Text Edit Plus supports 2 text engines: SynEdit and Classic
The default text engine is SynEdit, however users are free to switch between them. Use the Options menu, then click Text Engine.
SynEdit is an open-source editor component, not based on the Windows common controls. Vovsoft's SynEdit implementation has some pros and cons.
Gutter (line numbers)
Highlight active line
Vertical selection (using Alt key)
Only supports fixed-pitch (monospaced) fonts
Classic text engine uses the common control provided by Windows. It's basically the same text engine in Notepad.
Supports all kinds of fonts
Does not support SynEdit's advantages