After selecting an item, combobox's dropdown will scroll to right automatically

Steps to Reproduce

1. run attached zul
2. select an item in combobox
3. click to expand the dropdown

Actual:

scrollbar moves to the right automatically:

every time you click combobox to expand the dropdown, the scrollbar scrolls to the right.

Expected:

scrollbar remains being on leftmost position.

Extra Information

It might relate to ZK-2193.

I have tested 6.5.6.FL.20140430, and it also has this issue.