I like the scene in part II where Marty picks up Marty Jr. and places him against the wall behind the counter. That was almost seamless.
When there's no more room in Hollywood, remakes shall walk the Earth.

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Well in any case, that was still done pretty well for the late 80s/1990. Also think about the kitchen scene from part II- a really good job using whatever methods were available.
You'll kill everyone!
But Ice Cream Cake!

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Actually, you never physically see him hand it off. At the point, where Marty takes the baby from Seamus, Maggie walks in front of the shot. Technology today would make it readily available to shoot it better, but it wasn't around in 1990. Bob Zemeckis used a device called a VistaGlide that made it possible to shoot 2 or 3 different characters played by the same actor. That's what they used there, but for the shot where Seamus hands off the baby, Maggie McFly walks by to eliminate you actually seeing them hand it off to each other.
If you're not taking any steps forward, you're not moving at all.

They had to actually invent time travel to make this scene work.
Complicated, but I think it was worth it.