In page host mode a plain Save of a .figcraft the host opened used the document's internal name, so the host got a different file name with saveAs true instead of an update to the file it opened. Services gains host_file(); Save uses it for a .figcraft host file, Save As keeps the document name, and a missing, .json or .fig host file stays a Save As.
The transports, the message types and the origin checks come from the shared crate (storytold/craft-libs#420); FigCraft keeps what is its own: which saved files become the document, the open size limit and the author.
A page that embeds the web build can hand it a file and get every save back instead of a browser download, and hears about unsaved changes: ?host=parent for a same-origin iframe (postMessage, the protocol the GridCraft, PdfCraft, WordCraft and CADCraft bridges use) and data-host="page" on the canvas for a page that loads the module into its own document (non-bubbling CustomEvents on the canvas). The bridge holds a Transport (Parent or Page) with one send; the pure rules are unit-tested; the hosting guide documents both modes.
Helen Kwok