How to map fallback in ASP .NET Core Web API so that Blazor WASM app only intercepts requests that are not to the API

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To recap the problem, when somebody makes a request to:

https://yourapp.com/api/someendpoint

and /api/someendpoint can’t be found, they’re taken to a Blazor page. This default behaviour is weird. For requests starting with /api, they were expecting an HTTP Status Code and probably a JSON object too, but instead, they got HTML. Maybe they don’t even use your app. Maybe they’re not even human (more likely they’re a piece of software).

This is how you send them an HTTP Status Code instead.
On your controllers:

[Route("api/[controller]")]
public class SampleController : ControllerBase
{
    // ...
}

In Startup.cs:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    // ...
    app.UseStaticFiles();
    app.UseRouting();
    app.UseEndpoints(endpoints =>
    {
        endpoints.MapRazorPages();
        endpoints.MapControllers();
        endpoints.Map("api/{**slug}", HandleApiFallback);
        endpoints.MapFallbackToFile("{**slug}", "index.html");
    });
}

private Task HandleApiFallback(HttpContext context)
{
    context.Response.StatusCode = StatusCodes.Status404NotFound;
    return Task.CompletedTask;
}

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