Well in the portal games I don’t think portals have ever moved

But the way I see it the “distance” you travel through the portal is the same. You don’t go through a tunnel it’s instantaneous. But one portal is moving so I think just and endpoint is moving

I’m thinking of it like a normal door

If a run through a door I will inherit my momentum like in the portal games

But if a door is quickly coming at me and I’m standing still the frame just goes around me I’m still