Semi.Avalonia/src/Semi.Avalonia.Demo/MainWindow.axaml

<Window
    Title="Semi.Demo"
    d:DesignHeight="450"
    d:DesignWidth="800"
    mc:Ignorable="d"
    x:Class="Semi.Avalonia.Demo.MainWindow"
    xmlns="https://github.com/avaloniaui"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:pages="using:Semi.Avalonia.Demo.Pages"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <TabControl TabStripPlacement="Left">
        <TabItem Header="Button">
            <pages:ButtonDemo />
        </TabItem>
        <TabItem Header="CheckBox">
            <pages:CheckBoxDemo />
        </TabItem>
    </TabControl>
</Window>