R – How to determine index of current ListBox item from DataTemplate

You can style your ContentControl such that a trigger fires when its container (the ListBoxItem) becomes selected:

<ContentControl 
    x:Name="designerContent"
    MinHeight="100"
    Margin="2,0,2,2"
    Content="{Binding Path=DesignerInstance}"
    Background="#FF999898">
    <ContentControl.Style>
        <Style TargetType="{x:Type ContentControl}">
            <Setter Property="Visibility" Value="Collapsed"/>
            <Style.Triggers>
                <DataTrigger
                        Binding="{Binding
                            RelativeSource={RelativeSource
                                Mode=FindAncestor,
                                AncestorType={x:Type ListBoxItem}},
                                Path=IsSelected}"
                        Value="True">
                    <Setter Property="Visibility" Value="Visible"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </ContentControl.Style>
</ContentControl>

Alternatively, I think you can add the trigger to the template itself and reference the control by name. I don't know this technique well enough to type it from memory and assume it'll work, but it's something like this:

<DataTemplate x:Key="ScenarioItemTemplate">
    <DataTemplate.Triggers>
        <DataTrigger
                Binding="{Binding
                    RelativeSource={RelativeSource
                        Mode=FindAncestor,
                        AncestorType={x:Type ListBoxItem}},
                        Path=IsSelected}"
                Value="True">
            <Setter
                TargetName="designerContent"
                Property="Visibility"
                Value="Visible"/>
        </DataTrigger>
    </DataTemplate.Triggers>

    ...
</DataTemplate>

I think an event trigger for the FrameworkElement.Loaded routed event could work. For example:

<DataTemplate DataType="{x:Type l:Foo}">
    <Button x:Name="Button" Content="{Binding Path=Bar}">
        <Button.Background>
            <SolidColorBrush x:Name="ButtonBrush" Color="Tan" />
        </Button.Background>
    </Button>
    <DataTemplate.Triggers>
        <EventTrigger RoutedEvent="FrameworkElement.Loaded" SourceName="Button">
            <BeginStoryboard>
                <Storyboard>
                    <ColorAnimation Storyboard.TargetName="ButtonBrush" Storyboard.TargetProperty="Color" To="Red" />
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
    </DataTemplate.Triggers>
</DataTemplate>