I tried some other technique to debug this and get to the root of the issue, I decided to put window.webkit.messageHandlers.handlerOne.postMessage(... and window.webkit.messageHandlers.handlerTwo.postMessage(... inside an if conditional and check which is the object that is `undefined. (function() { addEventListener('mousedown', function() { if (window.webkit && window.webkit.messageHandlers && window.webkit.messageHandlers.handlerOne) { window.webkit.messageHandlers.handlerOne.postMessage('WKUserScriptMouseDown'); } }, true); })(); After debugging this I found out that window.webkit.messageHandlers is the undefined object, which leads me to believe that webView.configuration.userContentController.add(handlerClass, name: "...") is broken on iOS 17.2 beta hence messageHandlers is never defined, I'd appreciate to hear if this is happening to someone else with their apps.

I tried removing the custom handlers and attempted this: (function() { addEventListener('mousedown', function() { window.webkit.messageHandlers.postMessage('WKUserScriptMouseDown'); }, true); })(); I would think this broadcast the message to all the handlers?? However now I get the error message: TypeError: undefined is not an object (evaluating 'window.webkit.messageHandlers.postMessage') My next attempt is to wrap this line into a (function() {})(); but if someone else has an idea or is facing similar issues with iOS beta 17.2 only it would be great to hear ideas