As discussed recently on the users mailing list [1], there's a strong precedent among GTK+ applications for F9 to toggle sidebar visibility. Examples cited were Nautilus, Evince, Totem, Rhythmbox, File-Roller and gThumb. Change the Send/Receive accelerator from F9 to F12, and designate F9 as the accelerator for View->Layout->Show Side Bar. [1] https://mail.gnome.org/archives/evolution-list/2013-May/msg00194.html