Resize browser window clears GMap?

<?xml version="1.0" encoding="UTF-8" ?>
<zk xmlns:zk="http://www.zkoss.org/2005/zul">
  <fwwindow id="${arg.windowId}"
            apply="com.pnl.fw.screens.map.freighttracker.FreightTrackerController"
            screenName="Freight Tracker"
            width="100%" height="100%">
    <zk:borderlayout>
      <zk:center title="Freight Tracker:" >
        <vbox width="100%" height="100%">
          <zk:gmaps id="gmap" mapType="map" lat="-30" lng="134" zoom="4" width="100%" height="100%" vflex="true" hflex="true" />
        </vbox>
      </zk:center>
    </zk:borderlayout>
  </fwwindow>
</zk>