<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="300" Width="300">
    <Window.Resources>
        <Style x:Key="ComboBoxTest2" TargetType="{x:Type ComboBox}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="ComboBox">
                            <ToggleButton x:Name="TGButton" Grid.Column="2" Focusable="false"
                          IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,
                          RelativeSource={RelativeSource TemplatedParent}}"
                          Padding="0,0,50,0">
                                <ToggleButton.Template>
                                    <ControlTemplate>
                                            <Grid.ColumnDefinitions>
                                                <ColumnDefinition Width="*" />
                                                <ColumnDefinition Width="30" />
                                            </Grid.ColumnDefinitions>
                                            <Border x:Name="Border" Grid.ColumnSpan="2" CornerRadius="5"
                         Background="LightGray" BorderBrush="Black" BorderThickness="1" />
                                            <Border x:Name="Border2" Grid.Column="0" CornerRadius="5,0,0,5"
                            Margin="1" Background="White" BorderBrush="Black"
                            BorderThickness="0,0,1,0" />
                                            <Path x:Name="Arrow" Grid.Column="1"  Fill="Black"
                          HorizontalAlignment="Center" VerticalAlignment="Center"
                          Data="M 0 0 L 4 4 L 8 0 Z"/>
                                        </Grid>
                                        <ControlTemplate.Triggers>
                                            <Trigger Property="ToggleButton.IsMouseOver" Value="true">
                                                <Setter TargetName="Border" Property="Background" Value="DarkGray" />
                                            </Trigger>
                                            <Trigger Property="ToggleButton.IsChecked" Value="true">
                                                <Setter TargetName="Border" Property="Background" Value="DarkGray" />
                                                <Setter TargetName="Border" Property="CornerRadius" Value="5,5,0,0" />
                                                <Setter TargetName="Border2" Property="CornerRadius" Value="5,0,0,0" />
                                            </Trigger>
                                        </ControlTemplate.Triggers>
                                    </ControlTemplate>
                                </ToggleButton.Template>
                            </ToggleButton>
                            <ContentPresenter Name="ContentSite" IsHitTestVisible="False"
                            Content="{TemplateBinding SelectionBoxItem}"
                            ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                            ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
                            Margin="3,3,40,3" />
                            <TextBox x:Name="PART_EditableTextBox" Visibility="Hidden"
                     IsReadOnly="{TemplateBinding IsReadOnly}" Width="50" />
                            <Popup Name="Popup" IsOpen="{TemplateBinding IsDropDownOpen}"
                   AllowsTransparency="True" Focusable="False" PopupAnimation="Slide"
                   Placement="Bottom">
                                <Grid Name="DropDown" SnapsToDevicePixels="True"
                   MinWidth="{TemplateBinding ActualWidth}"
                   MaxHeight="{TemplateBinding MaxDropDownHeight}">
                                    <Border x:Name="DropDownBorder" Background="White" BorderThickness="1"
                       BorderBrush="Black"/>
                                    <ListView ItemsSource="{TemplateBinding ItemsSource}"
                           SelectedItem="{Binding RelativeSource={RelativeSource TemplatedParent},
                           Path=SelectedItem}">
                                        <ListView.View>
                                            <GridView>
                                                <GridViewColumn Header="Key" DisplayMemberBinding="{Binding Key}" Width="80" />
                                                <GridViewColumn Header="Name" DisplayMemberBinding="{Binding Name}" Width="250"/>
                                            </GridView>
                                        </ListView.View>
                                    </ListView>
                                </Grid>
                            </Popup>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
        <ComboBox x:Name="cBox" Height="30" Width="200" ItemsSource="{Binding}"
            SelectedValue="Selected" DisplayMemberPath="Name" SelectedValuePath="Name"
            Style="{StaticResource ComboBoxTest2}" 
            SelectionChanged="Function_SelectionChanged" />
    </Grid>
</Window>

using System.Linq;
using System.Windows;
using System.Windows.Controls;
namespace WpfApp1
    public partial class MainWindow : Window
        public MainWindow()
            InitializeComponent();
            DataContext = Enumerable.Range(1, 100).Select(x => new { Key = x, Name = $"Person {x} with really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really long name" }).ToList();