Function materialize_and_move_boundary 

pub fn materialize_and_move_boundary(
    doc: &mut AutoCommit,
    left: DisplayedSession<'_>,
    right: DisplayedSession<'_>,
    new_left_end: &HistoryEntryId,
    new_right_start: &HistoryEntryId,
) -> Option<(String, String)>

Correct a seam even when one or both work sessions are still automatic projections. Missing records are materialized with the titles and bounds the person saw, then the bordering action is moved in the same document transaction. The atomic history entries themselves are never rewritten.