Focus Textbox using FocusManager.FocusedElement issue

I'm trying to set the keyboard focus to a textbox that is included in a stackpanel. When the IsEditMode becomes true i want the textbox to become, by default, focused.

I've tried this code:

<DataTemplate x:Key="LibraryItemTemplate">
<StackPanel Orientation="Vertical">
    <StackPanel.Style>
       <Style TargetType="StackPanel">
          <Style.Triggers>
               <DataTrigger Binding="{Binding IsEditMode}" Value="True">
                   <Setter Property="FocusManager.FocusedElement" Value="{Binding ElementName=TxtB}"/>
               </DataTrigger>
          </Style.Triggers>
       </Style>
    </StackPanel.Style>

    <TextBlock x:Name="TxtA" Text="A" />
    <TextBox x:Name="TxtB" Text="B" Visibility="{Binding IsEditMode, Converter={StaticResource BoolVisibilityCollapsed}}"/>
</StackPanel>
</DataTemplate>
....
<ListView x:Name="LibraryListView" SelectedItem="{Binding SelectedItem,   UpdateSourceTrigger=PropertyChanged}" >
<ListView.View>
    <GridView>
        <GridViewColumn CellTemplate="{StaticResource LibraryItemTemplate}"  Width="Auto"/>
    </GridView>
</ListView.View>

But the problem is the mouse doesn't marking seems the keyboard focus is not in textbox and I have to click by mouse once again to TextBox to be able to input some text in TextBox.

Any idea?

After FocusManager is setting the focus you have to handle this event and in the event you have to add

<TextBox x:Name="TxtB" 
         Text="B" 
         GotFocus="TxtB_GotFocus"  
         Visibility="{Binding IsEditMode
             , Converter={StaticResource BoolVisibilityCollapsed}}"/>

....
private void TxtB_GotFocus(object sender, RoutedEventArgs e)
{
    this.Dispatcher.BeginInvoke((Action)delegate
    {
       Keyboard.Focus(TxtB);
    }, DispatcherPriority.Render);
}

Thanks a lot to Darlene

And I'm adding the answer by myself to meet Sheridan's suggestion Thanks a lot