Avalonia/src/Windows/Perspex.Designer/PerspexDesigner.xaml

<UserControl x:Class="Perspex.Designer.PerspexDesigner"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <WindowsFormsHost Background="White" x:Name="NativeContainer" Panel.ZIndex="1"/> 
        <ScrollViewer VerticalScrollBarVisibility="Auto" Panel.ZIndex="0">
            <TextBox IsReadOnly="True" TextWrapping="WrapWithOverflow" x:Name="State"/>
        </ScrollViewer>
        <StackPanel x:Name="ErrorContainer">
            <TextBlock />
        </StackPanel>
    </Grid>
</UserControl>