From Trac ticket #1503:
All of the calls to compose.set_mode() should be calling compose.start()
instead. Nearly all are impossible to trigger while already composing,
and by calling compose.set_mode() they just do the stuff in set_mode()
twice. The only case where this would change behavior, that I can see,
is if you press C, Shift-Tab to unfocus the compose box without
dismissing it and then press c (or vice versa). I think it's okay if we
clear the input fields in that unlikely case.
(imported from commit ba7f181ec9d1df90a443b0a754462a3a201dcabb)
a7963a24f8