It is not possible to drag and drop items between two full listboxes. Seems to be a bug.
It also happens intermittently between one full listbox and one partially filled listbox.

I have tested this code in ZK Sandbox, so it is reproducible.

<window title="Listbox" border="normal">
<hbox>
<listbox onDrop="move(event.dragged)" droppable="true" fixedLayout="true" rows="3">
<listitem label="1" draggable="true" droppable="true"/>
<listitem label="2" draggable="true" droppable="true"/>
<listitem label="3" draggable="true" droppable="true"/>
<listitem label="4" draggable="true" droppable="true"/>
</listbox>
<listbox onDrop="move(event.dragged)" droppable="true" fixedLayout="true" rows="3">
<listitem label="5" draggable="true" droppable="true"/>
<listitem label="6" draggable="true" droppable="true"/>
<listitem label="7" draggable="true" droppable="true"/>
<listitem label="8" draggable="true" droppable="true"/>
</listbox>
</hbox>
<zscript>
void move(Component dragged) {
if (self instanceof Listitem)

{ self.parent.insertBefore(dragged, self); } else { self.appendChild(dragged); }
}
</zscript>
</window>


If the number of rows is changed to more than 4 then drag and drop works again.

<window title="Listbox" border="normal">
<hbox>
<listbox onDrop="move(event.dragged)" droppable="true" fixedLayout="true" rows="6">
<listitem label="1" draggable="true" droppable="true"/>
<listitem label="2" draggable="true" droppable="true"/>
<listitem label="3" draggable="true" droppable="true"/>
<listitem label="4" draggable="true" droppable="true"/>

</listbox>
<listbox onDrop="move(event.dragged)" droppable="true" fixedLayout="true" rows="6">
<listitem label="5" draggable="true" droppable="true"/>
<listitem label="6" draggable="true" droppable="true"/>
<listitem label="7" draggable="true" droppable="true"/>
<listitem label="8" draggable="true" droppable="true"/>

</listbox>
</hbox>
<zscript>
void move(Component dragged) {
if (self instanceof Listitem) { self.parent.insertBefore(dragged, self); }

else

{ self.appendChild(dragged); }

}
</zscript>
</window>