Navigate textboxes in listheader by tab key break frozen scroll behavior

<zk>
	<label multiline="true">
		1. Focus on first textbox
		2. Press "tab" key 3 times
	</label>
	<window title="Listbox" border="normal">
		<listbox width="300px">
			<frozen columns="1" />
			<listhead sizable="true">
				<listheader align="center" width="40px" />
				<listheader width="90px">
					<textbox value="filter 1" width="70px" />
				</listheader>
				<listheader width="90px">
					<textbox value="filter 2" width="70px" />
				</listheader>
				<listheader width="90px">
					<textbox value="filter 3" width="70px" />
				</listheader>
				<listheader width="90px">
					<textbox value="filter 4" width="70px" />
				</listheader>
				<listheader width="90px">
					<textbox value="filter 5" width="70px" />
				</listheader>
			</listhead>
			<listitem>
				<listcell />
				<listcell label="ZK Jet 0.8.0 is released" />
				<listcell label="2008/11/17 17:41:29" />
				<listcell label="cell" />
				<listcell label="cell" />
				<listcell label="cell" />
			</listitem>
			<listitem>
				<listcell />
				<listcell label="URLs for iPhone-Optimized Google Sites" />
				<listcell label="2008/11/17 15:56:37" />
				<listcell label="cell" />
				<listcell label="cell" />
				<listcell label="cell" />
			</listitem>
		</listbox>
	</window>
</zk>