it will happen when horizontal scroll appears.
<zk>
<tree width="100px">
<treecols>
<treecol width="200px"/>
</treecols>
<treechildren>
<treeitem forEach="1,2,3,4,5" label="item" open="false">
<treechildren>
<treeitem forEach="1,2,3,4,5" label="item"/>
</treechildren>
</treeitem>
</treechildren>
</tree>
</zk>
Fixed since 2011/7/13.