fromJSON(root.toJSON()) isn't lossless when a stylesheet has uneven spacing between its top-level nodes:

let css = 'a {}\nb {}\n\nc {}\n'
let root = postcss.parse(css)
postcss.fromJSON(JSON.parse(JSON.stringify(root.toJSON()))).toString()
// => 'a {}\nb {}\nc {}\n'   the blank line before `c` is gone

toJSON() serializes every raws.before correctly — the loss happens on the way back. fromJSON builds the child nodes and then passes them to the container constructor, which appends them; for root children that re-runs the insertion spacing logic in Root.normalize, overwriting each node's own raws.before with the previous sibling's. So every top-level node after the first inherits the first one's spacing.

It matters because round-tripping through JSON is meant to restore an AST exactly (e.g. caching a parsed AST), so a rehydrated root should stringify to the same CSS as the original.

I rehydrate the children and attach them directly instead of routing them through append, leaving their raws untouched. Runtime append/insertion behavior is unchanged. Found it round-tripping the postcss-parser-tests fixtures; added a regression test and pnpm test is green.