I should use style="overflow:auto;position:relative;padding:0px;" for groupbox.When I set height of groupbox with value, it couldn't collapse with none size. The simplest code is as follows:

<zk>
<groupbox width="100%" height="80px">
<caption label="Enable Backups"/>
<label value="Directory:" />
<textbox />
<checkbox label="Compress archived files" />
</groupbox>
<groupbox width="100%" height="80px">
<caption label="Enable Backups"/>
<label value="Directory:" />
<textbox />
<checkbox label="Compress archived files" />
</groupbox>
</zk>