Comment 18 for bug 390724

This is a valid bug. It is ugly for buttons to be nearly the same width but not quite: they should be either exactly the same width, or obviously and intentionally very different. This means that if you customize a toolbar (possible in other applications, but currently not in Nautilus) to add a button that has a label wider than any of those currently in the toolbar, the rest of the buttons should widen to match it.

I reported the equivalent bug in Mozilla in 2000 <https://bugzilla.mozilla.org/show_bug.cgi?id=60553>, and in Epiphany in 2005 <http://bugzilla.gnome.org/show_bug.cgi?id=169397>.