WPF TabIndex in a composite control

Question

I have a simple window with a simple composite control embedded within it.

(Main Window)

<Window x:Class="TabOrder.Window1"
xmlns:local="clr-namespace:TabOrder"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300">
<Grid>
    <Label HorizontalAlignment="Left" VerticalAlignment="Top">First</Label>
    <TextBox TabIndex="0" HorizontalAlignment="Stretch" VerticalAlignment="Top" Margin="80,0,0,0"/>

    <Label HorizontalAlignment="Left" VerticalAlignment="Top" Margin="0,30,0,0">Second</Label>
    <TextBox TabIndex="1" HorizontalAlignment="Stretch" VerticalAlignment="Top" Margin="80,30,0,0"/>

    <local:MyControl Margin="0,60,0,0" VerticalAlignment="Top" HorizontalAlignment="Stretch" TabIndex="2"/>
</Grid>

(Composite control)

<UserControl x:Class="TabOrder.MyControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"&gt;
<Grid>
    <Label HorizontalAlignment="Left" VerticalAlignment="Top">Third</Label>
    <TextBox HorizontalAlignment="Stretch" VerticalAlignment="Top" Margin="80,0,0,0"/>

    <Label HorizontalAlignment="Left" VerticalAlignment="Top" Margin="0,30,0,0">Fourth</Label>
    <TextBox HorizontalAlignment="Stretch" VerticalAlignment="Top" Margin="80,30,0,0"/>
</Grid>

As expected on my form I get 4 text boxes...

• First
• Second
• Third
• Fourth

But when "First" has focus and I hit tab the focus is switched to "Third". WPF seems to be seeing the tab list as a single flat list rather than as a tree where MyControl is TabIndex 3 and the text box "Third" the first tabbed control within it.

Is this a bug in WPF or is there another way of doing this? The composite control is used in many windows, it could even be used more than once on a single window.