     // internal field to identify the plugin config

packages/next/src/server/config-shared.ts

@@ -1355,6 +1355,7 @@ export const defaultConfig = {
       static: process.env.NEXT_STATIC_CACHE_HANDLER_PATH,
     },
     cssChunking: true,
+    devtoolNewPanelUI: process.env.__NEXT_DEVTOOL_NEW_PANEL_UI === 'true',
     multiZoneDraftMode: false,
     appNavFailHandling: false,
     prerenderEarlyExit: true,