Notifier never calls the authorization listener

Expected Behavior

Clear documentation that shows how to handle the callback with code

[REQUIRED] Describe expected behavior

It will be nice to show what happens when the (web)app is redirected to the callback URL with the code

Describe the problem

I've configured and started the SSO flow. My app gets redirected to the callback URL /auth?code=jsDWmM6u8ebY1wtDM.... From this point it is not clear what should happen. The library does nothing, it doesn't exchange the code for tokens, the authorization listener gets never called. Can I make a request to exchange the code? How do I get the verifier? From the electron sample the verifier is taken from the request in the listener.

 AuthorizationServiceConfiguration.fetchFromIssuer('/oauth')
            .then(response => {
                this.configuration = response;
                this.showMessage('Completed fetching configuration');
            })
            .catch(error => {
                console.log('Something bad happened', error);
                this.showMessage(`Something bad happened ${error}`);
            });
        this.authorizationHandler.setAuthorizationNotifier(this.notifier);
        this.notifier.setAuthorizationListener((request, response, error) => {
            log('Authorization request complete ', request, response, error);
            if (response) {
                this.code = response.code;
                this.showMessage(`Authorization Code ${response.code}`);
            }
        });

[REQUIRED] Environment

• AppAuth-JS version: 1.3.1
• AppAuth-JS Environment (Node, Browser (UserAgent), ...): Browser (Firefox)
• Source code snippts (inline or JSBin)

Start by comparing the browser callback flow with the electron sample, focusing on AuthorizationServiceConfiguration, authorizationHandler, and the notifier listener shown in the issue. Confirm how the callback code and verifier are expected to be handled; done means the listener is invoked reliably or the documented limitation and required exchange steps are clear.