The comment at the start of block_ext_overlap has a visual depiction of the eleven different cases of extent lists overlapping each other. It's a visual depiction and whitespace is significant, but The Big Clang Format helpfully rejigged it all, so now none of the overlaps line up visually.

The comment is difficult at best and misleading at worst - we should make the visual cases line up with the description. This is only a whitespace change.