Returns the insertion point immediately preceding the character visible in the upper left-hand corner of the screen. If a partial character is displayed, that character counts as the first visible character.
Note: In situations where no character is visible, because the text is scrolled to the right beyond the end of the top line, this returns the insertion point of the last character on the line (EOL not considered). In situations where there are no characters on the line, the insertion point at the beginning is returned.