You can test here - http://zkfiddle.org/sample/1m1gev5/1-Another-new-ZK-fiddle

Or with the following example,

<listbox sizedByContent="true"
style="border: 0;" hflex="min">
<listhead sizable="true">
<listheader label="Message Time" width="150px"></listheader>
<listheader label="Level" width="75px"
sclass="r1-listheader">
</listheader>
<listheader label="Source" width="75px"
sclass="r1-listheader">
</listheader>
<listheader label="Message" ></listheader>
</listhead>
<listitem>
<listcell label="6/28/10 4:19:18 PM" ></listcell>
<listcell label="Info" ></listcell>
<listcell label="Server" ></listcell>
<listcell
label="Merging recovery point 52 created 20 6/27/10 10 :11 PM" ></listcell>
</listitem>
<listitem>
<listcell label="6/28/10 4:19:18 PM" ></listcell>
<listcell label="Info" ></listcell>
<listcell label="Server" ></listcell>
<listcell label="Merging recovery point 52" ></listcell>
</listitem>
<listitem>
<listcell label="6/28/10 4:19:18 PM" ></listcell>
<listcell label="Info" ></listcell>
<listcell label="Server" ></listcell>
<listcell label="Merging recovery" ></listcell>
</listitem>
<listitem>
<listcell label="6/28/10 4:19:18 PM" ></listcell>
<listcell label="Info" ></listcell>
<listcell label="Server" ></listcell>
<listcell
label="Merging recovery------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------*" ></listcell>
</listitem>
</listbox>

If you remove the sizedByContent and then put a hflex="min" to the last listheader, and then it will work well.