It is a problem with IE9's engine, you can even try it without using the splitter and treeview, to reproduce it:
- Make a list with a max-height css attribute, overflow auto
- Make one of the items to be long enough as to make the horizontal scrollbar appear
- Apply a hover css to the list items.
The easiest way to fix this issue with the engine is to set to the ul element a min-height of 0%
I have updated the jsfiddle: http://jsfiddle.net/Krt9C/8/