diff --git a/IMPLEMENTATION.md b/IMPLEMENTATION.md
index 0b0c1ec..a163ff1 100644
--- a/IMPLEMENTATION.md
+++ b/IMPLEMENTATION.md
@@ -2,13 +2,13 @@
 
 A bridge application instance (`statusBridge`) is created to keep track of receivers’ statuses. This is expected to exist throughout the lifetime of the extension and will automatically reconnect if unexpectedly disconnected.
 
-The `shim/contentSetup.ts` content script is registered for all pages. It creates an empty `window.chrome` object in the page context since some sites may expect it to exist. It also intercepts any `src` attribute changes on `<script>` elements where the cast API may be loaded directly from a `chrome-extension://` URL, then sets them to the regular cast API script URL.
+The `shim/content.ts` content script is registered for all pages. It creates an empty `window.chrome` object in the page context since some sites may expect it to exist. It also intercepts any `src` attribute changes on `<script>` elements where the cast API may be loaded directly from a `chrome-extension://` URL, then sets them to the regular cast API script URL.
 
 ## Shim Initialization
 
 The background script registers a `webRequest.onBeforeRequest` listener that intercepts requests to Google’s Cast API library.
 
-When a request is intercepted, the `shim/content.ts` script is executed in the content script context. This facilitates any message passing across content/page script isolation (the shim itself is executed in the page context, both for convenience — since it interacts substantially with page scripts — and security reasons).
+When a request is intercepted, the `shim/contentBridge.ts` script is executed in the content script context. This facilitates any message passing across content/page script isolation (the shim itself is executed in the page context, both for convenience — since it interacts substantially with page scripts — and security reasons).
 
 Messages passed to the shim are custom events of type `__castMessage`. Messages passed back from the shim are custom events of type `__castMessageResponse`. Event listening and creation is handled by the `shim/messageBridge.ts` module.
 
@@ -23,6 +23,12 @@ The cast API is now available to the web app.
 
 The web app calls `chrome.cast.initialize` with an `ApiConfig` object containing the Chromecast receiver app ID to use. The shim sends a `main:/shimInitialized` message to the background script. The bridge sends `shim:/serviceUp` messages for any discovered devices with device info (address, port, label, etc…).
 
+### Extension Sender Apps
+
+The initialization for built-in sender apps (media/mirroring) works slightly differently. They run in the content script context and import the shim, rather than running in the page script context.
+
+The exported shim comes from `shim/export.ts`. Instead of setting the `window.__onGCastApiAvailable` callback, the module provides an init function which returns a promise. The `shim/contentBridge.ts` script is executed from that module and the initialization process is identical until the `shim:/initialized` message is received, at which point the promise is resolved.
+
 ## User Interaction
 
 A user will trigger casting through the web app interface and the app calls `chrome.cast.requestSession`. The shim sends a `main:/selectReceiverBegin` message to the background script to open the receiver selector.
diff --git a/ext/src/main.ts b/ext/src/main.ts
index 2cd1af5..f9c8172 100755
--- a/ext/src/main.ts
+++ b/ext/src/main.ts
@@ -258,14 +258,7 @@ browser.menus.onShown.addListener(info => {
 browser.webRequest.onBeforeRequest.addListener(
         async details => {
             await browser.tabs.executeScript(details.tabId, {
-                code: `window._isFramework = ${
-                        details.url === CAST_FRAMEWORK_LOADER_SCRIPT_URL}`
-              , frameId: details.frameId
-              , runAt: "document_start"
-            });
-
-            await browser.tabs.executeScript(details.tabId, {
-                file: "shim/content.js"
+                file: "shim/contentBridge.js"
               , frameId: details.frameId
               , runAt: "document_start"
             });
@@ -402,7 +395,7 @@ browser.runtime.onMessage.addListener(async message => {
 // Defines window.chrome for site compatibility
 browser.contentScripts.register({
     allFrames: true
-  , js: [{ file: "shim/contentSetup.js" }]
+  , js: [{ file: "shim/content.js" }]
   , matches: [ "<all_urls>" ]
   , runAt: "document_start"
 });
diff --git a/ext/src/shim/cast/classes/Session.ts b/ext/src/shim/cast/classes/Session.ts
index 838123f..168915a 100755
--- a/ext/src/shim/cast/classes/Session.ts
+++ b/ext/src/shim/cast/classes/Session.ts
@@ -18,7 +18,7 @@ import { ErrorCode
 
 import { ListenerObject
        , onMessage
-       , sendMessageResponse } from "../../messageBridge";
+       , sendMessageResponse } from "../../eventMessageChannel";
 
