Re: [w3ctag/design-reviews] early design review: Permissions-Policy: unload (Issue #738)

Unload is inherently unreliable on mobile because if a tab goes into the background, all kinds of things can occur that can result in the document being discarded without unload handlers running. E.g. it's common for the OS to kill a browser background process to reclaim memory, many people are in the habit of swiping apps closed when they are done with them to reduce background memory usage, this does not give the browser app time to correctly unload documents. This is quite different to desktop where the browser process is often alive for days or weeks and has an opportunity to run unload handlers if they exist when shutting down.

Unload is incompatible with BFCache. If a page enters BFCache, we can't run unload as the page enters BFCache because it might come back. If the user never comes back we will eventually evict the entry but we can't run it in the background then because that raises privacy problems. The page that they navigated away from minutes ago is suddenly running JS, maybe grabbing location information or making network requests etc.

The resolution on mobile was that anyone relying on unload was already broken, so it's fine to BFCache a page with unload, we just don't run it. Things get a bit more broken.

The resolution on desktop (for Chrome at least) was that things are still relying on unload and we should not unilaterally break it. Right now, for 16% of history navigations the only reason blocking from entering BFCache is that they have an unload handler in some frame. There are lots more pages blocked by unload+some other reasons, so other browsers could be seeing far higher than 16% that are only blocked by unload. If we were to just ignore it and BFCache anyway, we would be dropping the reliability of unload from something close to 100% down to 84% and even further as we make more and more features compatible with BFCache.


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