I have also experienced this, and I’m pretty sure it’s not an after-image. It’s not “visual”, but “mental”, so to say. It happens to me not only with code, but

I have also experienced this, and I’m pretty sure it’s not an after-image. It’s not “visual”, but “mental”, so to say. It happens to me not only with code, but also some games; for example after a lot of time playing Tetris, I’ll sort of imagine blocks whenever I’m not more focused on something else. I suppose it has to do with repetitive patterns. Perhaps some sort of loop takes root in the brain and these patterns are re-experienced in the way you mention; or maybe the brain was so absorbed with the task that it keeps working on it at a subconscious level, with occasional incursions into conscious experience. I’m wildly speculating, though. It would be great to know of a scientific explanation for this.