In Cheese we have three buttons that behave like a RadioButton group.
They are actually three toggle buttons related to three radio actions the make them behave like some sort of "mode selector" (they switch Cheese mode to "Photo", "Video", "TimeLapse").
See [1] for a screenshot.

Now it would be great to have a way in gtk to tell the engine to visually group those buttons making them look like a single widget.
See for example [2] and [3] for an idea about the desired look.

The reason for a change like this is that those buttons really behave like a single widget, something like a radio button group, and it would be a lot nicer to have some visual hint that stresses this aspect. IMHO it would be a lot sexier but also better from a usability point of view.

Why don't we just use radio buttons? I don't think they are really suited for use in a toolbar.

How much popular it is? well in GNOME nobody has yet implemented anything like this. I guess that's because it's difficult to have the engines style custom widgets from each applications so it would be better to have this directly in gtk+. It is popular elsewhere though, see for example this screenshot of OSX Finder toolbar[4]. GMail groups related buttons this way too.

How could this widget be implemented? There has been a brief discussion in gtk-devel[5,6] and, from what I understood, there seem to be essentially two ways:
a) illustrated in [7], point 3: export some kind of information to the theme engine about "related" widgets so that the theme can choose either to visually bind them or just to ignore that hint.
b) add a real button container that the engine *is supposed* to style properly to visually connect its children in a similar way it connects the arrow button to the text entry in GtkComboBoxEntry


1. http://people.gnome.org/~fargiolas/togglegroup.png 
2. http://people.gnome.org/~fargiolas/toggle-button-mockup.png 
3. http://audidude.com/?p=61
4. http://people.gnome.org/~fargiolas/finder-toolbar.png
5. http://mail.gnome.org/archives/gtk-devel-list/2010-March/msg00014.html
6. http://mail.gnome.org/archives/usability/2010-March/msg00025.html
7. http://blogs.gnome.org/carlosg/2009/02/20/dublin-theming-hackfest/