diff --git a/ext/bg/js/backend.js b/ext/bg/js/backend.js
index 08ce82a2..5eb7982d 100644
--- a/ext/bg/js/backend.js
+++ b/ext/bg/js/backend.js
@@ -861,6 +861,7 @@ class Backend {
 
     _createActionListenerPort(port, sender, handlers) {
         let hasStarted = false;
+        let messageString = '';
 
         const onProgress = (...data) => {
             try {
@@ -871,12 +872,34 @@ class Backend {
             }
         };
 
-        const onMessage = async ({action, params}) => {
+        const onMessage = (message) => {
             if (hasStarted) { return; }
-            hasStarted = true;
-            port.onMessage.removeListener(onMessage);
 
             try {
+                const {action, data} = message;
+                switch (action) {
+                    case 'fragment':
+                        messageString += data;
+                        break;
+                    case 'invoke':
+                        {
+                            hasStarted = true;
+                            port.onMessage.removeListener(onMessage);
+
+                            const messageData = JSON.parse(messageString);
+                            messageString = null;
+                            onMessageComplete(messageData);
+                        }
+                        break;
+                }
+            } catch (e) {
+                cleanup(e);
+            }
+        };
+
+        const onMessageComplete = async (message) => {
+            try {
+                const {action, params} = message;
                 port.postMessage({type: 'ack'});
 
                 const messageHandler = handlers.get(action);
@@ -893,25 +916,29 @@ class Backend {
                 const result = async ? await promiseOrResult : promiseOrResult;
                 port.postMessage({type: 'complete', data: result});
             } catch (e) {
-                if (port !== null) {
-                    port.postMessage({type: 'error', data: errorToJson(e)});
-                }
-                cleanup();
+                cleanup(e);
             }
         };
 
-        const cleanup = () => {
+        const onDisconnect = () => {
+            cleanup(null);
+        };
+
+        const cleanup = (error) => {
             if (port === null) { return; }
+            if (error !== null) {
+                port.postMessage({type: 'error', data: errorToJson(error)});
+            }
             if (!hasStarted) {
                 port.onMessage.removeListener(onMessage);
             }
-            port.onDisconnect.removeListener(cleanup);
+            port.onDisconnect.removeListener(onDisconnect);
             port = null;
             handlers = null;
         };
 
         port.onMessage.addListener(onMessage);
-        port.onDisconnect.addListener(cleanup);
+        port.onDisconnect.addListener(onDisconnect);
     }
 
     _getErrorLevelValue(errorLevel) {
diff --git a/ext/mixed/js/api.js b/ext/mixed/js/api.js
index 075ea545..5e3195d6 100644
--- a/ext/mixed/js/api.js
+++ b/ext/mixed/js/api.js
@@ -236,7 +236,6 @@ const api = (() => {
 
         _invokeWithProgress(action, params, onProgress, timeout=5000) {
             return new Promise((resolve, reject) => {
-                let timer = null;
                 let port = null;
 
                 if (typeof onProgress !== 'function') {
@@ -245,12 +244,6 @@ const api = (() => {
 
                 const onMessage = (message) => {
                     switch (message.type) {
-                        case 'ack':
-                            if (timer !== null) {
-                                clearTimeout(timer);
-                                timer = null;
-                            }
-                            break;
                         case 'progress':
                             try {
                                 onProgress(...message.data);
@@ -275,10 +268,6 @@ const api = (() => {
                 };
 
                 const cleanup = () => {
-                    if (timer !== null) {
-                        clearTimeout(timer);
-                        timer = null;
-                    }
                     if (port !== null) {
                         port.onMessage.removeListener(onMessage);
                         port.onDisconnect.removeListener(onDisconnect);
@@ -288,17 +277,20 @@ const api = (() => {
                     onProgress = null;
                 };
 
-                timer = setTimeout(() => {
-                    cleanup();
-                    reject(new Error('Timeout'));
-                }, timeout);
-
                 (async () => {
                     try {
                         port = await this._createActionPort(timeout);
                         port.onMessage.addListener(onMessage);
                         port.onDisconnect.addListener(onDisconnect);
-                        port.postMessage({action, params});
+
+                        // Chrome has a maximum message size that can be sent, so longer messages must be fragmented.
+                        const messageString = JSON.stringify({action, params});
+                        const fragmentSize = 1e7; // 10 MB
+                        for (let i = 0, ii = messageString.length; i < ii; i += fragmentSize) {
+                            const data = messageString.substring(i, i + fragmentSize);
+                            port.postMessage({action: 'fragment', data});
+                        }
+                        port.postMessage({action: 'invoke'});
                     } catch (e) {
                         cleanup();
                         reject(e);