 import { Callbacks
        , CallbacksMap
diff --git a/ext/src/shim/cast/index.ts b/ext/src/shim/cast/index.ts
index 0d5f5db..ac5fc5c 100755
--- a/ext/src/shim/cast/index.ts
+++ b/ext/src/shim/cast/index.ts
@@ -28,7 +28,7 @@ import * as media from "./media";
 
 import { ReceiverSelectorMediaType }
         from "../../receiver_selectors/ReceiverSelector";
-import { onMessage, sendMessageResponse } from "../messageBridge";
+import { onMessage, sendMessageResponse } from "../eventMessageChannel";
 
 
 type ReceiverActionListener = (
diff --git a/ext/src/shim/cast/media/classes/Media.ts b/ext/src/shim/cast/media/classes/Media.ts
index 757da65..8958b21 100644
--- a/ext/src/shim/cast/media/classes/Media.ts
+++ b/ext/src/shim/cast/media/classes/Media.ts
@@ -23,7 +23,7 @@ import { PlayerState
 import _Error from "../../classes/Error";
 import { ErrorCode } from "../../enums";
 
-import { onMessage, sendMessageResponse } from "../../../messageBridge";
+import { onMessage, sendMessageResponse } from "../../../eventMessageChannel";
 
 import { Callbacks
        , CallbacksMap
diff --git a/ext/src/shim/content.ts b/ext/src/shim/content.ts
index 4d01ba9..2de200f 100644
--- a/ext/src/shim/content.ts
+++ b/ext/src/shim/content.ts
@@ -1,42 +1,38 @@
 "use strict";
 
-import { Message } from "../types";
-import { onMessageResponse, sendMessage } from "./messageBridge";
-
-import { loadScript } from "../lib/utils";
+import { CAST_LOADER_SCRIPT_URL
+       , CAST_SCRIPT_URLS } from "../endpoints";
 
 
-if ((window as any)._isFramework) {
-    loadScript(browser.runtime.getURL("vendor/webcomponents-lite.js"));
-}
+const _window = (window.wrappedJSObject as any);
+
+_window.chrome = cloneInto({}, window);
+_window.navigator.presentation = cloneInto({}, window);
 
 
-// Message ports
-const backgroundPort = browser.runtime.connect({ name: "shim" });
-let popupPort: browser.runtime.Port;
+/**
+ * Replace the src property setter on <script> elements to
+ * intercept the new value.
+ *
+ * If it matches one of Chrome's cast extension sender script
+ * URLs, replace it with the standard API URL, the request for
+ * which is handled in the main script.
+ */
+const { get, set } = Reflect.getOwnPropertyDescriptor(
+        HTMLScriptElement.prototype.wrappedJSObject, "src");
 
+Reflect.defineProperty(
+        HTMLScriptElement.prototype.wrappedJSObject, "src", {
 
-// Set popupPort once it connects
-browser.runtime.onConnect.addListener(port => {
-    if (port.name === "popup") {
-        popupPort = port;
-    }
+    configurable: true
+  , enumerable: true
+  , get
 
