Header of grid scroll fail after remove the Frozen

https://sourceforge.net/tracker/?func=detail&aid=3293433&group_id=152762&atid=785191

1. drag the scroll bar, the header will scroll well.
2. click add Frozen, then drag the scroll bar, the header will scroll well.
3. click delete Frozen, then drag the scroll bar, the header will not scroll.

<zk>
<button label="add Frozen ">
<attribute name="onClick"><![CDATA[
Frozen fz = new Frozen();
fz.setColumns(1);
grid.appendChild(fz);
]]></attribute>
</button>
<button label="delete Frozen ">
<attribute name="onClick"><![CDATA[
grid.getFrozen().detach();

]]></attribute>
</button>
<grid id="grid" width="300px">
<columns>
<column label="column ${each}" forEach="1,2,3,4,5"
width="100px" />
</columns>
<rows>
<row forEach="1,2,3,4,5">
<label value="item ${each}" forEach="1,2,3,4,5" />
</row>
</rows>
</grid>
</zk>