@Dave -- Another reason for wishing we had a "mark as read" option...
I have ATP as one of my home tabs in my firefox. I'm on this site 24/7/365. If my PC's up and connected to the internet, ATP is open on one of my tabs. Every time I go back to the home page, or refresh the home page, it tells me there is new content in my watched forums. But there's NOT. It counts all the unread threads as new content.
8-10 times yesterday, I opened a watched forum because the home page told me "new content," and there was nothing new in them. I don't remember it acting this way before, and it's a little annoying, honestly. If it's a glitch, then ignore this post. But if it's standard, then it's another reason for wanting to be able to mark an entire forum as read.
