I add lines at the end of the textview and would like it to scroll down to 
the bottom so I can see the new lines. I can't get it to work. When it have
scrolled there are still some non-visible lines in the end.

I'll attach a test program that shows the problem here. The text that is
added to the textbuffer have to be long for the problem to show up. So I
took part of the text that my application tried to show when I saw the
failure. It's not just the number of lines in the text that is added that
matters, because if I just add 20 one-character lines, then I don't see the
problem. 

As far as I can tell from the docs it should work when one use a text-mark
to do this, while gtk_text_view_scroll_to_iter () is not guaranteed to work.