Avalonia/samples/IntegrationTestApp/TopmostWindowTest.axaml

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="IntegrationTestApp.TopmostWindowTest"
        Title="TopmostWindowTest"
        Width="640"
        Height="480">
  <Grid>
    <TextBox Name="CurrentPosition"
             Grid.Column="1"
             Grid.Row="3"
             IsReadOnly="True" />
    <Button HorizontalAlignment="Center" Name="MoveButton" VerticalAlignment="Center" Click="Button_OnClick">Move</Button>
  </Grid>
</Window>