maxscroll (TextField.maxscroll プロパティ)

this.createTextField("scroll_txt", this.getNextHighestDepth(), 10, 10, 160, 20);
this.createTextField("my_txt", this.getNextHighestDepth(), 10, 30, 320, 240);
my_txt.multiline = true;
my_txt.wordWrap = true;
for (var i = 0; i<10; i++) {
    my_txt.text += "Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh "
            + "euismod tincidunt ut laoreet dolore magna aliquam erat volutpat.";
}
scrollUp_btn.onRelease = function() {
    my_txt.scroll--;
    scroll_txt.text = my_txt.scroll+" of "+my_txt.maxscroll;
};
scrollDown_btn.onRelease = function() {
    my_txt.scroll++;
    scroll_txt.text = my_txt.scroll+" of "+my_txt.maxscroll;
};