I think I am able to reproduce that behavior (I'm using IDLE from a python.org 10.9+ installer).  What I'm seeing is that if you scroll to the bottom of a file that exceeds the length of the open text window and then click on and try to move the position indicator in the scroll bar, the position indicator does not move nor does the file scroll.  But if I then click elsewhere in the scroll bar and then try to grab and move the indicator, it works OK.  So perhaps IDLE is overriding a text widget event or some such.  FWIW a quick test in the wish8.6 text demo seems to work OK.