feat(web-extension): detect bg script when on firefox

Firefox runs background script in a hidden page,
so it has a window object.

packages/web-extension/src/messaging/index.ts

@@ -1,8 +1,9 @@
 import { BackgroundMessenger, createBackgroundMessenger, generalizeBackgroundMessenger } from './BackgroundMessenger';
 import { ChannelName, ConsumeRemoteApiOptions, ExposeApiProps, MessengerDependencies } from './types';
-import { FinalizationRegistryDestructor } from './util';
+import { FinalizationRegistryDestructor, isBackgroundProcess } from './util';
 import { consumeMessengerRemoteApi, exposeMessengerApi } from './remoteApi';
 import { createNonBackgroundMessenger } from './NonBackgroundMessenger';
+import { runtime } from 'webextension-polyfill';
 
 export * from './BackgroundMessenger';
 export * from './NonBackgroundMessenger';
@@ -15,7 +16,7 @@ export * from './errors';
 
 export type BaseChannel = { baseChannel: ChannelName };
 
-const isInBackgroundProcess = typeof window === 'undefined';
+const isInBackgroundProcess = isBackgroundProcess(runtime);
 
 const getBackgroundMessenger = (() => {
   let backgroundMessenger: BackgroundMessenger | null = null;

packages/web-extension/src/messaging/types.ts

@@ -60,6 +60,7 @@ export interface MessengerPort {
 export interface MinimalRuntime {
   connect(connectInfo: Runtime.ConnectConnectInfoType): MessengerPort;
   onConnect: MinimalEvent<(port: MessengerPort) => void>;
+  getBackgroundPage?: unknown;
   lastError?: unknown;
 }
 

packages/web-extension/src/messaging/util.ts

@@ -9,6 +9,7 @@ import {
   FactoryCallMessage,
   InternalMsg,
   MethodRequest,
+  MinimalRuntime,
   RequestMessage,
   ResponseMessage
 } from './types';
@@ -77,3 +78,6 @@ export class FinalizationRegistryDestructor implements Destructor {
     this.#registry.register(obj, objectId);
   }
 }
+
+export const isBackgroundProcess = (runtime: MinimalRuntime) =>
+  typeof window === 'undefined' || typeof runtime.getBackgroundPage === 'function';