refer to zul code below

ZK 5.0.7.1 works well, but not after ZK 5.0.8 (tested use zk fiddle)

<zk>
<div id="main" vflex="true" width="100%">
<div height="200px" width="100%" style="background: #5CB3FF;">
</div>
<div id="toolbarMask" visible="false">
</div>
<div vflex="true" width="100%" style="background: #9E7BFF;">
</div>
</div>
</zk>