Flash Player 6.
my_txt.bottomScroll
Property (read-only); an integer (1-based index) that indicates the bottommost line that is currently visible in my_txt. Think of the text field as "a window" onto a block of text. The property TextField.scroll is the 1-based index of the topmost visible line in the window.
All the text between lines TextField.scroll and TextField.bottomScroll is currently visible in the text field.