CSS file not loading Core 3.0

After upgrading my project to the release of Core 3.0 the css file no longer loads and just returns an empty page.

I was able to reproduce this on the demo project in the repo just by upgrading it to core 3.0. I had to comment out

     app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");
            });

to run the project.

EDIT: setting o.ConfigureKestrel(options => options.AllowSynchronousIO = true); fixes the issue, but i am guessing is not a good solution

[NickBranstein]

I had this same issue when using IIS so I do not think it is fully fixed. Instead of turning on SyncIO for the entire server you can combine the two above solutions and turn it off for specific requests.

app.UseWhen(context => context.Request.Path.StartsWithSegments("/elmah", StringComparison.OrdinalIgnoreCase), appBuilder =>
            {
                appBuilder.Use(next =>
                {
                    return async ctx =>
                    {
                        ctx.Features.Get<IHttpBodyControlFeature>().AllowSynchronousIO = true;

                        await next(ctx);
                    };
                });
            });

            app.UseElmah();

[JonBraund]

I found this problem wasn’t solved by the keerthiraja1988’s code snippet when using IIS with the .net core 3 hosting bundle as kestrel doesn’t seem to be relevant. Instead this similar snippet worked for me in ConfigureServices in startup.cs

services.Configure<IISServerOptions>(options => { options.AllowSynchronousIO = true; });

Hope this helps someone