Out of the box AvalonEdit supports XML syntax highlighting, line numbers and block indent (select several lines and press Tab of Shift+Tab). I’ve also added the Ctrl+OemQuestion keyboard shortcut to comment and uncomment selected text.
AvalonEdit is easily extensible – more features like code completion and folding can be added later. Or completely different style editor control can be added to Tabs Studio in a separate add-in.
Download link: AvalonStyleEditor v1.0.0.