offscreen.js

// This URL must point to the public site
const _URL = 'https://webmaker.app/signup';
const iframe = document.createElement('iframe');
iframe.src = _URL;
document.documentElement.appendChild(iframe);
chrome.runtime.onMessage.addListener(handleChromeMessages);

function handleChromeMessages(message, sender, sendResponse) {
	// Extensions may have an number of other reasons to send messages, so you
	// should filter out any that are not meant for the offscreen document.
	if (message.target !== 'offscreen') {
		return false;
	}

	function handleIframeMessage({ data }) {
		console.log('got postmessage in offscreen doc from iframe');

		try {
			if (data.startsWith('!_{')) {
				// Other parts of the Firebase library send messages using postMessage.
				// You don't care about them in this context, so return early.
				return;
			}
			data = JSON.parse(data);
			self.removeEventListener('message', handleIframeMessage);

			// being sent back to worker
			sendResponse(data);
		} catch (e) {
			console.log(`json parse failed - ${e.message}`);
		}
	}

	globalThis.addEventListener('message', handleIframeMessage, false);

	// Initialize the authentication flow in the iframed document. You must set the
	// second argument (targetOrigin) of the message in order for it to be successfully
	// delivered.
	iframe.contentWindow.postMessage(
		{ initAuth: true, providerName: message.providerName },
		new URL(_URL).origin
	);
	return true;
}