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