I ran testcase.py with stock 3.1.2 on winxp (under IDLE).
Changes needed (one place each) were
Tkinter -> tkinter
print x -> print(x)
<>      -> !=    # <> is long since deprecated!

I get 3 line box windowing 10 lines. If I enlarge window, there are still only 3 lines displayed.

With triangle arrows at top and bottom of scroll bar box, I can scroll up and down a line at a time, with lines 1 to 8 at top. Nothing prints. There is a slight flash with redisplay.

With click and move on scroll bar itself, I can get down to where line 4 is at the top. Motion is not really jerky, rather it jumps one line at a time, just as with triangles. Going farther, box goes crazy flashing. When I unclick mouse, it settles all the way down with line 8 on box. Repetitious print looks like
0.4 0.4 4
0.7001 0.7001 7
0.4334 0.4334 4
0.7 0.7 7
0.4333 0.4333 4
0.7 0.7 7
...
So confirmed behavior is, if anything, worse. Just to make sure that print was not causing a problem, I commented it out and got same behavior.

I do not know enough tkinter (tk) to know if there is a bug in your code. I recommend posting to python-list, subject: Problem with tkinter box, with this link and short explanation. There are some people there who know tkinter much better than me who might comment.