If you run the code below in the sandbox http://www.zkoss.org/zksandbox using Firefox, you will notice that the underline on the word "Help" extents 1 character beyond the "p". Using firebug, you can see that a " " character is being appended to the label in line:

<menuitem label="Help" style="text-decoration:underline;"/>

This extra spaces causes the underline to extend past the text of the label.

<zk>
	<menubar id="menubar" width="100%">
		<menu label="Project" >
			<menupopup>
				<menuitem  label="New"  />
				<menuitem  label="Open"  />
			</menupopup>
		</menu>
		<menuitem label="Help" style="text-decoration:underline;"/>
		<menu>
			<menupopup>
				<menuitem label="Index"  />
			</menupopup>
		</menu>
	</menubar>
</zk>