Mark posts above currently displayed post as read (similar to "older than" but not by date)
This is a little similar to https://theoldreader.uservoice.com/forums/187017-feature-requests/suggestions/4149552-list-view-bulk-action-mark-some-as-read, except that I think having to mark checkboxes on each post would be incredibly tedious for the use case I have in mind (especially on mobile). https://theoldreader.uservoice.com/forums/187017-feature-requests/suggestions/3803731-provide-a-partial-mark-as-read-mechanism-for-list is also somewhat similar, although that appears to have been "completed" in a way other than what I have in mind here.
As described in the second post, I would like to be able to mark only part of a folder as read, with a behavior similar to "Mark all as read Older than..." feature, but without reference to a specific age of post, rather by simply drawing an imaginary line and marking everything above it as read. It could be something like "mark posts above currently displayed post as read". It should really mean "above" and not "older than", so if posts are sorted newest first it would mark those as read -- it's about how far I made it down the page, not the age of the posts.
For example, I have about 300 posts right now in a feed from a newspaper, which go back about 3 days. I am reading it in list view because I don't read every article, so as I scroll through the feed most of the posts are not being marked read (which is normal and desired behavior). I don't have time to scroll past all 300 posts right now, so I would like to be able to mark the place where I stop, wherever it happens to be, and mark everything above it as read, so that I don't have to scroll past them a second time when I am able to come back to the feed and continue browsing, nor do I have to try to make it through at least however many there are up until 1 day ago so that "older than 1 day" would have the same effect (which itself doesn't work as a workaround if all the many posts I can only get partway through are within the past day).