I get that it may have *seemed* to work, but it was under
no obligation to, and you started to see the evidence of
that. :P
Making a GTK+ instance global alone seems unable to solve the
problem: what you need is to stop manipulating it from threads
that didn't initialise GTK+ / aren't running the GLib/GTK+
main loop.
Not that I immediately have a ready-made alternative, but the
architecture you described seems somewhat less than ideal to me.
I would personally try for a model/view architecture, i.e. have
whatever thread you want calculating the next frame of the cell
grid, then sending a signal to the GUI thread when it is done
and the GUI should update to reflect the new data.
Alternatively, have GTK+ periodically check whether a new frame
is ready and, if so, update from the world data.
