Libsigc++ is not thread safe. That is, it is not safe to connect/disconnect signals from multiple threads. I noticed however that is used widely in synfig-core. Is there any special way this is handled in synfig? It seems to me that this will eventually cause very hard to catch bugs.
https://developer-old.gnome.org/gtkmm-tutorial/stable/sec-the-constraints.html.en
https://ardour.org/development/signals
) used, but AFAIK signal connection/disconnection is not.