Run the following zul:
<zk>
<window>
<vlayout style="height:1000px" vflex="1">
<div vflex="5" style="background-color:yellow">xxxxxxxoooooooxxxxxx</div>
<div vflex="5" style="background-color:pink">xxodxx</div>
<div vflex="5" style="background-color:red">xxodxx</div>
</vlayout>
</window>
</zk>
Then resize browser, the size of the divs will change