wpf ComboBox无法通过Tab键获得焦点

Question

这是我的GUI。

当我专注于TextBox时，我可以通过Tab键移动焦点。

但是，当我专注于电话号码文本框时，我按Tab键但焦点只是停留而不是移动。

我想通过按Tab键将焦点移动到ComboBox（可编辑）。

.xaml文件

 <TextBlock Text="{Resx incidentCallName }" Grid.Row="2" Style="{DynamicResource TextBlockSubStyle}"/>
            <TextBox x:Name="txtbox_who" Grid.Row="2" Grid.Column="1" Style="{DynamicResource TextInputStyle}" Validation.Error="Validation_Error"
             Text="{Binding UpdateSourceTrigger=PropertyChanged, Path=Person, ValidatesOnDataErrors=true, NotifyOnValidationError=true}"/>
            <TextBlock Text="{Resx incidentCallNo}" Grid.Row="3" Style="{DynamicResource TextBlockSubStyle}"/>
            <TextBox x:Name="txtbox_tel" Grid.Row="3" Grid.Column="1" Style="{DynamicResource TextInputStyle}" LostFocus="txtbox_tel_LostFocus" PreviewKeyDown="txtbox_tel_PreviewKeyDown" Validation.Error="Validation_Error"
             Text="{Binding UpdateSourceTrigger=PropertyChanged, Path=Tel, ValidatesOnDataErrors=true, NotifyOnValidationError=true}"/>
            <Button x:Name="btn_history" IsTabStop="True" Grid.Row="3" Grid.Column="2" Content="!" Margin="1" IsEnabled="False" Click="btn_history_Click" FontSize="18"/>
            <TextBlock Text="{Resx incidentList_type}" Grid.Row="4" Style="{DynamicResource TextBlockSubStyle}"/>
            <ComboBox x:Name="cb_type" Grid.Row="4" Grid.Column="1" Style="{DynamicResource ComboBoxStyle}" 
                      ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}},Path=TypeCombo, UpdateSourceTrigger=PropertyChanged}" 
                      DisplayMemberPath="Code_name" SelectedValuePath="Comm_code_id" IsEditable="True" IsReadOnly="True" Validation.Error="Validation_Error"
                      SelectedValue="{Binding UpdateSourceTrigger=PropertyChanged, Path=TypeCode, ValidatesOnDataErrors=true, NotifyOnValidationError=true}"
                      SelectedIndex="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}},Path=DataContext.CbTypeSelect, UpdateSourceTrigger=PropertyChanged}"/>

App.xaml（代表风格）

<Style x:Key="ComboBoxStyle" TargetType="ComboBox" BasedOn="{StaticResource {x:Type ComboBox}}">
            <Setter Property="FontSize" Value="12"/>
            <Setter Property="Margin" Value="5,5,5,5"/>
            <Setter Property="VerticalAlignment" Value="Center"/>
            <Setter Property="IsTabStop" Value="True"/>
            <Setter Property="KeyboardNavigation.TabNavigation" Value="Continue"/>
            <Style.Triggers>
                <Trigger Property="IsSelectionBoxHighlighted" Value="True">
                    <Setter Property="IsDropDownOpen" Value="true" />
                </Trigger> 
                <Trigger Property="IsKeyboardFocusWithin" Value="True">
                    <Setter Property="IsDropDownOpen" Value="true" />
                </Trigger>
            </Style.Triggers>
        </Style>

有什么问题？请帮帮我。