Function visible_fold_edge 

pub fn visible_fold_edge(
    folds: &[Fold],
    foldenable: bool,
    line: u32,
    forward: bool,
) -> Option<u32>

VM.3i: vim’s zj / zk edge from line — the nearest fold START after it (forward) or fold END before it — among edges that are VISIBLE.

“A closed fold is counted as one fold”: folds nested inside a closed one render on its row, so they aren’t stops. Candidates are compared by the row they’re displayed on (FoldIndex::visible_anchor), and an edge on the cursor’s own row isn’t a destination. The edges returned are still line numbers, so zk from below a collapsed section lands on its last line.

One rule for both callers: the zj / zk motions (through the host’s fold resolver) and Editor::do_goto_fold (the WIT action path).