ComboBox ToggleButton の FocusVisualElement が表示されない理由がわかりません。Focused Storyboard を MouseOver に移動してみましたが、動作するので、ストーリーボードが動作すると確信しています。おそらく、フォーカスされた VisualState は起動していませんか? 以下に私のスタイルを含めました。ご協力いただきありがとうございます！
<Style TargetType="ComboBox">
<Setter Property="Height" Value="25"/>
<Setter Property="Padding" Value="5,5,25,5"/>
<Setter Property="FontSize" Value="12"/>
<Setter Property="FontWeight" Value="Thin"/>
<Setter Property="FontFamily" Value="Arial"/>
<Setter Property="Foreground" Value="#FFCCCCCC"/>
<Setter Property="Background" Value="#FF222222"/>
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="TabNavigation" Value="Once" />
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
<Setter Property="BorderBrush" Value="#FF666666"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBox">
<Grid>
<Grid.Resources>
<Style x:Name="comboToggleStyle" TargetType="ToggleButton">
<Setter Property="Foreground" Value="#FFFFFFFF"/>
<Setter Property="Background" Value="#FF000000"/>
<Setter Property="BorderBrush" Value="#FFFFFFFF"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="Padding" Value="5"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ToggleButton">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="MouseOverBackground" Storyboard.TargetProperty="Opacity" To="1"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="PressedBackground" Storyboard.TargetProperty="Opacity" To="1"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="DisabledBackground" Storyboard.TargetProperty="Opacity" To="1"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="CheckStates">
<VisualState x:Name="Checked">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="CheckedBackground" Storyboard.TargetProperty="(UIElement.Opacity)" To="1"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Unchecked"/>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetProperty="Visibility" Storyboard.TargetName="FocusVisualElement">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Unfocused"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Rectangle x:Name="Background" Fill="{TemplateBinding Background}" StrokeThickness="{TemplateBinding BorderThickness}" Stroke="{TemplateBinding BorderBrush}"/>
<Rectangle x:Name="MouseOverBackground" Opacity="0" Stroke="#FF888888" StrokeThickness="{TemplateBinding BorderThickness}"/>
<Rectangle x:Name="PressedBackground" Opacity="0" Fill="#FF0096db" StrokeThickness="{TemplateBinding BorderThickness}" Stroke="#00000000"/>
<Rectangle x:Name="DisabledBackground" Opacity="0" Fill="#FF222222" StrokeThickness="{TemplateBinding BorderThickness}" Stroke="#00000000"/>
<Rectangle x:Name="CheckedBackground" Opacity="0" Fill="{TemplateBinding Background}" StrokeThickness="{TemplateBinding BorderThickness}" Stroke="#00000000"/>
<ContentPresenter
x:Name="contentPresenter"
Content="{TemplateBinding Content}"
ContentTemplate="{TemplateBinding ContentTemplate}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
Margin="{TemplateBinding Padding}"/>
<Rectangle x:Name="FocusVisualElement" Stroke="#FF0096db" StrokeThickness="{TemplateBinding BorderThickness}" Visibility="Collapsed" IsHitTestVisible="false"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Grid.Resources>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal" />
<VisualState x:Name="MouseOver" />
<VisualState x:Name="Disabled"/>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Focused" />
<VisualState x:Name="Unfocused"/>
<VisualState x:Name="FocusedDropDown">
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="00:00:00" Storyboard.TargetName="PopupBorder" Storyboard.TargetProperty="(UIElement.Visibility)">
<DiscreteObjectKeyFrame KeyTime="00:00:00">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="ValidationStates">
<VisualState x:Name="Valid"/>
<VisualState x:Name="InvalidUnfocused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ValidationErrorElement" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0" >
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="InvalidFocused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ValidationErrorElement" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0" >
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="validationTooltip" Storyboard.TargetProperty="IsOpen">
<DiscreteObjectKeyFrame KeyTime="0" >
<DiscreteObjectKeyFrame.Value>
<sys:Boolean>True</sys:Boolean>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="ContentPresenterBorder">
<Grid>
<ToggleButton x:Name="DropDownToggle" Style="{StaticResource comboToggleStyle}" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Background="{TemplateBinding Background}" Margin="0" HorizontalContentAlignment="Right" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}">
<Path x:Name="BtnArrow" Height="4" Width="8" Stretch="Uniform" Data="F1 M 301.14,-189.041L 311.57,-189.041L 306.355,-182.942L 301.14,-189.041 Z "
Margin="0,0,6,0" HorizontalAlignment="Right" Fill="#FFCCCCCC"/>
</ToggleButton>
<ContentPresenter x:Name="ContentPresenter"
Margin="{TemplateBinding Padding}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
<TextBlock Text=" " />
</ContentPresenter>
</Grid>
</Border>
<Rectangle x:Name="DisabledVisualElement" Fill="White" Opacity="0" IsHitTestVisible="false" />
<Border x:Name="ValidationErrorElement" BorderThickness="1" BorderBrush="#FFDB000C" Visibility="Collapsed">
<ToolTipService.ToolTip>
<ToolTip x:Name="validationTooltip" Template="{StaticResource CommonValidationToolTipTemplate}" Placement="Right"
PlacementTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}"
DataContext="{Binding RelativeSource={RelativeSource TemplatedParent}}">
<ToolTip.Triggers>
<EventTrigger RoutedEvent="Canvas.Loaded">
<BeginStoryboard>
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="validationTooltip" Storyboard.TargetProperty="IsHitTestVisible">
<DiscreteObjectKeyFrame KeyTime="0" >
<DiscreteObjectKeyFrame.Value>
<sys:Boolean>true</sys:Boolean>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</ToolTip.Triggers>
</ToolTip>
</ToolTipService.ToolTip>
<Grid Width="12" Height="12" HorizontalAlignment="Right" Margin="1,-4,-4,0" VerticalAlignment="Top" Background="Transparent">
<Path Margin="1,3,0,0" Data="M 1,0 L6,0 A 2,2 90 0 1 8,2 L8,7 z" Fill="#FFDC000C"/>
<Path Margin="1,3,0,0" Data="M 0,0 L2,0 L 8,6 L8,8" Fill="#ffffff"/>
</Grid>
</Border>
<Popup x:Name="Popup">
<Border x:Name="PopupBorder" HorizontalAlignment="Stretch" Height="Auto" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}"
Background="{TemplateBinding Background}">
<ScrollViewer x:Name="ScrollViewer" BorderThickness="0" Padding="0">
<ItemsPresenter/>
</ScrollViewer>
</Border>
</Popup>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>