Optional_e: EventWrite the spine width, in mm, through the canvas (TYP-759).
⚠️ It goes through set_spine_width() rather than touching the record, because changing the
spine moves the front cover and everything on it — the canvas carries the elements with
move_native() and grows the container. Writing the record here would leave the artwork
behind and the container the wrong size.
Optional_e: EventThrough the canvas (TYP-976): one undo step, the page bar and canvas refreshed, and
typograph_keep_pages_together sent for the host. Writing allow_page_shuffle here recorded
no undo step — so no typograph_document_changed, and autosave missed it — and told nobody.
Which way the cover's spine reads (TYP-789). One undo step; nothing on the page moves.