Upd: instead of shared worker, it could be easier and better for browser support to use Web Locks API

Notes about using Web Locks API here.

We don't have actually a "resource" that we want to block. But we can use a Leader pattern.
With Locks API we can lock a tab to be a "Leader" who is responsible for everything that should be done only in one tab. Then let's call "being a leader" a resource that is lock for infinity time (until the tab is not closed).

Then we can create a helper like

async function onTalkLeaderTab(() => {
  // Promise of waiting to be a leader
  return new Promise((resolve) =>
    // Request "being a leader"
    navigator.locks.request('talk:leader', () => {
      // Since we got this "resource" this tab is a leader. Resolve promise
      resolve()
      
      // Infinity promise, resource is blocked forever, while tab is not closed
      return new Promise(() => {})
  }
}

onTalkLeaderTab()
  .then(() => {
    // Now we are the leader
    // Start the setInterval for re-fetching or anything
  })

One important thing: it only works with HTTPS ⚠️

Then we can use the BroadcastChannel to inform other tabs about new conversations (cc nextcloud-libraries/nextcloud-event-bus#600)

Follow-up: because x-talk-hash header is checked not in any HTTP response, but only in re-fetching conversations for the LeftSidebar, it is not checked not in sub-tabs.

Follow-up: because x-talk-hash header is checked not in any HTTP response, but only in re-fetching conversations for the LeftSidebar, it is not checked not in sub-tabs.

Rebased, follow-up fixed with additional broadcasted message specifically for talk hash.
Also a logic has been moved to the store and App.vue, to be able to use a 'leader' locked status within different processes in one tab.

Follow-ups:

• Don't cache conversations when nothing actually changed (we know it from the patching action)
• Add methods to post specific messages into the broadcast channel into the talkBroadcastChannel service as well as methods to add message handlers
• Think about centralized way to control what and when is synced, when to post messages

/backport to stable27