Общие вопросы

http://msdn.microsoft.com/library/en-us/dnwebteam/html/webteam12032001.asp, глава O Cursor, Where Art Thou?

Альтернативное решение - имеем два TextRange, один из selection, другой из всего поля. Первому делаем moveStart, пока compareEndPoints не скажет 0.

PS. Ср.: The second thing to know is that in order to represent the location in the window where typed text will appear, the Terminal component uses the Windows "caret" which some folks also call the "insertion point" and in text mode screens is traditionally referred to as the "cursor." Under Windows that latter term has (for good or ill) been appropriated to refer to the mouse pointer.