Relay had the option to "mark as read" on scrolling and a separate option to "mark as read" on individual posts along with the option to clear read items from the currently displayed list, very handy if you wanted to clear stuff you weren't interested in while keeping the list populated with posts you might want to go back to.
I agree. I don't want all read posts to automatically be hidden as I enjoy reading new comments. But I do want the ability to hide read posts when I choose.