There are a lot of threads on this and who knows which one will catch on, so here’s my take again:
In my opinion, the ideal state of a poster is that of an unblemished one. The more markers visible on a given screen, the more visual clutter.
Which is better, a marker for watched or unwatched, comes with another question: Which majority is more common? For me, it’s drastically in favor of watched media. Thanks to this change, a majority of my posters are now marked with an intrusive green checkmark.
I too would prefer an option to switch this state.
