I don't think there was any offscreen, actually. This was literally two seconds after the last panel last chapter, so we only missed the slash going through him and his body falling apart.
I read that post but it doesn't make much sense to me: Gojo isn't in a separate realm or dimension. He's in the same one protected by a barrier of sorts.
I need Sukuna actually explaining something in this chapter.
Clearly he isn't in a separate dimension, the illustrations are there just to, well, illustrate the point somewhat
And yeah, it was literally off screened since we only saw the aftermath of it.
It was that "call an ambulance, but not for me" meme, with Sukuna going from bending over in pain and Gojo being on the up and up to Gojo being cleaved in half.
Twxtbook off screen