Avalonia/src/Avalonia.Themes.Simple/Controls/SelectableTextBlock.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Design.PreviewWith>
    <SelectableTextBlock Text="Preview" />
  </Design.PreviewWith>

  <MenuFlyout x:Key="SelectableTextBlockContextFlyout" Placement="Bottom">
    <MenuItem x:Name="SelectableTextBlockContextFlyoutCopyItem" Header="Copy" Command="{Binding $parent[SelectableTextBlock].Copy}"
              IsEnabled="{Binding $parent[SelectableTextBlock].CanCopy}" InputGesture="{x:Static TextBox.CopyGesture}" />
  </MenuFlyout>

  <ControlTheme x:Key="{x:Type SelectableTextBlock}" TargetType="SelectableTextBlock">
    <Style Selector="^[IsEnabled=True]">
      <Setter Property="Cursor" Value="IBeam" />
      <Setter Property="ContextFlyout" Value="{StaticResource SelectableTextBlockContextFlyout}" />
    </Style>
  </ControlTheme>
</ResourceDictionary>