PowerToys/src/modules/fancyzones/editor/FancyZonesEditor/LayoutPreview.xaml

<UserControl x:Class="FancyZonesEditor.LayoutPreview"
             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" 
             xmlns:local="clr-namespace:FancyZonesEditor"
             mc:Ignorable="d" 
             Loaded="OnLoaded"
             d:DesignHeight="450"
             d:DesignWidth="800">
    <Grid>
        <Border CornerRadius="4"
                Background="{DynamicResource LayoutPreviewBackgroundBrush}" />
        <Grid x:Name="Body"
              Background="Transparent" />
    </Grid>
</UserControl>