Except my point is I'm completely okay with this because its not modal at an application level if its HTML. I just want to be able to leave the site. Change the way the page is rendered all you want, disable interactivity etc but I don't feel a website should be able to prevent me from closing it which is what a modal dialog does.

The most common use for onwindowunload dialog boxes is to warn the user of unsaved changes and prevent their loss.

Our web app autosaves so the dialog box should never appear, but our metrics show that it appears surprisingly often -- sometimes it can take a few seconds for changes to flush through the websocket, and for the save to get acknowledged.

If you took that away, our users would lose changes. I imagine they would get pretty irate.

If this is absolutely necessary, you can force the site to request permission to do this, just like getting location.

How about making it 'harder' for the user to close the browser when there is an alert being shown by a website ? For instance, if google docs throws an alert about the data not being saved, your browser could throw you a msgbox notifying you that such tab has an alert you haven't dealt with when you try to close it.

Yes, that's a lot of msgboxes, but it's a decent compromise I think. It's not like that would happen very often anyway.

Google docs and Gmail and many other top 100 sites use on window unload. It wouldn't fly.

See: https://news.ycombinator.com/item?id=11747335