Version of Sitecore's GoToLoginPage processor for sites using CallbackAuthority setting

CustomGoToLoginPage.cs

using Sitecore.Abstractions;
using Sitecore.Diagnostics;
using Sitecore.Owin.Authentication.IdentityServer.Extensions;
using Sitecore.Owin.Authentication.Pipelines.CookieAuthentication.ApplyRedirect;
using System;
using System.Collections.Specialized;
using System.Web;

namespace Identity.Pipelines.CookieAuthentication.ApplyRedirect
{
    public class CustomGoToLoginPage : GoToLoginPage
    {
        BaseSettings Settings { get; }

        public CustomGoToLoginPage(BaseSettings settings)
        {
            Settings = settings;
        }

        public override void Process(ApplyRedirectArgs args)
        {
            Assert.ArgumentNotNull(args, nameof(args));

            string callbackAuthority = Settings.IdentityServer().CallbackAuthority;
            Uri baseUrl = args.Context.Request.Uri;
            if (!string.IsNullOrEmpty(callbackAuthority))
                Uri.TryCreate(callbackAuthority, UriKind.Absolute, out baseUrl);

            Uri uri = new Uri(baseUrl, new Uri(args.Context.RedirectUri, UriKind.RelativeOrAbsolute));
            if (!string.Equals(args.Context.Options.LoginPath.Value, uri.LocalPath, StringComparison.OrdinalIgnoreCase) || string.IsNullOrWhiteSpace(args.Site?.LoginPage))
                return;
            NameValueCollection queryString = HttpUtility.ParseQueryString(uri.Query);
            queryString[args.Context.Options.ReturnUrlParameter] = HttpContext.Current.Request.RawUrl;
            UriBuilder uriBuilder = new UriBuilder(uri)
            {
                Path = args.Site.LoginPage,
                Query = queryString.ToString()
            };
            args.Context.RedirectUri = uriBuilder.ToString();
        }
    }
}