In the origin-private file system API, everything is basically asynchronous. You can get a synchronous handle for reading, writing or truncating a file, but it still passes through a Promise.
Documents are immediately closed in the browser. Is there any guarantee that an active handle whose Promise is not yet resolved will continue until it's used for writing to a game save file? For example, can the developer do FS operations in the beforeunload event and will the browser keep the document finishing its process in the background until there's no active FS promise?