This was broken in 0796d7b6ff9393746d.
https://bugzilla.gnome.org/show_bug.cgi?id=751018
0026994 GtkMenuButton: Make programmatic toggling work again
gtk/gtkmenubutton.c | 36 +++++++++++++++++++++++-------------
1 file changed, 23 insertions(+), 13 deletions(-)
Upstream: git.gnome.org