Repeat after me: nobody can judge of a pope.
That means that nobody is invested with the authority level official judgments against a pope. It's not possible to drag someone recognised as the Pope before the Inquisition or to have a body of bishops judge him. He can be corrected, but he cannot be judged.
Pope St. Nicholas, epistle (8), Proposueramus quidem, 865: “… Neither by Augustus, nor by all the clergy, nor by religious, not by the people will the judge be judged… ‘The first seat will not be judged by anyone.’"
Pope St. Leo IX, In terra pax hominibus, Sept. 2, 1053, Chap. 32: “… As the hinge while remaining immoveable opens and closes the door, so Peter and his successors have free judgment over all the Church, since no one should remove their status because ‘the highest See is judged by no one.’”
Canon 1556, 1917 Code of Canon Law, On trials in general: “The First See is judged by no one.”
We're not talking about a juridical sentencing but a discernment by the Church about the orthodoxy or lack thereof of a particular Pontiff. Quite a few theologians hold that the Church would have to make a declaratory statement regarding this discernment. Rejection of a Pope as a non-Catholic cannot come from Joe Sixpack in the pew. Yes, yes; once it's been determined that the Pope is a heretic he loses office regardless of whether or not he had received Universal Acceptance as pope. Problem at hand is how does that determination get made and by whom?