Although, I would go even farther in some instances than this article it is another great resource at defending the faith from Sedevacantist – like positions.
The Cassiciacum thesis was developed by Fr. Guerard and says that the Pope is materially Pope but does not have full Papal powers (like to teach infallibly) Fortunately, for Vatican II the Popes did not use extraordinary Papal infallibility dogmatic pronouncements but taught at the level of the ordinary magisterium. The ordinary magisterium does not rely only on the Pope but on all the bishops in line with the Pope, so it would really not matter if the Pope could be materially but not formally a Pope for Vatican II to be valid, true and guarded from error through the ordinary magisterium alone.
In the linked article by Griff Ruby the flaws in the Cassiciacum thesis are revealed which like so many theological errors and heresies try to limit the Pope’s powers and privileges.
It is a long read but a more or less in depth treatment! See the link Here.