Looks like an infinite loop, possibly caused by having multiple tabs open? If you close one of those pop-ups, another identical one takes it place, so I'm guessing whatever loop is going through the posts to find new ones is hitting the first one and never ending.
Also, is there a way to disable this feature? Personally, I like to open new forum posts in new tabs and work my way through them like that so this feature is doing noting for me but blocking off the far right of the screen; can we have an option to disable it?
EDIT; If you make a thread, it also lists the first post as a reply to that thread (i.e it informs you of new pots even if it was you who made the new post).
Also, sometimes it isn't just one post, I've gotten an endless loop of about 8 notifications that just go round and round; when you close one, it just goes back to the bottom of the stack.
It also lists an edit to a post as a new post, I think.