After fixing your 'testcase', it works as expected:
ScrollBar is regular widget, so Splitter does what is supposed to do - use it as splitter part.
BTW, would be so hard to provide working testcase, one that I only unpack in MyApps, select and run? Not the bunch of files that are not in proper folder and do not compile? Perhaps add a screenshot of the problem even?
I am adding 'fixed' testcase as I am not 100% sure that I have edited it the right way.