You can also test with zktest/test2/B50-3313071.zul

For example, (if you remove the vflex in panelchildren, the result is correct)

<window mode="modal" title="Edit Appointment" border="normal"
position="center" hflex="min">
<panel hflex="min">
<panelchildren vflex="min" hflex="min">
<borderlayout vflex="min" hflex="min">
<center vflex="min" hflex="min">
<div style="background-color:red; width:100px;height:100px">
You should see a square red color.(100px * 100px)
</div>
</center>
</borderlayout>
</panelchildren>
</panel>
</window>