Why are some TemplateBinding values not shown in designer but others are? [with repro code]

Question

I've got an odd situation: Following sample code replaces the Content of the window, uses the setter's background color and provides a stack panel with two labels. One for the background and one for the title.

The thing is, that they during design time <Label Content="{TemplateBinding Background}" /> does perfectly show the background value however <Label Content="{TemplateBinding Title}" /> does that only when running the app. What is the difference here?

I've played around with the TargetType (by setting it to MainWindow, no effect)

This is the full sample:

<Window x:Class="TBinding.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Height="350" Width="525">
    <Window.Style>
        <Style TargetType="Window">
            <Setter Property="Background" Value="LawnGreen" />
            <Setter Property="Title" Value="The title of this window" />
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="Window">
                        <StackPanel Margin="50" Background="{TemplateBinding Background}">
                                <Label Content="{TemplateBinding Background}" />
                                <Label Content="{TemplateBinding Title}" />
                        </StackPanel>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </Window.Style>
</Window>

Answer 1

Found on connect: https://connect.microsoft.com/VisualStudio/feedback/details/770067/wpf-designer-not-sho

Designer creates a proxy for Window classes and not the Window itself. The substitute does not fully reflect all properties therefore has limited functionality.

This issues occurs in VS2012 as well in VS2013.