I have an enormously long list of recent "Activity" (under the bell icon); when I switch to looking at the "Unread" tab, it seems to be only aware of cases where I've explicitly clicked through from this menu.

So for example if there are ten new replies to a single comment, and I read through all of them when I see the first one, this doesn't seem to mark anything as read. The only way to remove them from the "Unread" tab seems to be clicking through each one from that tab/menu/list. Given how hard it can be to toggle back and forth, it would be great if it could recognize when the entire message is presented on the screen (perhaps with some minimum duration, if that gives better results) and then marked as read.