I don't have tried this application, I just came across this bug, but I guess this is due to structure change written in ./module/gtk3privates.h (yes, this is originally "private" header in gtk3, so the structures may change in gtk3).
For example, with gtk+ 3.20, struct _GtkMenuPrivate has more members than what is written in ./module/gtk3privates.h in topmenu-gtk.
I don't have tried this application, I just came across this bug, but I guess this is due to structure change written in ./module/ gtk3privates. h (yes, this is originally "private" header in gtk3, so the structures may change in gtk3).
For example, with gtk+ 3.20, struct _GtkMenuPrivate has more members than what is written in ./module/ gtk3privates. h in topmenu-gtk.