One of the toughest problems I faced when I built Notepad Classic was an issue where many functions like Go To & Find were always off a few characters. After a bit of experimenting I noticed a pattern, it was off by the number of characters equal to the line number (0 based).
i.e.: no issues on the first line (line index 0); Off by one on the second line (line index 1); Off by two on the third line (line index 2); Off by three on the forth line etc…
It turned out that the way the string functions count a line break (i.e. \r\n) as two characters, and rightly so – it is two characters a \r & a \n. However the TextBox functions like Select treat \r\n as a single character because that is what is displayed visually – a line break is one character visually. So there is a difference between the two scenarios and thus the “off by one x line count” error I found.
The solution I used is to compensate for it by working out the number of lines to the point (i.e. count all the line breaks before the point) and adjusting the results (adding +1 for each \r\n) as needed.
Sample application to show this problem and the solution can be found at: https://bitbucket.org/rmaclean/off-by-one-x-sample-code