In both the iOS app and the website, if you press the “mark all as read” button, all stories for that feed will be marked read - even if they were just fetched a second ago, and you hadn’t seen them in the list (and thus “next” wouldn’t read them).
Example:
- Read “Boing Boing’s” feed. See 10 new stories.
- Read 9 new stories.
- Go get coffee, lunch, call from neighbour,
- Without noticing, an hour has passed
- Notice that the headline of the last story is uninteresting, so don’t read it, but …
- (3 new stories have been fetched; you can see them in the feed count, but not in the “list of stories pane”, which only has one unread and uninteresting story. But you don’t notice that …)
- You press the “mark all as read”. And those 3 stories, you’ve never seen or even noticed are also marked as read, so you are unlikely to notice them (especially if you use “unread only”)
Even if you are vigilant and always look at the count just before pressing “mark all as read”, there’s a race condition in which this happens without any feedback.
IMO, either the “mark all as read” should only mark those visible (or selected as part of) the existing window, or it shouldn’t be in that part of the screen.