On further thought, trying to force the text scroll view to a line number, while something else is sending it to the 'end', is asking for "jitter". I think your intent is to be able to "freeze and read" something of interest in the output. What your best
option would be, is to have a button which makes a copy of the the text, at the instant, and opens it as a copy in a second text box in another toplevel window. That way you can take your time to read it, and not interfere with the output. If you need help on how to open a toplevel, with a copy of the text, let us know.