Since we now FINALLY have push notifications on iOS PWA 
I was looking into the delay that is applied before a push notification is being sent - because it seemed to take longer than expected.
The code checks if a user was online recently (less than push_notification_time_window_mins ago) and if they were then the notification is delayed for push_notification_time_window_mins
if user.seen_since?(SiteSetting.push_notification_time_window_mins.minutes.ago)
Jobs.enqueue_in(
SiteSetting.push_notification_time_window_mins.minutes,
...
)
else
Jobs.enqueue(:send_push_notification, user_id: user.id, payload: payload)
end
But this means that in practice this can take up to as twice as long as expected since the push_notification_time_window_mins is applied both in the online check as in the delay.
Example:
push_notification_time_window_mins is set to 10
13:00
I leave the site
13:10
Something happens that triggers a notification.
Code checks whether I was online less than 10 minutes ago.
I was, so the notification will be sent at 13:20.
12:59
I leave the site
13:10
Something happens that triggers a notification.
Code checks whether I was online less than 10 minutes ago.
I was not, so the notification will be sent immediately, at 13:10.
So one minute difference in my online time can change the notification delivery with almost push_notification_time_window_mins. Especially if the setting is changed into a higher value this can make a significant difference.
I would expect the delay to be push_notification_time_window_mins - (now - seen_since)