When using `Actor.ease_property` if the property starts with '@' and the
duration of the transition is zero (which may happen if the actor is not
mapped even if a non-zero duration was passed to `ease_property`), the
impl will try getting the actual target object where the property should
be set.
This works fine for most cases but it currently throws an error when
passing '@content.*' properties. Fix this by handling '@content' as
a property of `actor.content` (used by MetaBackgroundActor when
showing the overview).
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1461
return [actor.get_action(name), prop];
case '@constraints':
return [actor.get_constraint(name), prop];
+ case '@content':
+ return [actor.content, name];
case '@effects':
return [actor.get_effect(name), prop];
}