[KevinH]

I think there is a QtWebEngineURl Scheme setting that enables the fetch api. If the fetch is foreign though you would have to enable the remote access in Sigil Preferences too.

Is that js opening any Windows, as I think that is disabled as well?

What scheme does the url passed to fetch() use? Is it a private scheme or http or https?

Can you please post a very simple epub test case with it and I will see if I can get things to work on Sigil too by changing the QWebEngineProfile, QWebEngineSettings, and/or our Sigil url scheme handler settings settings that Sigil uses.