Winforms Invalidate doesn't fire up OnPaint

Question

I'm trying to move a form smoothly using code given on question http://stackoverflow.com/questions/967968/how-to-smoothly-animate-windows-forms-location-with-different-speeds

But for some reason my this.Invalidate() call will never fire up the OnPaint event. Is there some configuration that's required on the form for this to be possible?

Edit:

Threading is involved, as it runs in a backgroundworker with it's own messageloop. Here's the code :

public class PopupWorker
{
    public event PopupRelocateEventHandler RelocateEvent;

    private BackgroundWorker worker;
    private MyPopup popupForm;

    public PopupWorker()
    {
     worker = new BackgroundWorker();
     worker.DoWork += worker_DoWork;
    }

    void worker_DoWork(object sender, DoWorkEventArgs e)
    {
     popupForm = PopupCreator.CreatePopup("Title", "BodyText");
     this.RelocateEvent += popupForm.OnRelocate;
     popupForm.CustomShow();
     Application.Run();
    }

    public void Show()
    {
     worker.RunWorkerAsync();
    }

    public void PopupRelocate(object sender, Point newLocation)
    {
     if (popupForm.InvokeRequired)
      popupForm.Invoke(new PopupRelocateEventHandler(PopupRelocate), new object[] {sender, newLocation});
     else
      RelocateEvent(this, newLocation);
    }
}

Form :

public void OnRelocate(object sender, Point newLocation)
{
    targetLocation = newLocation;
    this.Invalidate();
}

protected override void OnPaint(PaintEventArgs e)
{
    base.OnPaint(e);
    if (Location.Y != targetLocation.Y)
    {
        Location = new Point(Location.X, Location.Y + 10);
        if (Location.Y > targetLocation.Y)
            Location = targetLocation;
        this.Invalidate();
    }
}