Hi,
There might be some helpful info in MacBook Pro with Retina display - strange behaviour
Specifically, it’s not clear that passing the fullscreen resolution to SDL_CreateWindow() is correct - the docs at https://wiki.libsdl.org/SDL_CreateWindow specifically state that the width/height params are ignored if you call it with SDL_WINDOW_FULLSCREEN (even though they are not ignored in practice).
It might be worth seeing if you can reproduce the problem by using SDL_SetCurrentDisplayMode instead of SDL_CreateWindow() .
I have a retina display Mac as well so would be interested in looking at this, mind posting the code for your test program?