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