Vbox width grows when containing a Tabbox

<zk>
	<div>
		Resize browser window several times. The width of vbox should not grow.
	</div>
	<div width="400px" style="border: 1px solid #EE1111">Div</div>
	<vbox id="vbox" width="400px" height="400px" style="border: 1px solid #EE1111">
		<tabbox id="tabbox" vflex="1" hflex="1" style="border: 1px solid #1111EE">
			<tabs>
				<tab label="Tab" />
			</tabs>
			<tabpanels>
				<tabpanel style="overflow:auto">
					Tabpanel Content
				</tabpanel>
			</tabpanels>
		</tabbox>
	</vbox>
</zk>