Demonstrates how to host a WPF application in your test then verify visual things.

InProcessAutomation.cs

public static class TestHost
  {
    Application _app;
    Thread _appThread;
    
    public static void Start()
    {
      _appThread = new Thread(StartDispatcher);
    }
    
    public static void StartDispatcher()
    {
      _app = new Application { ShutdownOptions = ShutdownOptions.OnExplicitShutdown };
      _app.Run();
    }
    
    public static SwitchToUIThreadAwaiter SwitchToAppThread()
    {
      return new SwitchToUIThreadAwaiter(_app.Dispatcher);
    }
    
    public static void Shutdown()
    {
      _app.InvokeShutdown();
      _appThread.Join();
    }
  }
  
  public class SwitchContextToUiThreadAwaiter : INotifyCompletion
{
    private readonly Dispatcher _uiContext;
 
    public SwitchContextToUiThreadAwaiter(Dispatcher uiContext)
    {
        _uiContext = uiContext;
    }

    public SwitchContextToUiThreadAwaiter GetAwaiter()
    {
        return this;
    }
 
    public bool IsCompleted { get { return false; } }
 
    public void OnCompleted(Action continuation)
    {
        _uiContext.Invoke(s => ((Action)s)(), continuation); 
    }
 
    public void GetResult() { }
}
  
  public class AutomationTestBase
  {
    static AutomationTestBase()
    {
      TestHost.Start();
      
      AppDomain.Unload += Unload;
    }
    
    public static Unload(..)
    {
      TestHost.Shutdown();
    }
  }

public class AutomationTests : AutomationTestBase
{
  [Fact]
  public async Task VerifyBrushes()
  {
    // Instead of this you could have TestHost.InvokeOnUIThread(()=> ...); for all interactions
    // This will basically run the rest of the method on the TestHost ui thread
    await TestHost.SwitchToUIThread();
    
    var window = new MahAppsWindow();
    window.Show();
    
    // Then just use approval tests to verify it hasn't changed.
    // Approval tests will render the control as an image, then do a image diff
    // You can also grab the visual tree and do stuff with it
    Approvals.Verify(window.FindControl("SomeControl"));
  }
}

Nice, thanks for useful snippet. Problem that it is not compiling.