Narrowed it down further to the "opacity" property:
obj_props[PROP_OPACITY] = g_param_spec_uint ("opacity", P_("Opacity"), P_("Opacity of an actor"), 0, 255, 255, G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS | G_PARAM_EXPLICIT_NOTIFY | // HERE CLUTTER_PARAM_ANIMATABLE);
Narrowed it down further to the "opacity" property:
obj_props[ PROP_OPACITY] = spec_uint ("opacity",
P_("Opacity" ),
P_("Opacity of an actor"),
0, 255,
255,
G_PARAM_ READWRITE |
G_PARAM_ STATIC_ STRINGS |
G_PARAM_ EXPLICIT_ NOTIFY | // HERE
CLUTTER_ PARAM_ANIMATABL E);
g_param_