aistudio-wpf-diagram/AIStudio.Wpf.BaseDiagram/Controls/MultiSelectComboBox.xaml

<UserControl x:Class="AIStudio.Wpf.BaseDiagram.Controls.MultiSelectComboBox"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ComboBox Background="Transparent" BorderBrush="Transparent"
        x:Name="MultiSelectCombo"  
        SnapsToDevicePixels="True"
        OverridesDefaultStyle="True"
        ScrollViewer.HorizontalScrollBarVisibility="Auto"
        ScrollViewer.VerticalScrollBarVisibility="Auto"
        ScrollViewer.CanContentScroll="True"
        IsSynchronizedWithCurrentItem="True"
                           >
        <ComboBox.ItemTemplate>
            <DataTemplate>
                <CheckBox Content="{Binding Title}"
                          IsChecked="{Binding Path=IsSelected, Mode=TwoWay}"
                          Tag="{RelativeSource FindAncestor, AncestorType={x:Type ComboBox}}"
                           Click="CheckBox_Click"           />
            </DataTemplate>
        </ComboBox.ItemTemplate>
        <ComboBox.Template>
            <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=UserControl}}" 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 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>
        </ComboBox.Template>
    </ComboBox>
</UserControl>