Listbox: multiple="true" checkmark="true" issue

Scroll in the huge listbox is loosing the selected check boxes.

steps:

1) Select 1st Listitem
2) scroll down to last listitem
3) select last or items near to it.
4) Scroll up to first selected listitem and we see its now unchecked.

I have attached the sample code taken form zk sandbox to recreate this issue