Function rooted_at 

pub fn rooted_at(tree: &NavTree, focus: Option<BlockId>) -> Scope

The scope the tree hangs from: the focused block, or the tree’s own root when nothing is focused or the focused block has since left the document.

[Scope::Root] is the tree’s root here — the document’s top block, whose children the tree lists — since the top is never a row of its own.