Avalonia/tests/Avalonia.DesignerSupport.TestApp/MainWindow.xaml

<Window xmlns="https://github.com/avaloniaui"
        xmlns:pages="clr-namespace:ControlCatalog.Pages;assembly=ControlCatalog"
        Title="TESTAPP"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        x:Class="Avalonia.DesignerSupport.TestApp.MainWindow">
    <Button/>
</Window>