Fix multi-tab session lock on Firefox not being cleared

[ManuelHu]

Fixes element-hq/element-web#26165

As described in the issue, the pagehide event is currently not reliably called in Firefox, which will lead to the session lock not being cleared. If Element was re-opened quickly after closing, this leads to confusing messages shown to the user and additional waiting time at startup.

By my tests, the unload event is fired more reliably. Chrome docs discourage the usage of the unload event, and MDN states that it might not be called at all by Chrome - so my change also keeps the pagehide handler, and only adds the unload handler.

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

Notes: Fix multi-tab session lock on Firefox not being cleared

Type: defect

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Fix multi-tab session lock on Firefox not being cleared (#11800). Fixes Confusing session lock error message shown if tab re-opened shortly after closing it element-hq/element-web#26165. Contributed by @ManuelHu.

[florianduros]

Thanks for your contribution!
Tested on both chrome & firefox

[richvdh]

Looks like this is new.

On balance, adding it is probably a good idea.

Previously I left the lockServicer in place, which would mean if the page came back to life, we would reclaim the lock. But that is racy: if we clear our claim, but the lockServicer fires again before the tab actually dies, we'll end up with a stuck claim.

[ManuelHu]

Yes, this was my motivation - with the two handlers the risk to have a racy access might be higher. But I did not think about the page after being restored.

To close the bfache hole, couldn't we listen to pageshow with event.persisted==true and immediately re-start the lock servicer? But this might induce other problems, e.g.

1. have one Element instance in bfcache
2. open another instance in another tab (which works after the lock is expired)
3. restore the first instance of the bfcache

Maybe listening for pageshow with event.persisted==true and somehow forcing the session to be re-initialized would be better (or just reloading the whole app?).

[richvdh]

Yeah, there are no ideal solutions here. To be honest, my understanding is that the presence of an unload handler prevents the app being eligible for the bfcache anyway, so it's probably fine in practice. Longer term, the "right" solution is to fix multi-tab loading so we don't need any of this locking business (element-hq/element-web#26231).