33761eb2af
While the `get_key_focus()` method returned the stage itself when no explicit focus was set, it made sense for the corresponding setter to accept the stage as synonym for NULL. But now that the getter always returns the property value, it makes more sense to expect NULL to unset the key focus. Keep the current behavior of normalizing the key focus to NULL in that case to minimize breakage, but print a warning to use NULL instead. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4256>