Visual Studio Designer shows empty window with custom ContentPropertyAttribute

My application has a lot of windows and most of them share some basic features. Because of that I extended theWindow class to create a base for all my windows.

Everything compiles and displays fine but the designer just shows an empty window when I use my window class.

I made a basic working example that can be easily used, my real window is much more complex but this shows the problem. Here is the code:

using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Markup;

namespace WpfApplication1
{
    [ContentProperty("ContentElement")]
    public class MyWindow : Window
    {
        public ToolBar ToolBar { get; private set; }
        public StatusBar StatusBar { get; private set; }
        public Border ContentBorder { get; private set; }

        public UIElement ContentElement
        {
            get { return (UIElement)GetValue(ContentElementProperty); }
            set { SetValue(ContentElementProperty, value); }
        }
        public static readonly DependencyProperty ContentElementProperty = DependencyProperty.Register(
 "ContentElement", typeof(UIElement), typeof(MyWindow),
            new PropertyMetadata(null, (d, e) =>
             {
                 MyWindow w = (MyWindow)d;
                 w.ContentBorder.Child = (UIElement)e.NewValue;
             }));

        public MyWindow() : base()
        {
            ToolBar = new ToolBar();
            ToolBar.Height = 30;
            ToolBar.VerticalAlignment = VerticalAlignment.Top;

            StatusBar = new StatusBar();
            StatusBar.Height = 20;
            StatusBar.VerticalAlignment = VerticalAlignment.Bottom;

            ContentBorder = new Border();
            ContentBorder.SetValue(MarginProperty, new Thickness(0, 30, 0, 20));

            Grid grid = new Grid();
            grid.Children.Add(ToolBar);
            grid.Children.Add(ContentBorder);
            grid.Children.Add(StatusBar);
            Content = grid;
        }
    }
}

XAML:

<local:MyWindow x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="300" Width="300"><Grid><Rectangle Fill="Blue" /></Grid></local:MyWindow>

Doing the exact same thing with a UserControl works just fine, also in the designer. Just replace every occurance ofMyWindow with MyUserControl and extend from UserControl if you want to try that.

Is there any way I can get a custom Window like that to work with the designer, or do i have to make aUserControl and use that in every window? Also, is this some kind of bug or intended behavior?

Addional info: I'm running Visual Studio 2015 Community and I'm using .net 4.6

I Also tried another approach. Instead of using the ContentPropertyAttribute i have overwritten the ContentProperty like this:

new public object Content {
    get { return GetValue(ContentProperty); }
    set { SetValue(ContentProperty, value); }
}
new public static DependencyProperty ContentProperty = DependencyProperty.Register("Content", typeof(object), typeof(BaseUserControl), new PropertyMetadata(null, (s, e) =>
{
    MyWindow bw = (MyWindow)s;
    bw.ContentBorder.Child = (UIElement)e.NewValue;
}));

Again this works completely fine with a UserControl. With the Window I can at least see the Content in the designer, but the ToolBar and StatusBar are still not visible in the Designer. When running it everything works correctly