Since I don't get any answers I try to provide some more info and code.

Basically it is a matter that the AssignedContentItem is not set, but I can't figure out a way around this or to set it.

Here's the code

I hijack the route by this code

u.EndpointRouteBuilder.MapControllerRoute(
"Category view",
"/category/{shortname}",
new { Controller = "Category", Action = "Category" });

Then I have a CategoryController

public class CategoryController : UmbracoPageController, IVirtualPageController
{
   public IPublishedContent? FindContent(ActionExecutingContext actionExecutingContext)
    {
        if (_umbracoContextAccessor.TryGetUmbracoContext(out var umbracoContext))
        {
            var categoryRoot = umbracoContext.Content.GetById(1083); // todo: hämta listnamn
            if (categoryRoot != null)
            {
                return categoryRoot;
            }
        }

        return null;
    } 

    public IActionResult Category(string shortname)
    {
        // retrieve data from db etc
        var viewmodel = new MyViewModel();
        return View(viewmodel);
    }
}

The view then inherits an master template that have the following line

 @(await Component.InvokeAsync("TopNavigation", new { selected = Model.Selected }))

The TopNavigation component looks something like this

[ViewComponent(Name = "TopNavigation")]
public class TopNavigationViewComponent : ViewComponent
{
    private readonly INavigationService navigationService;

    public TopNavigationViewComponent(INavigationService navigationService)
    {
        this.navigationService = navigationService;
    }

    public IViewComponentResult Invoke(int selected)
    {
        var model = new TopNavigationViewModel()
        {
            Items = this.navigationService.GetTopNavigation(),
        };
        return View(model);
    }

}

The Navigation Service

public class NavigationService : INavigationService
{
    private readonly UmbracoHelper umbracoHelper;
    private readonly ISiteSettings siteSettings;

    public NavigationService(UmbracoHelper umbracoHelper, ISiteSettings siteSettings)
    {
        this.umbracoHelper = umbracoHelper;
        this.siteSettings = siteSettings;
    }

    public List<IPublishedContent> GetSubNavigation(int currentId)
    {
        var currentPage = this.umbracoHelper.Content(currentId);
        var parentOrSelf = currentPage.AncestorOrSelf(2);
        return parentOrSelf.Children?.ToList();
    }

    public IEnumerable<UmbNavItem> GetTopNavigation()
    {
        var menu = this.siteSettings.Menu;
        return menu;
    }
}

And last the site settings. Here I have tried to use IUmbracoContextFactory to retrieve the data without luck. It's the following line

this.settings.GetProperty(PropertyAlias.HeaderMenu).GetValue() as IEnumerable<UmbNavItem>

that seems to don't have an umbraco context. I thought that using FindContent in IVirtualPageController was used to set the current node, but I suppose it does'nt?

public class SiteSettings : ISiteSettings
{
    private readonly UmbracoHelper umbracoHelper;
    private readonly IUmbracoContextFactory umbracoContextFactory;
    private readonly IContentService contentService;

    public SiteSettings(UmbracoHelper umbracoHelper, IUmbracoContextFactory umbracoContextFactory, IContentService contentService)
    {
        this.umbracoHelper = umbracoHelper;
        this.umbracoContextFactory = umbracoContextFactory;
        this.contentService = contentService;
    }

    private IPublishedContent home => this.umbracoHelper.ContentAtXPath($"//{ContentTypeAlias.Home}")?.FirstOrDefault();
    private IPublishedContent settings  => this.umbracoHelper.ContentAtXPath($"//{ContentTypeAlias.SiteSettings}")?.FirstOrDefault();

    public string SiteName => this.home.Name;

    public IEnumerable<UmbNavItem> Menu => this.settings.GetProperty(PropertyAlias.HeaderMenu).GetValue() as IEnumerable<UmbNavItem>;

}

So what is the way to accually get the IPublishedContent from that settings when umbracoHelper does not have a Current?

Calling this will return that Current is null, but only when my custom route is used

this.settings.GetProperty(PropertyAlias.HeaderMenu).GetValue() as IEnumerable<UmbNavItem>
Count = 0
    Current: null
    Results View: Expanding the Results View will enumerate the IEnumerable

And I think it's because of AssignedContentItem is not set

umbracoHelper.AssignedContentItem
'umbracoHelper.AssignedContentItem' threw an exception of type 
'System.InvalidOperationException'
Data: {System.Collections.ListDictionaryInternal}
HResult: -2146233079
HelpLink: null
InnerException: null
Message: "Cannot return the IPublishedContent because the UmbracoHelper was not constructed with an IPublishedContent."
Source: "Umbraco.Web.Common"
StackTrace: "   at Umbraco.Cms.Web.Common.UmbracoHelper.get_AssignedContentItem()"
TargetSite: {Umbraco.Cms.Core.Models.PublishedContent.IPublishedContent get_AssignedContentItem()}

Ok, more test have been done. I found that HasValue returns true, and if I inspect the value of the property it has a private sourcevalue of

[{"udi":"umb://document/0dea73210cd349c492846538f436bb33","key":"0dea7321-0cd3-49c4-9284-6538f436bb33","name":"Om oss","description":"/om-oss/","title":"Om oss","url":"/om-oss/","children":[],"icon":"icon-umb-content color-blue","published":true,"naviHide":false,"culture":null,"itemType":"link","id":1075,"collapsed":true},{"udi":"umb://document/bdec7357fd824e36bb112bf0a490614b","key":"bdec7357-fd82-4e36-bb11-2bf0a490614b","name":"Min sida","description":"/min-sida/","url":"/min-sida/","children":[],"icon":"icon-umb-content color-blue","published":true,"naviHide":false,"culture":null,"itemType":"link","id":1080,"collapsed":true}]

So what I did was I created a simple text field in the settings, and I could retrieve that without problem. So it turns out it's not about the context, but about the property somehow.

The property is of type "AaronSadler.UmbNav" so I guess I may have to look up that implementation?

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