i tried with =directx as well, no difference.

here are some results from different output= settings:

surface = better movement but still jumpy to the point that i can't select anything
overlay = waaay too jumpy
ddraw = better movement but still jumpy to the point that i can't select anything
opengl = waaay too jumpy
openglnb = waaay too jumpy

different resolutions make no difference.

Are posx and posy in some sensible range at some time? As other SDL
games use only those values, and they worked fine you said.

I dont know. What are sensible values?

Yes i tried several other SDL games which run fullscreen and they worked fine.

If you move the mouse slowly, x/y should modify slowly as well, as it
happens in windowed mode.
As other games seem to work fine, there must be some configuration
where the x/y are usable.