Leaving a dropped down ComboBox swallows focus in WPF

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><ComboBoxVerticalAlignment="Top"HorizontalAlignment="Left"Width="100"IsSynchronizedWithCurrentItem="True"><ComboBoxItemContent="item 1"/><ComboBoxItemContent="item 2"/><ComboBoxItemContent="item 3"/></ComboBox><ButtonContent="Test"VerticalAlignment="Bottom"HorizontalAlignment="Right"Width="100"Click="Button_Click"/></Grid></Window>

and the code behind:

privatevoidButton_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