Avalonia/src/Avalonia.Themes.Simple/Controls/AutoCompleteBox.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:ClassModifier="internal">
  <ControlTheme x:Key="{x:Type AutoCompleteBox}"
                TargetType="AutoCompleteBox">
    <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}" />
    <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}" />
    <Setter Property="BorderThickness" Value="{DynamicResource ThemeBorderThickness}" />
    <Setter Property="Padding" Value="4" />
    <Setter Property="Template">
      <ControlTemplate>
        <Panel>
          <TextBox Name="PART_TextBox"
                   Padding="{TemplateBinding Padding}"
                   Background="{TemplateBinding Background}"
                   BorderBrush="{TemplateBinding BorderBrush}"
                   BorderThickness="{TemplateBinding BorderThickness}"
                   CornerRadius="{TemplateBinding CornerRadius}"
                   CaretIndex="{TemplateBinding CaretIndex, Mode=TwoWay}"
                   DataValidationErrors.Errors="{TemplateBinding (DataValidationErrors.Errors)}"
                   Watermark="{TemplateBinding Watermark}"
                   MaxLength="{TemplateBinding MaxLength}"                   
                   InnerLeftContent="{TemplateBinding InnerLeftContent}"
                   InnerRightContent="{TemplateBinding InnerRightContent}"
                   />

          <Popup Name="PART_Popup"
                 MinWidth="{Binding Bounds.Width, RelativeSource={RelativeSource TemplatedParent}}"
                 MaxHeight="{TemplateBinding MaxDropDownHeight}"
                 IsLightDismissEnabled="True"
                 PlacementTarget="{TemplateBinding}">
            <Border Background="{DynamicResource ThemeBackgroundBrush}"
                    BorderBrush="{DynamicResource ThemeBorderMidBrush}"
                    BorderThickness="1">
              <ListBox Name="PART_SelectingItemsControl"
                       Background="{TemplateBinding Background}"
                       BorderThickness="0"
                       Foreground="{TemplateBinding Foreground}"
                       ItemTemplate="{TemplateBinding ItemTemplate}"
                       ScrollViewer.HorizontalScrollBarVisibility="Auto"
                       ScrollViewer.VerticalScrollBarVisibility="Auto" />
            </Border>
          </Popup>
        </Panel>
      </ControlTemplate>
    </Setter>
  </ControlTheme>
</ResourceDictionary>