I can see this post from .world. Federation looks to be pretty close to in-sync (those activities behind values are only estimates since not necessarily all of those would be bound for lemm.ee). In my experience, "activities behind" value from .world less than 500 is as pretty close to fully in sync as you can really get (sometimes it drops lower, but average for me is 200-500).
Do you have a specific post/comment link that isn't showing up on .world?
fwiw, the estimate number only states the max amount of activities behind. the real number can be lower, but not higher (unless sending is entirely broken on the instance being checked).
each activity being sent has a numeric id in the database. lemmy has an api that returns the id of the last activity that was either successfully sent to an instance or skipped when it didn't need to get sent (e.g. pm to a user on a different instance). there may also be holes in activity ids due to postgres implementation details for auto-incrementing sequence ids.
for determining the highest known activity id to compare it with the last activity id sent to a specific instance, you can just go through the successfully sent ids for all instances in the response and find the highest number across them all. then you can calculate the difference between the highest number and the number for the specific instance.
depending on the lemmy version and timing of the action, it can take up to 30 seconds for the activity queue to deal with new activities, so on a somewhat busy instance the delta is likely rarely going to be zero.
I had to resolve the post, but it's there now.
I don't notice a lot of stuff disappearing during federation with LW, but when I do notice it, I just un-upvote my comment and re-upvote it. Seems to kick it in gear. Never tried that with posts, but might work similarly.
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