windows state maximised method cuts maximised window into half

Question

I'm trying to maximize a window when the user clicks on the maximize button. The problem is that the method that I use cuts the window in half whenever it's maximized:

Here is the method that I used :

void OnStateChanged(object sender, EventArgs args)
{
    if (WindowState == WindowState.Minimized)
    {
        if (Messenger.Option.showlearnetlogo)
        {
            Logo.Visibility = Visibility.Hidden;
            this.Title = Properties.Resources.MainView_OnStateChanged_Learnet_Communicator;
        }

        for (int i = App.Current.Windows.Count - 1; i >= 0; i--)
        {
            if (App.Current.Windows[i].IsActive)
                App.Current.Windows[i].WindowState = WindowState.Minimized;
        }
    }
    else if (WindowState == WindowState.Maximized)
    {
        if (prevwinstate == WindowState.Minimized)
            WindowState = WindowState.Normal;

        m_storedWindowState = WindowState;
    }
    else
    {
        if (Messenger.Option.showlearnetlogo)
        {
            this.Title = "";
            Logo.Visibility = Visibility.Visible;
        }

        m_storedWindowState = WindowState;
    }

    prevwinstate = WindowState;
}

What is causing this and is there anyway to fix the maximizing problem?