Leaving a dropped down ComboBox swallows focus in WPF

[cabmar]

I have the following problem: on a WPF window I placed a combo box and a button. I drop down the combo box and while the combo box is still dropped down I click on the button. The button does not react on the mouse click but the dropped down combo box closes. After a second mouse click the button reacts. And here's the code sample:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Height="100" Width="300">
    <Grid>
        <ComboBox VerticalAlignment="Top" HorizontalAlignment="Left"
                  Width="100" IsSynchronizedWithCurrentItem="True" >
            <ComboBoxItem Content="item 1"/>
            <ComboBoxItem Content="item 2"/>
            <ComboBoxItem Content="item 3"/>
        </ComboBox>
        <Button Content="Test" VerticalAlignment="Bottom" HorizontalAlignment="Right"
                Width="100" Click="Button_Click" />
    </Grid>
</Window>

and the code behind:

private void Button_Click(object sender, RoutedEventArgs e)
{
    MessageBox.Show("Button clicked");
}

How can I prevent the combo box to swallow the focus on the button? Thanks.

regards mc