Sometimes a single reply to a thread generates two bubble notifications like:
You have 1 new reply to Thread from X
You have 2 new replies to Thread
Without refreshing the forum root page with the notification, visit the bubble link in a new tab, do something with it, then return to find:
You have 1 new reply to Thread from X
You have 3 new replies to Thread
Again, without refreshing the forum root page go visit the page as above to find again on return:
You have 1 new reply to Thread from X
You have 2 new replies to Thread
Subsequent actions don’t appear to change the reply count, no matter what the “do something with it” mentioned above was.
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