Where the browser has the File System Access API (Chromium), Open
Notebook asks for a folder rather than files to copy in: the notebook in
it opens where it is, its handle kept to reach it after a reload, with a
Reconnect button where the browser wants the permission asked again.

notebook::fs mounts such a folder: the page mirrors it and reads what
other apps write to it every few seconds. A browser takes no locks, so a
commit to a section there goes to the page and stays uncertain, as one
whose answer was lost, until the page has written it, provided nothing
else wrote the file since it was read; otherwise the file as it now
stands comes back and the edits go again on top. The sync popup says the
folder is not locked.

Assisted-by: claude-opus-5.5