new/Views/MainWindow.axaml

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:YtYtAvalonia.ViewModels"
        xmlns:v="using:YtYtAvalonia.Views"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="1920" d:DesignHeight="1080"
        x:Class="YtYtAvalonia.Views.MainWindow"
        x:DataType="vm:MainWindowViewModel"
        Icon="/Assets/avalonia-logo.ico"
        Title="YtYtAvalonia"
		WindowState ="Maximized">

    <Design.DataContext>
        <!-- This only sets the DataContext for the previewer in an IDE,
             to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
        <vm:MainWindowViewModel/>
    </Design.DataContext>

	<Grid >
		<ScrollViewer>
			<StackPanel>
				<ContentControl Content="{Binding UC}"></ContentControl>
			</StackPanel>
		</ScrollViewer>
	</Grid>

</Window>