Those two functions are helpful, unfortunately not as much as I would love to.
get_line_count() gives me all lines in the Label.
get_visible_line_count() gives me number of visible lines in the Label, BUT (!) counting even characters that are not yet visible.
That means, when showing one character after another, I have no way of knowing if I show characters from the first four lines or from the others that are out of textbox boundaries.