Why is the first item of a ComboBox is selected after switch to another TabItem?

Question

I have a TabControl with three TabItems. In each of the TabItems is one ComboBox. If I switch through the TabItems the first entry of the ComboBoxes is selected. But I want that nothing is selected. How to do it?

Here are some screenshots:

After the first call of the form, there is nothing selected

After the switch to the second tab, the first element of the combobox is selected

Update: This is the code for this example

   <Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Width="525" SizeToContent="Height">
<Grid>
    <TabControl>
        <TabItem Header="Test 1">
            <ComboBox>
                <ComboBoxItem>Test 1</ComboBoxItem>
                <ComboBoxItem>Test 2</ComboBoxItem>
            </ComboBox>
        </TabItem>
        <TabItem Header="Test 2">
            <ComboBox>
                <ComboBoxItem>Test 1</ComboBoxItem>
                <ComboBoxItem>Test 2</ComboBoxItem>
            </ComboBox>
        </TabItem>
        <TabItem Header="Test 3">
            <ComboBox>
                <ComboBoxItem>Test 1</ComboBoxItem>
                <ComboBoxItem>Test 2</ComboBoxItem>
            </ComboBox>
        </TabItem>
    </TabControl>
</Grid>

No binding. A brand new project in Visual Studio Express 2010.

And here is the code behind:

using System.Windows;
namespace WpfApplication1
{
   public partial class MainWindow : Window
   {
      public MainWindow()
      {
        InitializeComponent();
      }
   }
}

Answer 1

With your help and a long journey through the web, I now know, that there is no way to avoid the behavior, that the first focusable child on tabitem gets focused, without some code behind :(

Thank you all for your answers, especially @makc for his advice, that the blue background means, that the combobox is focused.