Razor Runtime View Compilation does not work in Razor Class Libraries

Which exact Umbraco version are you using? For example: 9.0.1 - don't just write v9

9.4.3

Bug summary

If Razor Runtime View Compilation is configured for Razor Class Libraries (RCL) (e.g., in the RCL's Composer) that is consumed by an Umbraco 9 project, runtime view compilation doesn't work.

Specifics

Seems to be related to the logic in the AddInMemoryModelsRazorEngine method, where it takes a copy of the container's service registrations, but composers are registered by default after the copy has been made so any new FileProviders registered in MvcRazorRuntimeCompilationOptions in composers aren't known to the RefreshingRazorViewEngine's view engine factory function.

Replacing the view engine with the default one using the following code works around the issue. Call the following code after the UmbracoBuilder calls. However, this will break re-compilation of views when in-memory models are re-generated.

    public static IServiceCollection FixRuntimeViewCompilationForRazorClassLibrariesInUmbraco(this IServiceCollection services)
    {
        if (services.FirstOrDefault(x => x is
            {
                ServiceType: { } serviceType,
                ImplementationFactory.Method.Name: { } factoryName, // requires C#10 langversion
            } && serviceType == typeof(IRazorViewEngine) && factoryName.Contains("AddInMemoryModelsRazorEngine")
        ) is { } faultyViewEngineRegistration)
        {
            services.Remove(faultyViewEngineRegistration);
        }

        return services;
    }

Another work around would be to configure runtime view compilation options before the IUmbracoBuilder.AddWebsite call where ModelsBuilder is registered. This can be done manually, or by moving AddComposers above AddWebsite (which changes DI registration order)

I think we should only enable the RefreshingRazorViewEngine if in-memory models are being used, so that RCLs work for source code models projects, and perhaps document this gotcha in the Models Builder docs. The model mode check would need to be done using IConfiguration as the IOptions<ModelsBuilderSettings> won't be available at the time where the check is needed.

I'm happy to look into implementing the above suggestion once triaged and if it's up for grabs.

Steps to reproduce

I've created a repo @ https://github.com/xdjoshuaaz/UmbracoRCLRuntimeViewCompilationBug

1) Create an Umbraco 9 solution 2) Create a Razor Class Library project in the solution 3) Reference the RCL project from the Umbraco project 4) Create a Composer in the RCL with runtime view compilation configured as documented in these Microsoft docs

You won't need to call AddRuntimeViewCompilation as Umbraco does this already 5) Create and render the RCL's view somehow. The most typical Umbraco-like steps would be:
Build & run the project
Create a content type, allowed at the root, with a template
Stop the project
Move the Razor template view file into the RCL
- Note: Umbraco template view files in RCLs might not a supported thing in Umbraco, but I'd expect partials and non-template views to work as it would in plain old ASP.NET Core
Populate the view with some dummy text that we'll try to change at runtime later
Build & run the project
Create and publish a content node of the created content type
Visit the newly created content node 6) Observe the dummy text 7) Try changing the dummy text in the RCL's view while Umbraco is running 8) Refresh the page

Expected result / actual result

Expected: Dummy text should have changed (i.e. the Razor view is re-compiled) Actual: Dummy text does not change (i.e. the Razor view did not re-compile)

umbraco / Umbraco-CMS