thank you!! i thought i used physics_process in the game, but now that i think of it i might've forgotten to O.O thanks for pointing that out!
edit: checked the code and i did use physics_process haha
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Have you also tried using the same value for Project Settings->Debug->Settings->Force Fps and Project Settings->Physics->Common->Physics Fps? For example 60 for both of them. When I change one of them the movement jitter appears but when they are the same, it is buttery smooth. If you haven't tried this, maybe give it a go.