procedure at:
http://www.zkoss.org/zkdemo/effects/drag_n_drop

with listbox i don't encounter this issue
http://www.zkoss.org/zkdemo/effects/drag_n_drop

i see it's by text in listbox item inside a div.
text in tree node is inside a span.

i hot fix it by:
in TreeitemRenderer.render

add:
Treecell tc = new Treecell(Objects.toString(node));
tc.setWidgetListener("onMouseDown", "preventSelectTextInMoveTreeItem();");

preventSelectTextInMoveTreeItem is a javascript function
function preventSelectTextInMoveTreeItem (){
event.preventDefault();
}