Mock @/utils/postMessage (unused)

It ended up not being necessary

src/__mocks__/@/utils/postMessage.ts

@@ -0,0 +1,69 @@
+/*
+ * Copyright (C) 2022 PixieBrix, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/**
+ * @file and security
+ * 1. The content script generates an iframe with a local document.
+ * 2. postMessage only works with `"*"` in this direction
+ *    https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage#using_window.postmessage_in_extensions_non-standard
+ * 3. The iframe is safe because it's local and wrapped in a Shadow DOM,
+ *    thus inaccessible/not-alterable by the host website.
+ * 4. Each content script message includes a private channel port that the
+ *    iframe can use to respond exclusively to the content script.
+ * 5. The channel is closed immediately after the response.
+ *
+ * Prior art: https://groups.google.com/a/chromium.org/g/chromium-extensions/c/IPJSfjNSgh8/m/Dh35-tZPAgAJ
+ * Relevant discussion: https://github.com/w3c/webextensions/issues/78
+ */
+
+import {
+  PostMessageInfo,
+  PostMessageListener,
+  RequestPacket,
+} from "@/utils/postMessage";
+
+const window = new EventTarget();
+export default async function postMessage({ type, payload }: PostMessageInfo) {
+  const packet: RequestPacket = {
+    type,
+    payload,
+  };
+  console.log("will post", packet);
+
+  window.dispatchEvent(
+    new MessageEvent("message", {
+      data: packet,
+    })
+  );
+}
+
+export function addPostMessageListener(
+  type: string,
+  listener: PostMessageListener,
+  { signal }: { signal?: AbortSignal } = {}
+): void {
+  console.log("will listen to", type);
+  window.addEventListener(
+    "message",
+    (event: MessageEvent<RequestPacket>) => {
+      if (event.data.type === type) {
+        listener(event.data.payload);
+      }
+    },
+    { signal }
+  );
+}