The court renders a decision in response to an appeal; the appealing parties request the decision. The court may reach its decision based on prior law and the Constitution by rigourous logic in which case no other valid decision would be possible. In that case, that necessary decision would have been reached by any court at any time provided only they made no logical error. More often their decision is not logically implicit in the Constitution and existing law but rather is contingent and creates new law which cannot be considered to pre-exist the decision, as in the case of Texas v. White or Roe v. Wade. To apply that law to convict a defendent, other than those directly involved in the case that gave rise to the decision, whose case predated the decision, raises exactly the same objection as does the use of an ex post facto law. I repeat, to apply a contingent decision retroactively is certainly unjust; Constitutional law must certainly prohibit it.
We have been through most of this argument before.