Why unsubscribing an event handler from an event from inside the event handler itself makes Visual Studio think that app is running when app closes?

Question

I am writing an WPF application using PRISM. I am running it through debug from Visual Studio.

When I am creating the Shell, I register an event handler for the Shell's ContentRendered event. This event handler does something unrelated (that I can see is completed on the UI, just filling a ListBox with strings) then unregister the event from the event handler (which I know, in practice, is unnecessary).

If I close the application with the event handler self-unsubscribing, a few threads (that I see on the Visual Studio debug tools) are left alive seemingly forever (>10 min waiting, but the .exe seems to be gone on process explorer), so I have to manually press "Stop debugging" before being able to start the app again.

I can't figure out why this happens. Can someone please explain what is happening?

If I comment out the unsubscription on the code below, the issue is gone:

using Prism.Ioc;
using Prism.Modularity;
using Prism.Unity;
using System.Windows;
using MyTest.Views;

namespace MyTest
{
    public partial class App : PrismApplication
    {
        protected override IModuleCatalog CreateModuleCatalog()
        {
            return new DirectoryModuleCatalog() { ModulePath = @".\Modules" };
        }

        protected override Window CreateShell()
        {
            var Shell = Container.Resolve<MainWindow>();
            Shell.ContentRendered += On_Shell_First_Presentation;
            return Shell;
        }

        protected override Window On_Shell_First_Presentation(object sender, EventArgs e)
        {
            Container.Resolve<IModuleManager>().Run();
            
            // Uncommenting line below gets visual studio stuck
            //Container.Resolve<Shell>().ContentRendered -= On_Shell_First_Presentation;
        }
    }
}

BTW, Am I triggering the loading of the modules the wrong way? If I don't do this, the modules are never loaded, as in, the class that inherits from IModule are constructed but neither functions defined are ever called. I sent OnDemand value on the Module attribute to False. I expected PRISM to trigger it, but I really haven't figured out why it doesn't.

EDIT: Per suggestion on comments, I confirmed that the way it is written actually makes a new Window object to be created but I still do not understand why this would cause this behavior. Fixing the commented line to user a proper reference to the Window instantiated on CreateShell eliminates the issue.