I have spent some free time to port a control from VB.Net to C# and refactor it a little bit. This UI control attaches to RichTextBox and displays line numbers for the text. The control has many settings and re-paints quickly and exactly. You may get it here: https://github.com/antgraf/C--Numbered-Lines-Control-for-RichTextBox--LineNumbersControlForRichTextBox-