aistudio-wpf-diagram/AIStudio.Wpf.DiagramDesigner/Styles/ComboBox.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Style x:Key="ComboBoxStyle" TargetType="ComboBox">
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="BorderBrush" Value="Transparent"/>
        <Setter Property="SnapsToDevicePixels" Value="True"/>
        <Setter Property="OverridesDefaultStyle" Value="True"/>
        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
        <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
        <Setter Property="ScrollViewer.CanContentScroll" Value="True"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="ComboBox">
                    <Grid>
                        <ToggleButton x:Name="ToggleButton" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}"
                        Grid.Column="2" IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}"
                        Focusable="false"                           
                        ClickMode="Press" HorizontalContentAlignment="Left" >
                            <ToggleButton.Template>
                                <ControlTemplate>
                                    <Grid>
                                        <Grid.ColumnDefinitions>
                                            <ColumnDefinition Width="*"/>
                                            <ColumnDefinition Width="12"/>
                                        </Grid.ColumnDefinitions>
                                        <Border
                                          x:Name="Border" 
                                          Grid.ColumnSpan="2"
                                          Background="{TemplateBinding Background}"
                                          BorderBrush="{TemplateBinding BorderBrush}"
                                          BorderThickness="{TemplateBinding BorderThickness}" />
                                        <Border 
                                        x:Name="BorderComp" 
                                        Grid.Column="0"
                                        CornerRadius="2" 
                                        Margin="1" 
                                        BorderThickness="0,0,0,0" >
                                            <TextBlock Text="{Binding Path=Text,RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ComboBox}}" Padding="3" />                                          
                                        </Border>
                                        <Border x:Name="ArrowBorder"  Grid.Column="1"   >
                                            <Path 
                                          x:Name="Arrow"                                         
                                          Stretch="Fill" Width="5" Height="3"
                                          Fill="Black" 
                                          HorizontalAlignment="Center"
                                          VerticalAlignment="Center"
                                          Data="M 0 0 L 4 4 L 8 0 Z"/>
                                        </Border>
                                    </Grid>
                                    <ControlTemplate.Triggers>
                                        <Trigger Property="IsMouseOver" Value="true">
                                            <Setter TargetName="ArrowBorder" Property="Background" Value="{DynamicResource Fluent.Ribbon.Brushes.Button.MouseOver.Background}" />
                                        </Trigger>
                                    </ControlTemplate.Triggers>
                                </ControlTemplate>
                            </ToggleButton.Template>
                        </ToggleButton>                       
                        <Popup x:Name="Popup"
                            Placement="Bottom"                        
                            AllowsTransparency="True" 
                            Focusable="False"  IsOpen="{TemplateBinding IsDropDownOpen}"
                            PopupAnimation="Slide">
                            <Grid x:Name="DropDown"
                            SnapsToDevicePixels="True"  
                            MinWidth="{TemplateBinding ActualWidth}"
                            MaxHeight="{TemplateBinding MaxDropDownHeight}">
                                <Border x:Name="DropDownBorder" 
                                   BorderThickness="1" Background="{DynamicResource WhiteBrush}"
                                   BorderBrush="{DynamicResource GrayBrush8}"/>
                                <ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True" DataContext="{Binding}">
                                    <StackPanel Background="AliceBlue" IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
                                </ScrollViewer>
                            </Grid>
                        </Popup>

                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="HasItems" Value="false">
                            <Setter TargetName="DropDownBorder" Property="MinHeight" Value="95"/>
                        </Trigger>
                        <Trigger SourceName="Popup" Property="Popup.AllowsTransparency" Value="true">
                            <Setter TargetName="DropDownBorder" Property="Margin" Value="0,2,0,0"/>
                        </Trigger>
                        <Trigger Property="IsDropDownOpen" Value="true">
                            <Setter TargetName="ToggleButton" Property="Background" Value="{DynamicResource Fluent.Ribbon.Brushes.Button.MouseOver.Background}" />
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>