using the freshly 6.5.1 zk you can see that if you use portalchildren and renderdeferer you will get a js error and a "loading" forever.
An example:

<zk>
<portallayout id="myptl" maximizedMode="whole">
<portalchildren renderdefer="100">
<panel id="paneltables" height="352px" border="normal" collapsible="true" maximizable="true" style="margin-bottom:10px">
<panelchildren>
<hlayout>
<label>ciao</label>
</hlayout>
</panelchildren>
</panel>
</portalchildren>
</portallayout>
</zk>