Some notes for the loaded/total props for ProgressEvent: they are todo_wine for `loadend` because wine-gecko returns them as 0 instead of their actual values. This is an issue in wine-gecko's nsXMLHttpRequest where OnProgress never gets called in some cases (such as our tests in xhr.js), so it needs a fix on gecko side.
[This MR](https://gitlab.winehq.org/wine/wine-gecko/-/merge_requests/7) in wine-gecko solves that issue (the first commit), but it needs to remain todo_wine until next release.