This is an interesting feature. Back in the day we implemented something similar in ocPortal using JavaScript, but it turned out way too complicated and buggy and was removed.

One problem is that it could confuse the user if things jump around unexpectedly. Ideally it should show that a page is stale, like making it less opaque or shadowed or something.

Also there is potential for issue if JS code is running. It could run twice for example.

So, if we do this, it needs a lot of care and testing.

This is not what I thought (https://web.dev/stale-while-revalidate/). It populates the future cache record with a newer version while using the stale version - but there's no switcheroo.