Only forward messages in the background page (#224)

source/targetLogic.test.ts

@@ -1,7 +1,7 @@
 import { assert, describe, test, vi } from "vitest";
 import { getActionForMessage } from "./targetLogic.js";
 import { type Tabs } from "webextension-polyfill";
-import { isContentScript } from "webext-detect-page";
+import { isContentScript, isBackground } from "webext-detect-page";
 
 vi.mock("webext-detect-page");
 
@@ -55,7 +55,7 @@ describe("getActionForMessage", async () => {
     ["contentScript", "somePage", "frame", "respond"], // Wrong, but won't happen
 
     ["contentScript", "thisTab", "background", "forward"],
-    ["contentScript", "thisTab", "somePage", "forward"],
+    ["contentScript", "thisTab", "somePage", "ignore"],
     ["contentScript", "thisTab", "tab", "respond"], // Won't happen, content scripts cannot target tabs
     ["contentScript", "thisTab", "frame", "respond"], // Won't happen, content scripts cannot target tabs
   ] satisfies Array<
@@ -70,6 +70,7 @@ describe("getActionForMessage", async () => {
     async (from, to, receiver, expected) => {
       const isCs = receiver === "tab" || receiver === "frame";
       vi.mocked(isContentScript).mockReturnValueOnce(isCs);
+      vi.mocked(isBackground).mockReturnValueOnce(receiver === "background");
       vi.stubGlobal("location", {
         origin: isCs ? "http://example.com" : "chrome-extension://extension-id",
       });

source/targetLogic.ts

@@ -1,4 +1,4 @@
-import { isContentScript } from "webext-detect-page";
+import { isBackground, isContentScript } from "webext-detect-page";
 import { type AnyTarget, type Sender } from "./types.js";
 import { type Entries } from "type-fest";
 
@@ -47,7 +47,12 @@ export function getActionForMessage(
 
   // We're in an extension page, but the target is not one.
   if (!to.page) {
-    return "forward";
+    // Only the background page can forward messages at the moment
+    // https://github.com/pixiebrix/webext-messenger/issues/219#issuecomment-2011000477
+    // If this condition is changed, it might also need to ensure that messages are not
+    // forwarded to itself, e.g. when using the tabs API in an iframed extension page.
+    // https://github.com/pixiebrix/webext-messenger/issues/221#issuecomment-2010165690
+    return isBackground() ? "forward" : "ignore";
   }
 
   // Set "this" tab to the current tabId