I just committed http://core.tcl.tk/tk/info/26a029b4a88ff97f, which fixes the scrolling issue in Tk. Running the test scripts here indicate the behavior is now correct; clicking several pixels below the bottom part of the scroll button causes the scroll to jump, instead of smoothly scrolling. (One must click the scroll button directly for smooth scrolling, which is the expected behavior.) The fix involved removing support for a small scroll button variant, which was causing the confusion; by sticking with a single variant, the normal size scroller, the behavior is correct and consistent.