I’ve noticed something new in ROTJ in that Vader has Luke’s lightsaber during the duel on the 2nd Death Star. It’s never mentioned in the film or referenced to except the novel.
Apparently after Vader knocked him out of the rafters, he drops his saber and Vader picks it up. Which is why he was hiding under the stairs. He gets it back with the force and the fight continues.
I’ve always just assumed he was meditating or something under those stairs. But this is another change I think would be simple for ROTJ Revisited