This was a feature that I fell in love with on Boost. I like to sort by top day and do short quick bursts of scrolling through the feed, so it helped a lot.
I’m not even sure if it’s possible on lemmy, or if anyone else feels they’d get use from it as well.
Or at least it would be nice to color differently posts that have already been seen. And when clicking on already-read posts only expand the new comments.