<ScrollViewer VerticalScrollBarVisibility="Visible">
    <Avalon:TextEditor ShowLineNumbers="True"
        Text="">
    </Avalon:TextEditor>
</ScrollViewer>
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<Window x:Class="WpfApp1.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:WpfApp1"
        xmlns:Avalon="http://icsharpcode.net/sharpdevelop/avalonedit"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <ScrollViewer VerticalScrollBarVisibility="Visible">
            <Avalon:TextEditor ShowLineNumbers="True"
                Text="">
            </Avalon:TextEditor>
        </ScrollViewer>
    </Grid>
</Window>
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