AFAIK, there isn't a way to currently do this in a quick bulk operation. I agree that this would definitely be useful.
If I want to mark a bunch of threads as read, I'll just double click on the expand caret next to the thread to open and close it (quickly enough so that the thread doesn't actually expand), which will mark it as read. I don't know of any way to set threads
back to unread though.
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