A Menuitem cannot display itself completely after shrinking Menubar's width

steps to reproduce

1. load the zul
2. click the button "shrink width"
3. scroll to the rightmost menu item

current result

the rightmost menu item cannot display itself completely.

debug info

• this issue happens under Chrome, firefox, and safari.
• it's caused by incorrectly calculated width of z-menubar-body
• another related issue: if we don't call Clients.resize(bar);, Menubar doesn't resize itself when shrinking its parent <div>