Tk Source Code

I think what you are describing is the correct behavior of the scrollbar. If you select the thumb scroller directly, that allows line by line scrolling of the text widget. If you click in the trough, the scrollbar gets focus and the scrolling behavior is "paging" (larger chunks of text). If there are large amounts of text, the scrollbar gets smaller, and scrolling performance is slower, but this is still the intended/correct behavior.