Because I use TaskbarRadWindow from this article: http://www.telerik.com/forums/can-t-see-radwindow-in-the-task-bar. The workaround is easy:

this.Loaded += TaskbarRadWindow_Loaded;

void TaskbarRadWindow_Loaded(object sender, RoutedEventArgs e)
        {
            _window = this.ParentOfType<Window>();
            if (_window != null)
            {
                var childrens = this.ChildrenOfType<Grid>();
                var header = childrens.FirstOrDefault(x => x.Name == "Header");
                if (header != null)
                {
                    _window.MaxWidth = SystemParameters.PrimaryScreenWidth;
                    _window.MaxHeight = SystemParameters.PrimaryScreenHeight - header.ActualHeight;
                }

                _window.ShowInTaskbar = true;
                _window.StateChanged += window_StateChanged;
                _window.Focus();
                this.Loaded -= TaskbarRadWindow_Loaded;
            }
        }

Not only header buttons. The whole right and bottom side of the window goes out of screen.