Portals are not discontinuous and the cube is not "launched"; its trajectory is not a change but a continuation of its motion seen from the far side of the portal. The frame of reference is not "agnostic", it is a frame of reference, a construct applied to our observations, and the cube necessarily exists in two different ones simultaneously. That is to say, "what is happening on the side it came from" isn't an objective physical state of the cube, but a relative one, and the fact that it appears stationary in one frame of reference does not prevent it from being in motion in another. Your description of portals is actually more accurate under "A" if you ask me, except then out of nowhere you conclude "the box doesn't launch because there is nothing that could have caused it to" which seems to me to be a misunderstanding of what you actually described.

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