Mesen2/UI/Styles/AvaloniaComboBoxStyles.xaml

<Styles
	xmlns="https://github.com/avaloniaui"
	xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	xmlns:sys="clr-namespace:System;assembly=netstandard"
>
	<Design.PreviewWith>
		<StackPanel Width="200" Height="200">
			<ComboBox>
				<ComboBoxItem>Test</ComboBoxItem>
			</ComboBox>
		</StackPanel>
	</Design.PreviewWith>
	
	<Style Selector="ComboBox">
		<Setter Property="Template">
			<ControlTemplate>
				<DataValidationErrors>
					<Grid RowDefinitions="Auto, *, Auto" ColumnDefinitions="*,32">
						<ContentPresenter x:Name="HeaderContentPresenter"
												Grid.Row="0"
												Grid.Column="0"
												Grid.ColumnSpan="2"
												IsVisible="False"
												FontWeight="{DynamicResource ComboBoxHeaderThemeFontWeight}"
												Margin="{DynamicResource ComboBoxTopHeaderMargin}"
												VerticalAlignment="Top" />
						<Border x:Name="Background"
								  Grid.Row="1"
								  Grid.Column="0"
								  Grid.ColumnSpan="2"
								  Background="{TemplateBinding Background}"
								  BorderBrush="{TemplateBinding BorderBrush}"
								  BorderThickness="{TemplateBinding BorderThickness}"
								  CornerRadius="{TemplateBinding CornerRadius}"
								  MinWidth="{DynamicResource ComboBoxThemeMinWidth}" />
						<Border x:Name="HighlightBackground"
								  Grid.Row="1"
								  Grid.Column="0"
								  Grid.ColumnSpan="2"
								  Background="{DynamicResource ComboBoxBackgroundUnfocused}"
								  BorderBrush="{DynamicResource ComboBoxBackgroundBorderBrushUnfocused}"
								  BorderThickness="{TemplateBinding BorderThickness}"
								  CornerRadius="{TemplateBinding CornerRadius}"
								  IsVisible="False"/>
						<TextBlock x:Name="PlaceholderTextBlock"
									  Grid.Row="1"
									  Grid.Column="0"
									  HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
									  VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
									  Margin="{TemplateBinding Padding}"
									  Text="{TemplateBinding PlaceholderText}"
									  Foreground="{TemplateBinding PlaceholderForeground}"
									  IsVisible="{TemplateBinding SelectionBoxItem, Converter={x:Static ObjectConverters.IsNull}}" />
						<ContentControl x:Name="ContentPresenter"
											 Content="{TemplateBinding SelectionBoxItem}"
											 ContentTemplate="{TemplateBinding ItemTemplate}"
											 Grid.Row="1"
											 Grid.Column="0"
											 Margin="{TemplateBinding Padding}"
											 HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
											 VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" />

						<Border x:Name="DropDownOverlay"
								  Grid.Row="1"
								  Grid.Column="1"
								  Background="Transparent"
								  Margin="0,1,1,1"
								  Width="30"
								  IsVisible="False"
								  HorizontalAlignment="Right" />

						<PathIcon x:Name="DropDownGlyph"
									 Grid.Row="1"
									 Grid.Column="1"
									 UseLayoutRounding="False"
									 IsHitTestVisible="False"
									 Height="12"
									 Width="12"
									 Margin="0,0,10,0"
									 HorizontalAlignment="Right"
									 VerticalAlignment="Center"
									 Foreground="{DynamicResource ComboBoxDropDownGlyphForeground}"
									 Data="M1939 486L2029 576L1024 1581L19 576L109 486L1024 1401L1939 486Z"/>

						<Popup Name="PART_Popup"
								 WindowManagerAddShadowHint="False"
								 IsOpen="{TemplateBinding IsDropDownOpen, Mode=TwoWay}"
								 HorizontalOffset="-1"
								 VerticalOffset="-1"
								 MinWidth="{Binding Bounds.Width, RelativeSource={RelativeSource TemplatedParent}}"
								 MaxHeight="{TemplateBinding MaxDropDownHeight}"
								 PlacementTarget="Background"
								 Placement="BottomEdgeAlignedLeft"
								 IsLightDismissEnabled="True"
								 InheritsTransform="True">
							<Border x:Name="PopupBorder"
									  Background="{DynamicResource ComboBoxDropDownBackground}"
									  BorderBrush="{DynamicResource ComboBoxDropDownBorderBrush}"
									  BorderThickness="{DynamicResource ComboBoxDropdownBorderThickness}"
									  Margin="0"
									  Padding="{DynamicResource ComboBoxDropdownBorderPadding}"
									  HorizontalAlignment="Stretch"
									  CornerRadius="{DynamicResource OverlayCornerRadius}">
								<ScrollViewer HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
												  VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">
									<ItemsPresenter Name="PART_ItemsPresenter"
                                  Margin="{DynamicResource ComboBoxDropdownContentMargin}"
                                  ItemsPanel="{TemplateBinding ItemsPanel}" />
								</ScrollViewer>
							</Border>
						</Popup>
					</Grid>
				</DataValidationErrors>
			</ControlTemplate>
		</Setter>
	</Style>
</Styles>