IdentityServer4 Windows Authentication Missing Callback implementation

The documentation to setup Windows Authentication is here: https://docs.identityserver.io/en/latest/topics/windows.html

But I have no idea how to configure the Callback() method referred to in the line RedirectUri = Url.Action("Callback"), or wethere or not I'm even supposed to use that.

I tried manually redirecting back to the https://<client:port>/auth-callback route of my angular app but I get the error:

Error: No state in response
    at UserManager.processSigninResponse (oidc-client.js:8308)

Does someone have a suggested Callback method I can use with an SPA using code + pkce ? I've tried searching Google but there are no current example apps using Windows Authentication and the ones that do exist are old.

Take a look at the ExternalLoginCallback method. I've also pasted the version of the code as of 26 Oct 2020 below for future reference incase the repo goes away.

    /// <summary>
    /// Post processing of external authentication
    /// </summary>
    [HttpGet]
    public async Task<IActionResult> ExternalLoginCallback()
    {
        // read external identity from the temporary cookie
        var result = await HttpContext.AuthenticateAsync(IdentityConstants.ExternalScheme);
        if (result?.Succeeded != true)
        {
            throw new Exception("External authentication error");
        }

        // lookup our user and external provider info
        var (user, provider, providerUserId, claims) = await FindUserFromExternalProviderAsync(result);
        if (user == null)
        {
            // this might be where you might initiate a custom workflow for user registration
            // in this sample we don't show how that would be done, as our sample implementation
            // simply auto-provisions new external user
            user = await AutoProvisionUserAsync(provider, providerUserId, claims);
        }

        // this allows us to collect any additonal claims or properties
        // for the specific prtotocols used and store them in the local auth cookie.
        // this is typically used to store data needed for signout from those protocols.
        var additionalLocalClaims = new List<Claim>();
        additionalLocalClaims.AddRange(claims);

        var localSignInProps = new AuthenticationProperties();
        ProcessLoginCallbackForOidc(result, additionalLocalClaims, localSignInProps);
        ProcessLoginCallbackForWsFed(result, additionalLocalClaims, localSignInProps);
        ProcessLoginCallbackForSaml2p(result, additionalLocalClaims, localSignInProps);

        // issue authentication cookie for user
        // we must issue the cookie maually, and can't use the SignInManager because
        // it doesn't expose an API to issue additional claims from the login workflow
        var principal = await _signInManager.CreateUserPrincipalAsync(user);
        additionalLocalClaims.AddRange(principal.Claims);

        var name = principal.FindFirst(JwtClaimTypes.Name)?.Value ?? user.Id;
        await _events.RaiseAsync(new UserLoginSuccessEvent(provider, providerUserId, user.Id, name));

        // issue authentication cookie for user
        var isuser = new IdentityServerUser(principal.GetSubjectId())
        {
            DisplayName = name,
            IdentityProvider = provider,
            AdditionalClaims = additionalLocalClaims
        };

        await HttpContext.SignInAsync(isuser, localSignInProps);

        // delete temporary cookie used during external authentication
        await HttpContext.SignOutAsync(IdentityConstants.ExternalScheme);

        // validate return URL and redirect back to authorization endpoint or a local page
        var returnUrl = result.Properties.Items["returnUrl"];
        if (_interaction.IsValidReturnUrl(returnUrl) || Url.IsLocalUrl(returnUrl))
        {
            return Redirect(returnUrl);
        }

        return Redirect("~/");
    }