TabItem Storyboard affects neighboring TabItems

We have a TabItem Style which contains a very simple ControlTemplate:

<Border x:Name="border" BorderThickn开发者_如何学编程ess="0" Background="{DynamicResource MediumGray}" Margin="0,0,1,1" 
    SnapsToDevicePixels="True" TextBlock.Foreground="{DynamicResource DarkGray}">
    <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center" Text="{TemplateBinding Header}" />
</Border>

The ControlTemplate contains an EventTrigger which fires on MouseEnter. The storyboard uses a ColorAnimation:

<Storyboard>
    <ColorAnimation To="{StaticResource _LightGray}" BeginTime="0:00:00"
        Duration="0:00:00.200"
        Storyboard.TargetName="border"
        Storyboard.TargetProperty="(Border.Background).(SolidColorBrush.Color)" />
</Storyboard>

The Storyboard works as expected - except that it also affects the other TabItems in the TabControl. They all animate to the target color.

Moving the original background brush in to a ControlTemplate Resource cleared this issue:

<ControlTemplate TargetType="{x:Type TabItem}">
    <ControlTemplate.Resources>
        <SolidColorBrush x:Key="borderBackground" Color="{StaticResource _MediumGray}" />
    </ControlTemplate.Resources>

    <Border Background="{StaticResource borderBackground}" ... />
</ControlTemplate>