How do I detect a prerendering app in Blazor?

You can use the IHttpContextAccessor.HttpContext.Response.HasStarted property to check whether the application is pre-rendering or not. HasStarted specifies that the response header has been sent to the client. If HasStarted is set to false, it means that the application is still pre-rendering and client connection is not yet established.

@using Microsoft.AspNetCore.Http;
 
<button @onclick="@onClick"> Click </button>
 
@code {
    [Inject]
    protected IHttpContextAccessor httpContextAccessor { get; set; }
 
    private void onClick()
    {
        var isPreRendering = !this.httpContextAccessor.HttpContext.Response.HasStarted;
    }
}

The HttpContextAccessor service should be registered by calling the AddHttpContextAccessor method in the Startup.cs.

public class Startup
{
    . . . . .
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddRazorPages();
        services.AddServerSideBlazor();
        services.AddSingleton<WeatherForecastService>();
        services.AddHttpContextAccessor();
    }
    . . . . . 
}