FormField.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:ctrl="clr-namespace:Flithor_ReusableCodes.Controls">
    <Style TargetType="{x:Type ctrl:FormField}">
        <Setter Property="HorizontalContentAlignment" Value="Stretch" />
        <Setter Property="VerticalContentAlignment" Value="Center" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ctrl:FormField}">
                    <Border Padding="{TemplateBinding Padding}"
                            Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}">
                        <Grid x:Name="grid">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition x:Name="PART_FieldWidth" Width="Auto" />
                                <ColumnDefinition x:Name="HorizontalGap" Width="{TemplateBinding Gap}" />
                                <ColumnDefinition x:Name="HorizontalContainer" />
                            </Grid.ColumnDefinitions>
                            <Grid.RowDefinitions>
                                <RowDefinition x:Name="PART_FieldHigth" Height="Auto" />
                                <RowDefinition x:Name="VerticalGap" Height="0" />
                                <RowDefinition x:Name="VerticalContainer" />
                            </Grid.RowDefinitions>
                            <ContentPresenter x:Name="PART_Field"
                                              Grid.RowSpan="3"
                                              HorizontalAlignment="{TemplateBinding HorizontalHeaderAlignment}"
                                              VerticalAlignment="{TemplateBinding VerticalHeaderAlignment}"
                                              ContentSource="Header" />
                            <ContentPresenter x:Name="PART_FieldEditor"
                                              Grid.Row="0" Grid.RowSpan="3"
                                              Grid.Column="2"
                                              HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                              VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                              ContentSource="Content" />
                        </Grid>
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="ContainerPosition" Value="Bottom">
                            <Setter Property="VerticalContentAlignment" Value="Stretch" />
                            <Setter TargetName="PART_FieldEditor" Property="Grid.Column" Value="0" />
                            <Setter TargetName="PART_FieldEditor" Property="Grid.ColumnSpan" Value="3" />
                            <Setter TargetName="PART_FieldEditor" Property="Grid.Row" Value="2" />
                            <Setter TargetName="PART_FieldEditor" Property="Grid.RowSpan" Value="1" />

                            <Setter TargetName="PART_Field" Property="Grid.RowSpan" Value="1" />
                            <Setter TargetName="PART_Field" Property="Grid.ColumnSpan" Value="3" />

                            <Setter TargetName="HorizontalGap" Property="Width" Value="0" />
                            <Setter TargetName="VerticalGap" Property="Height" Value="{Binding Gap, RelativeSource={RelativeSource Mode=TemplatedParent}}" />
                            <Setter TargetName="VerticalContainer" Property="Height" Value="*" />
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>