feat: adjust background sync on user activity

[SebastianKrupinski]

Resolves #10717

Summary

• Added logic to track mail app activity
• Added logic to modify background sync job interval depending on user activity in the UI

Activity level -> Sync interval

• Did not use app for more than three days -> Sync every six hours
• Did not use the app for a day -> Sync every hour
• Used the app in the last 24 hours -> Sync every 15 minutes

[ChristophWurst]

From when we discussed the feature I remember saying that we want to track when a user accesses Mail. If they access it frequently we keep the hourly sync. If they don't, we drop down to one sync in four hours or even less often.

The current change tries to detect user activity, then skips syncing but also drops the interval from 60 minutes to 15 minutes.

Can you explain why the implementation was done so different to what we discussed? Are there new findings that make the original idea impossible?

[SebastianKrupinski]

From when we discussed the feature I remember saying that we want to track when a user accesses Mail. If they access it frequently we keep the hourly sync. If they don't, we drop down to one sync in four hours or even less often.

The current change tries to detect user activity, then skips syncing but also drops the interval from 60 minutes to 15 minutes.

Can you explain why the implementation was done so different to what we discussed? Are there new findings that make the original idea impossible?

Yes, that is what I am trying to do with this.

The idea here is that the background sync will not run as long as there is a up to date heart beat. So as long as the mail UI has been active the background sync will not run, but will revert back to the regular configured interval when there is no activity. Essentially if you are active all day the background sync should not execute. And if you are not active often then it will revert to the default once an hour to sync the mailbox to reduce UI waiting time, and to pick things like iMip.

It only reschdules to 15min when there is a heart beat to check for another heart beat, as the sync can be configure to run every 5min (app.mail.background-sync-interval), I thought it was more sane to check less frequently. We can probably change this to 30min.

[ChristophWurst]

The cron sync will sync all INBOXes and mailboxes marked for background sync. Frontend will sync INBOXes and the currently open mailbox. Therefore, the background sync should also run when a user has the app open all day.

[st3iny]

I discussed this PR with Sebastian and we derived a new logic. Please check the description for more information.