Running EW in two tabs may cause subtle nasty misbehaviour

[ara4n]

I think we’ve tied down the main splitbrain reliability problem with matrix-video-chat as being due to folks leaving instances running in background tabs. This means they keep a sync loop running for the same device in the other tabs, which race to consume to-device msgs with the fg tab. I think the race is something like:

• tab1 is calling /sync
• tab2 calls the sameish /sync, which returns with a todevice
• tab2 calls /sync again before tab1 returns, acknowleding and deleting the queued to device msg
• tab1 returns but without the todevice which has been “stolen” by tab2

This could affect device verif, possibly e2ee, and voip when msc3401 lands.

[aaronraimist]

Related: #11088

[jryans]

As one potential approach, Hydrogen's service worker tracks concurrent session access, ensuring only one tab can access a session at a time.