The handling of the module lists is a bit confusing here. There is
a global list that is populated with initial modules from GTK_MODULES,
and then there is a second list that is suppposed to hold the modules
loaded from the gtk-modules xsetting, which we attach to the GtkSettings
object. The blacklisting caused us to put the wrong modules into the
second list, and mess up the module refcounts, eventually double-freeing
memory when the setting changes repeatedly.

https://bugzilla.gnome.org/show_bug.cgi?id=659966