itayhac I've submitted a FB as FB8735832, but there has been no activity on it since my original submission on September 2020

Anyway, is this the right place to report bugs? This is 100% a bug. I don't know, but I assume it is as good a place as any. See my response to your question - https://developer.apple.com/forums/thread/659681?answerId=634052022#634052022, if you only need top frame responses there is a solution for your issue

You can get the responses to work by using the sendResponse callback. If you look at the browser compatibility table for onMessage listener - https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage#Browser_compatibility you will see that responses via promises only natively work on Firefox (and if you're using a browser. polyfill then the promise response works for chrome too). You can do something like this for safari: browser.runtime.onMessage.addListener((message, sender, sendResponse) => { 	if (x === 'hello') { 		console.warn(x); 		performAsyncOperation(function callback() { sendResponse('ok') }) 		// need to return true if you plan on sending a response asynchronously 		return true; 	} 	if (x === 'bye') { 		sendResponse('ok') 		// no need to return anything here, since we send the response synchronously 	} });