see
http://zkfiddle.org/sample/3itcb53/1-listbox-showing-horziontal-scollbar-when-using-hflex-min

When you use hflex="min" on the listbox and the listbox is heigh enough to show all items, then no scrollbar appears at all. If the listbox can not show all items a vertical scrollbar appears (that's expected behaviour), but since because of the verticall scrollbar, the listbox is not wide enough anymore, also a horizontal scrollbar appears (that's not expected behaviour).
When using hflex="min" either the listbox should be made wide enough to also show the vertical scroll bar. It looks very ugly when it show the horizontal scrollbar only for that few pixels.