They do? Then how do i do the forbidden stuff by accessing neighboring pixel data?

Have you actually done that recently?

do you mean accessing data outside of your app's framebuffer? or just accessing neighboring pixels during a shader pass, because those are _very_ different things. GPU MMUs mean that you cant access a buffer that doesn't belong to your app that's it, its not about restricting pixel access within your own buffers

TIL. Thank you.