Comment 28 for bug 36189

Firefox's solution to this problem (for its customizable toolbars) is to have a "flexible space" item that acts like a spring, expanding and contracting as needed to push the controls to its left and right all the way to the left and right ends of the toolbar, respectively. Firefox's toolbars also enforce order, so controls can't reorder themselves when the size of the window changes. Perhaps GNOME could use a similar technique.