Steps to Reproduce

Run fiddle: 

http://zkfiddle.org/sample/1tncn5c/2-Listbox-z-listbox-header-border

Move the scrollbar

Current Result

frozen listheader no longer have a bottom border

Expected Result

Listheader should still have a visible border-bottom when using frozen

Debug Information

When the scrollbar move, Listheaders receive a z-index:1; inline-css property

This puts them above the  <div class="z-listbox-header-border"></div> which provides the bottom border for the frozen listheader.

Due to this, the frozen listheader no longer display a border-bottom

Workaround

<style>
    .z-listbox-header-border {
        z-index: 1;
    }  
</style>