Avalonia/samples/ControlCatalog/MainWindow.xaml

<Window xmlns="https://github.com/avaloniaui" MinWidth="500" MinHeight="300"
        Width="1024" Height="800"
        xmlns:pages="clr-namespace:ControlCatalog.Pages"
        Title="Avalonia Control Gallery"
        Icon="/Assets/test_icon.ico"
        xmlns:local="clr-namespace:ControlCatalog"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="clr-namespace:ControlCatalog.ViewModels"
        xmlns:v="clr-namespace:ControlCatalog.Views"
        ExtendClientAreaToDecorationsHint="{Binding ExtendClientAreaEnabled}"
        ExtendClientAreaChromeHints="{Binding ChromeHints}"
        ExtendClientAreaTitleBarHeightHint="{Binding TitleBarHeight}"
        TransparencyLevelHint="{Binding TransparencyLevel}"        
        x:Name="MainWindow"
        x:Class="ControlCatalog.MainWindow" WindowState="{Binding WindowState, Mode=TwoWay}">
  <RelativePanel
    VerticalAlignment="Stretch"
    HorizontalAlignment="Stretch">
    <Rectangle
        Name="rectangle1"
        RelativePanel.AlignLeftWithPanel="True"
        Width="360"
        Height="50"
        Fill="Red"/>
    <Rectangle
        Name="rectangle2"
        RelativePanel.Below="rectangle1"
        Width="360"
        Height="50"
        Fill="Green" />
  </RelativePanel>
</Window>