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