Comment 2 for bug 162000

Thanks for your report, that's known upstream: http://bugzilla.gnome.org/show_bug.cgi?id=123994

comment from upstream:

"There are a couple of keys which are not candidates to be keyboard
accelerators, independently of the modifiers. Tab is one of those
keys, as it (with various combinations of modifiers) are reserved by
gtk for its own use and the window manager.

Note that validity is decided by calling gtk_accelerator_valid, but
the list of valid accelerator keys seems to be only in the source
(which you can find at
http://cvs.gnome.org/lxr/source/gtk+/gtk/gtkaccelgroup.c#790, in case
you feel like it)
"