- File attachments (#23): Attach File, drag and drop, open, and Save As, stored
  inline with a file-type icon as OneNote 2010 does; files on blank page are
  page-level objects that move, delete, and sync.
- Customize Tags (#8): OneNote's dialog with all 143 symbols and both palettes;
  the tag list lives in settings and drives the toolbar, menus, palette, and
  shortcuts; tag definitions store OneNote's status bits.
- Undo groups a run of typing until the caret moves or another edit, as
  OneNote 2010 does, on desktop and iOS.
- Background art is one undoable editor step that renders on the next frame,
  with an opaque backing that hides rule lines as OneNote's art does.
- Renderer: shadows are computed in the shader and the icon atlas evicts and
  grows instead of failing a frame.
- iOS: sync status and sheet, tags with the new artwork, Tags Summary, page
  background, cross-notebook search, and a replica creation race fix.
- An unset paragraph language is no longer written as language 0.

Assisted-by: claude-opus-5.5