Avalonia/samples/IntegrationTestApp/MainWindow.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.MainWindow"
        Title="IntegrationTestApp">
  <TabControl TabStripPlacement="Left" AutomationProperties.AutomationId="MainTabs">
    <TabItem Header="Button">
      <StackPanel>
        <Button AutomationProperties.AutomationId="BasicButton">
            Basic Button
        </Button>
        <Button AutomationProperties.AutomationId="ButtonWithTextBlock">
            <TextBlock>Button with TextBlock</TextBlock>
        </Button>
      </StackPanel>
    </TabItem>
    <TabItem Header="ComboBox">
    </TabItem>
  </TabControl>
</Window>