Make the Xceed WPF Toolkit SplitButton control look correct on a toolbar.

InverseBooleanConverter.vb

Imports System.Globalization

Namespace Utility

	Public Class InverseBooleanConverter
		Implements IValueConverter

		Public Function Convert(pValue As Object, pTargetType As Type, pParameter As Object, pCulture As CultureInfo) As Object Implements IValueConverter.Convert
			Return Not DirectCast(pValue, Boolean)
		End Function

		Public Function ConvertBack(pValue As Object, pTargetType As Type, pParameter As Object, pCulture As CultureInfo) As Object Implements IValueConverter.ConvertBack
			Return Not DirectCast(pValue, Boolean)
		End Function
	End Class

End Namespace

SplitButtonToolbarStyle.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
					xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
					xmlns:util="clr-namespace:Utility"
					xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit">
	<util:InverseBooleanConverter x:Key="InverseBooleanConverter" />

	<LinearGradientBrush x:Key="ColorPickerDarkBorderBrush" EndPoint="0.5,1" StartPoint="0.5,0">
		<GradientStop Color="#FFA3AEB9" Offset="0" />
		<GradientStop Color="#FF8399A9" Offset="0.375" />
		<GradientStop Color="#FF718597" Offset="0.375" />
		<GradientStop Color="#FF617584" Offset="1" />
	</LinearGradientBrush>

	<LinearGradientBrush x:Key="PopupBackgroundBrush" StartPoint="0,0" EndPoint="0,1">
		<LinearGradientBrush.GradientStops>
			<GradientStopCollection>
				<GradientStop Offset="0" Color="#FFffffff" />
				<GradientStop Offset="1" Color="#FFE8EBED" />
			</GradientStopCollection>
		</LinearGradientBrush.GradientStops>
	</LinearGradientBrush>

	<Style x:Key="SplitButtonToolbarStyle" TargetType="xctk:SplitButton">
		<Setter Property="IsTabStop" Value="False" />
		<Setter Property="HorizontalContentAlignment" Value="Center" />
		<Setter Property="VerticalContentAlignment" Value="Center" />
		<Setter Property="Template">
			<Setter.Value>
				<ControlTemplate TargetType="xctk:SplitButton">
					<Grid x:Name="MainGrid" SnapsToDevicePixels="True">
						<Grid>
							<Grid.ColumnDefinitions>
								<ColumnDefinition Width="*" />
								<ColumnDefinition Width="Auto" />
							</Grid.ColumnDefinitions>
							<Button x:Name="PART_ActionButton" Margin="0" Padding="{TemplateBinding Padding}" Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}"
										HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}">
								<ContentPresenter Name="ActionButtonContent" Margin="{TemplateBinding Padding}" Content="{TemplateBinding Content}"
													  ContentTemplate="{TemplateBinding ContentTemplate}" RecognizesAccessKey="true"
													  VerticalAlignment="{TemplateBinding VerticalContentAlignment}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" />
							</Button>
							<ToggleButton x:Name="PART_ToggleButton" Grid.Column="1" IsTabStop="False" IsChecked="{Binding IsOpen, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay}"
											  IsHitTestVisible="{Binding IsOpen, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource InverseBooleanConverter}}">
								<ToggleButton.Template>
									<ControlTemplate TargetType="ToggleButton">
										<ContentPresenter />
									</ControlTemplate>
								</ToggleButton.Template>
								<Grid>
									<xctk:ButtonChrome x:Name="ToggleButtonChrome" Background="Transparent" BorderThickness="1,0,0,0" BorderBrush="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}"
														   Padding="1,0,1,0" RenderChecked="{TemplateBinding IsOpen}" RenderEnabled="{TemplateBinding IsEnabled}"
														   RenderMouseOver="{Binding IsMouseOver, ElementName=PART_ToggleButton}" RenderPressed="{Binding IsPressed, ElementName=PART_ToggleButton}">
										<Grid x:Name="arrowGlyph" IsHitTestVisible="False" Margin="4,3,4,3">
											<Path x:Name="Arrow" Width="7" Height="4" Fill="Black" Data="M 0,0 L 4,4 8,0 Z" />
										</Grid>
									</xctk:ButtonChrome>
								</Grid>
							</ToggleButton>
						</Grid>

						<Popup x:Name="PART_Popup" HorizontalOffset="1" VerticalOffset="1" AllowsTransparency="True" StaysOpen="False" Placement="Bottom" Focusable="False"
								   IsOpen="{Binding IsChecked, ElementName=PART_ToggleButton}">
							<Border BorderThickness="1" Background="{StaticResource PopupBackgroundBrush}" BorderBrush="{StaticResource ColorPickerDarkBorderBrush}">
								<ContentPresenter x:Name="PART_ContentPresenter" Content="{TemplateBinding DropDownContent}" />
							</Border>
						</Popup>
					</Grid>
					<ControlTemplate.Triggers>
						<Trigger Property="IsEnabled" Value="False">
							<Setter Property="Fill" TargetName="Arrow" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}" />
							<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}" />
						</Trigger>
					</ControlTemplate.Triggers>
				</ControlTemplate>
			</Setter.Value>
		</Setter>
	</Style>