> In fact, we have yet to receive a single complaint regarding our multipart form posts resulting in actual browser navigation.

Just out of curiosity, how do you handle errors, specifically timeouts here? I'm guessing the HTTP server serves a custom error page, does it have a reference to the previous page with all the data they tried to submit and so on? Probably how I would have handled it.

Timeouts are a bit trickier, as the user would just see the browser error page and dassit.

Nice thing with decoupling server responses from what the browser actually renders is that you can handle things like that exactly like how you (as a developer) want to, without breaking the current page.

But, as always, it depends. Seems you folks are giving your users a lot of value from whatever you're doing regardless, so seems what you're doing is working :) Congrats.