-    port.onMessage.addListener(sendMessage);
-});
-
-// Forward background messages to shim
-backgroundPort.onMessage.addListener(sendMessage);
-
-// Forward shim messages to popup and background script
-onMessageResponse((message: Message) => {
-    const [ destination ] = message.subject.split(":/");
-
-    if (destination === "popup") {
-        if (popupPort) {
-            popupPort.postMessage(message);
+  , set: exportFunction(function (value) {
+        if (CAST_SCRIPT_URLS.includes(value)) {
+            return set.call(this, CAST_LOADER_SCRIPT_URL);
         }
-    } else {
-        backgroundPort.postMessage(message);
-    }
+
+        return set.call(this, value);
+    }, window)
 });
diff --git a/ext/src/shim/contentBridge.ts b/ext/src/shim/contentBridge.ts
new file mode 100644
index 0000000..fc1ea08
--- /dev/null
+++ b/ext/src/shim/contentBridge.ts
@@ -0,0 +1,15 @@
+"use strict";
+
+import { onMessageResponse, sendMessage } from "./eventMessageChannel";
+
+
+// Message port to background script
+const backgroundPort = browser.runtime.connect({ name: "shim" });
+
+// Forward background messages to shim
+backgroundPort.onMessage.addListener(sendMessage);
+
+// Forward shim messages to background
+onMessageResponse(message => {
+    backgroundPort.postMessage(message);
+});
diff --git a/ext/src/shim/contentSetup.ts b/ext/src/shim/contentSetup.ts
deleted file mode 100644
index 2de200f..0000000
--- a/ext/src/shim/contentSetup.ts
+++ /dev/null
@@ -1,38 +0,0 @@
-"use strict";
-
-import { CAST_LOADER_SCRIPT_URL
-       , CAST_SCRIPT_URLS } from "../endpoints";
-
-
-const _window = (window.wrappedJSObject as any);
-
-_window.chrome = cloneInto({}, window);
-_window.navigator.presentation = cloneInto({}, window);
-
-
-/**
- * Replace the src property setter on <script> elements to
- * intercept the new value.
- *
- * If it matches one of Chrome's cast extension sender script
- * URLs, replace it with the standard API URL, the request for
- * which is handled in the main script.
- */
-const { get, set } = Reflect.getOwnPropertyDescriptor(
-        HTMLScriptElement.prototype.wrappedJSObject, "src");
-
-Reflect.defineProperty(
-        HTMLScriptElement.prototype.wrappedJSObject, "src", {
-
-    configurable: true
-  , enumerable: true
-  , get
-
-  , set: exportFunction(function (value) {
-        if (CAST_SCRIPT_URLS.includes(value)) {
-            return set.call(this, CAST_LOADER_SCRIPT_URL);
-        }
-
-        return set.call(this, value);
-    }, window)
-});
diff --git a/ext/src/shim/messageBridge.ts b/ext/src/shim/eventMessageChannel.ts
similarity index 100%
rename from ext/src/shim/messageBridge.ts
rename to ext/src/shim/eventMessageChannel.ts
diff --git a/ext/src/shim/export.ts b/ext/src/shim/export.ts
index 5f4030d..79cd25a 100644
--- a/ext/src/shim/export.ts
+++ b/ext/src/shim/export.ts
@@ -4,7 +4,7 @@ import * as cast from "./cast";
 
 import { BridgeInfo } from "../lib/getBridgeInfo";
 import { Message } from "../types";
-import { onMessage } from "./messageBridge";
+import { onMessage } from "./eventMessageChannel";
 
 
 /**
@@ -18,7 +18,7 @@ export function init (): Promise<BridgeInfo> {
     return new Promise(async (resolve, reject) => {
 
         // Trigger message port setup side-effects
-        import("./content");
+        import("./contentBridge");
 
         onMessage(message => {
             switch (message.subject) {
diff --git a/ext/src/shim/framework/index.ts b/ext/src/shim/framework/index.ts
index faaf38c..e50a2ed 100644
--- a/ext/src/shim/framework/index.ts
+++ b/ext/src/shim/framework/index.ts
@@ -28,7 +28,7 @@ import { ActiveInputState
 
 import GoogleCastLauncher from "./GoogleCastLauncher";
 
-import { onMessage } from "../messageBridge";
+import { onMessage } from "../eventMessageChannel";
 
 
 export default {
diff --git a/ext/src/shim/index.ts b/ext/src/shim/index.ts
index 483a605..9ce90d1 100755
--- a/ext/src/shim/index.ts
+++ b/ext/src/shim/index.ts
@@ -4,7 +4,7 @@ import * as cast from "./cast";
 
 import { CAST_FRAMEWORK_SCRIPT_URL } from "../endpoints";
 import { loadScript } from "../lib/utils";
-import { onMessage } from "./messageBridge";
+import { onMessage } from "./eventMessageChannel";
 
 
 const _window = (window as any);
@@ -47,6 +47,12 @@ if (document.currentScript) {
 
         isFramework = true;
 
+        /**
+         * Framework API library requires webcomponents for the cast
+         * button custom element (<google-cast-launcher>).
+         */
+        loadScript(browser.runtime.getURL("vendor/webcomponents-lite.js"));
+
         const script = loadScript(CAST_FRAMEWORK_SCRIPT_URL);
         script.addEventListener("load", ev => {
             callPageReadyFunction();
diff --git a/ext/webpack.config.js b/ext/webpack.config.js
index d46c828..b028fec 100755
--- a/ext/webpack.config.js
+++ b/ext/webpack.config.js
@@ -26,7 +26,7 @@ module.exports = (env) => ({
         // Shim entries
       , "shim/bundle": `${env.includePath}/shim/index.ts`
       , "shim/content": `${env.includePath}/shim/content.ts`
-      , "shim/contentSetup": `${env.includePath}/shim/contentSetup.ts`
+      , "shim/contentBridge": `${env.includePath}/shim/contentBridge.ts`
     }
   , output: {
         filename: "[name].js"