From 872e92084704601b8a77023989970b9fe3165293 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rub=C3=A9n=20Norte?= <rubennorte@meta.com>
Date: Mon, 26 Oct 2026 04:50:59 -0700
Subject: [PATCH] Restore Fantom DevTools launching with `FANTOM_DEBUG_JS`
 (#59410)

Summary:

Restore DevTools launching when `FANTOM_DEBUG_JS` is set by allowing the existing native tester to launch through the shared browser launcher. Explicitly allow app launching for interactive test debugging, use a Chrome app window instead of the standalone shell, and run debugging sessions in band.

Changelog: [Internal]

Reviewed By: lenaic, andrewdacenko

Differential Revision: D127171074
---
 .../src/utils/DefaultToolLauncher.js          |  9 ++++--
 .../runner/global-setup/globalSetup.js        | 28 ++++++++++++++-----
 scripts/fantom.sh                             |  2 +-
 3 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/packages/dev-middleware/src/utils/DefaultToolLauncher.js b/packages/dev-middleware/src/utils/DefaultToolLauncher.js
index 8f8d89961d8b..ba654d4e872a 100644
--- a/packages/dev-middleware/src/utils/DefaultToolLauncher.js
+++ b/packages/dev-middleware/src/utils/DefaultToolLauncher.js
@@ -23,8 +23,13 @@ const {apps, openApp} = open;
  * local machine.
  */
 const DefaultToolLauncher = {
-  launchDebuggerAppWindow: async (url: string): Promise<void> => {
-    if (process.env.NODE_ENV === 'test') {
+  launchDebuggerAppWindow: async (
+    url: string,
+    options?: Readonly<{
+      allowInTests?: boolean,
+    }>,
+  ): Promise<void> => {
+    if (process.env.NODE_ENV === 'test' && options?.allowInTests !== true) {
       assertMockedInTests();
     }
 
diff --git a/private/react-native-fantom/runner/global-setup/globalSetup.js b/private/react-native-fantom/runner/global-setup/globalSetup.js
index b8c7e4cf5fda..92d2c5e43a79 100644
--- a/private/react-native-fantom/runner/global-setup/globalSetup.js
+++ b/private/react-native-fantom/runner/global-setup/globalSetup.js
@@ -10,10 +10,17 @@
 
 import type {DevToolLauncher} from '@react-native/dev-middleware';
 
-import {isOSS, validateEnvironmentVariables} from '../EnvironmentOptions';
+import {
+  debugJS,
+  isOSS,
+  validateEnvironmentVariables,
+} from '../EnvironmentOptions';
 import {getTestBuildOutputPath} from '../paths';
 import build from './build';
-import {createDevMiddleware} from '@react-native/dev-middleware';
+import {
+  createDevMiddleware,
+  unstable_DefaultToolLauncher as DefaultToolLauncher,
+} from '@react-native/dev-middleware';
 import * as Metro from 'metro';
 import {Server} from 'node:net';
 import path from 'node:path';
@@ -55,19 +62,26 @@ async function startMetroServer() {
   // $FlowExpectedError[cannot-write]
   metroConfig.server.port = Number(process.env.__FANTOM_METRO_PORT__);
 
-  // No-op app launcher to prevent debugger-shell launches during tests
+  const serverBaseUrl = `http://localhost:${metroConfig.server.port}`;
   const devToolLauncher: DevToolLauncher = {
-    launchDebuggerAppWindow: async (_url: string) => {},
-    launchDebuggerShell: async () => {},
-    prepareDebuggerShell: async () => ({code: 'not_implemented'}),
+    launchDebuggerAppWindow: async (url: string) => {
+      if (debugJS) {
+        await DefaultToolLauncher.launchDebuggerAppWindow(url, {
+          allowInTests: true,
+        });
+      }
+    },
   };
 
   const {
     middleware: devMiddleware,
     websocketEndpoints: debuggerWebsocketEndpoints,
   } = createDevMiddleware({
-    serverBaseUrl: `http://localhost:${metroConfig.server.port}`,
+    serverBaseUrl,
     unstable_toolLauncher: devToolLauncher,
+    unstable_experiments: {
+      enableStandaloneFuseboxShell: false,
+    },
   });
 
   const server = await Metro.runServer(metroConfig, {
diff --git a/scripts/fantom.sh b/scripts/fantom.sh
index 29f54d6e95c8..3cfa32231979 100755
--- a/scripts/fantom.sh
+++ b/scripts/fantom.sh
@@ -33,7 +33,7 @@ for arg in "$@"; do
   esac
 done
 
-if [[ -n "$FANTOM_RUN_BENCHMARKS" ]]; then
+if [[ -n "$FANTOM_RUN_BENCHMARKS" || -n "$FANTOM_DEBUG_JS" ]]; then
   ARGS+=("--runInBand")
 fi