Zul listboxes in molds other than dropdown are rendered as html tables and are thus not focusable. I guess an empty list (i.e., an empty table) is not directly focusable, but if they're populated its first item definetely ist.

C1 LB C2

I think it would be desirable that when the focus is on component C1 and the user presses 'tab' the focus gets transferred to
1) The LB, if it's on dropdown mold
2) The first item in LB, if it's on a mold other than dropdown and it's not empty
3) C2, if none of the above is true

(it looks like Vincent didn't actually post it as a new feature request http://www.zkoss.org/forum/listComment/16897-Can-t-use-tab-keys-to-select-the-first-list-item-in-a-list-box)