How do I prevent Stable Diffusion from tilting the background or having a broken horizon line(see image).
It's impossible, because stable diffusion generation process is not separated into background -> characters -> post-effects line. All happens at the same time, so, Stable Diffusion doesn't know what to do with image, divided by character in the center. And that's the reason, why it can't do proper indoors backgrounds with character on the screen. But i'm sure, that horizon line should be fine for most gens. If it's not - better to find less broken model, maybe. Or optimize prompt, if it is too big ≥150 positive tokens and ≥75 negative tokens.
As has been said before: there are no prompts to prevent this from happening. If you don't want to manually edit the image or try to fix it somehow in inpaint but the horizons bother you, try making small adjustments to the generation parameters. If this image was generated with 20 sampling steps look what is generated with 18,19,21,22 steps and play around with highres fix at different denoising strengths.