Questions

I figured it out. Use a custom TextAnnotation and then set the Horizontal and Vertical Alignment properties.

    <Style x:Key="TextAnnotationTopLeft" TargetType="s:TextAnnotation">

        <Setter Property="HorizontalAlignment" Value="Left" />
        <Setter Property="VerticalAlignment" Value="Top" />


        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="s:TextAnnotation">
                    <Border x:Name="PART_TextAnnotationRoot"
                        Margin="{TemplateBinding Margin}"
                        Background="White"
                        BorderBrush="Purple"
                        BorderThickness="0"
                        CornerRadius="{TemplateBinding CornerRadius}">
                        <TextBox x:Name="PART_InputTextArea"
                                 Margin="10"
                                 VerticalAlignment="{TemplateBinding VerticalAlignment}"
                                 AcceptsReturn="True"
                                 Background="White"                                    
                                 FontFamily="{TemplateBinding FontFamily}"
                                 FontSize="{TemplateBinding FontSize}"
                                 FontWeight="Bold"
                                 Foreground="Black"                                     
                                 IsTabStop="{TemplateBinding IsEditable}"
                                 Text="{Binding Text,
                                            RelativeSource={RelativeSource TemplatedParent},
                                            Mode=TwoWay}"
                                 TextAlignment="{TemplateBinding TextAlignment}"
                                 TextWrapping="Wrap" />
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>