What is the difference between HasEffectiveKeyboardFocus & IsKeyboardFocused in wpf

Question

We have an editabled Combobox(Cb1) which has a customised Popup with another editable Combobox(Cb2). Below is the sample code

 public class ExComboBox : ComboBox
    {            
      public static readonly DependencyProperty CustomPopupProperty = DependencyProperty.Register(
        "CustomPopup", 
        typeof(FrameworkElement), 
        typeof(ExComboBox),
        new PropertyMetadata((d, e) =>
        {

        }));       
   
       public FrameworkElement CustomPopup
       {
          get { return (FrameworkElement)GetValue(CustomPopupProperty); }
          set { SetValue(CustomPopupProperty, value); }
       }

      static ExComboBox()
      {
          DefaultStyleKeyProperty.OverrideMetadata(typeof(ExComboBox), new FrameworkPropertyMetadata(typeof(ExComboBox)));
      }
}

With in the XAML we have a CustomControl which will show the binded 'Popup'.

<ControlTemplate x:Key="ExComboBoxTemplate" TargetType="{x:Type local:ExComboBox}">
<Grid x:Name="templateRoot" SnapsToDevicePixels="true" >
  <Grid.ColumnDefinitions>
    <ColumnDefinition Width="*" />
    <ColumnDefinition MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" Width="0" />
  </Grid.ColumnDefinitions>
  <Popup x:Name="PART_Popup" AllowsTransparency="true" Grid.ColumnSpan="2" IsOpen="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Margin="1" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Placement="Bottom" >
    <Themes:SystemDropShadowChrome x:Name="shadow" Color="Transparent" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=templateRoot}" >
      <Border x:Name="dropDownBorder" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="1" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}">
        <ContentControl Content="{TemplateBinding CustomPopup}"  />
      </Border>
    </Themes:SystemDropShadowChrome>
  </Popup>
  <ToggleButton x:Name="toggleButton" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ComboBoxToggleButton}"  />
  <ContentPresenter x:Name="contentPresenter" ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Content="{TemplateBinding SelectionBoxItem}" />
</Grid>

  <ExComboBox x:Name="Cb1" IsEditable="True" Text="{Binding Value1, UpdateSourceTrigger=LostFocus}" 
                                 HorizontalAlignment="Left">
                        <ExComboBox.CustomPopup >                                   
                            <StackPanel Margin="5">
                                <GroupBox>
                                    <GroupBox.Header>
                                        <TextBlock Text="My Header" />
                                    </GroupBox.Header>
                                    <StackPanel Margin="5">
                                        <TextBlock Text="Select or Enter value" />
                                        
                                        <ComboBox x:Name="Cb2" 
                                                  IsEditable="True" 
                                                  Text="{Binding Value2}"                               
                                                  IsTextSearchEnabled="True" 
                                                  IsSynchronizedWithCurrentItem="True" 
                                                  ItemsSource="{Binding Value2Collection}" 
                                                  VerticalAlignment="Center"  />               
                                           <!-- Other controls -->

                                    </StackPanel>
                                </GroupBox>

                            </StackPanel>
                        </ExComboBox.CustomPopup >
                    </ExComboBox>

We also has some validation on the values that has been entered on both the comboboxes. When I tried to enter the value in Cb2, Cb1's previewTextInput is getting called first, instead of Cb2. I am expecting Cb2 will handle first as Cb2 got the keyboardfocus. When looked into the properties 'HasEffectiveKeyboardFocus ' is always showing the right value, but not 'IsKeyboardFocused'.

Could you please let me know the difference between HasEffectiveKeyboardFocus & IskeyboardFocused. Or how can I get the right focused element, so that I can ignore Cb1's previewTextInput while entering value in Cb2. ( I can't use 'HasEffectiveKeyboardFocus as it is protected proeprty) Thanks in advance.