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This is the default behavior. But if you use the inline style I mentioned above, you can set the width to be wider or narrower than this if you wish.(I'm not sure why you'd want to hide some of the text and destroy the scrollbar, but you can if you really want to!)
It's a bit different for NN4 -- the box will alway be at least as wide as the widest item, but you can increase the width by adding the width attribute.