MainWindow.xaml

<Window x:Class="WPF_XAML_Islands_WinUI3.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        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"
        xmlns:local="clr-namespace:WPF_XAML_Islands_WinUI3"
        mc:Ignorable="d"
        Title="Test WinUI 3 DesktopWindowXamlSource" Height="680" Width="1120">
    <Grid>
        <Button x:Name="btn1" Content="Set XAML from code" HorizontalAlignment="Left" Margin="120,20,0,0" VerticalAlignment="Top" Height="30" Width="130" Click="btn1_Click"/>
        <Button x:Name="btn2" Content="Set XAML from string" HorizontalAlignment="Left" Margin="120,70,0,0" VerticalAlignment="Top" Height="30" Width="130" Click="btn2_Click"/>
        <RichTextBox x:Name="rtb1" HorizontalAlignment="Left" Height="400" Margin="20,120,0,0" VerticalAlignment="Top" Width="350">
            <FlowDocument>
                <Paragraph>
                    <Run Text="RichTextBox"/>
                </Paragraph>
            </FlowDocument>
        </RichTextBox>

    </Grid>
</Window>