(I sincerely hope I haven't already made this request before, I know I've at least thought about it, but I have a bad memory and couldn't find anything when I searched)

Sometimes I lose track of what posts I've already engaged with, for example in busy forums where discussion topics get repeated, or for things that come back across my feed when someone I follow comments (see above comment on bad memory lol). It isn't a huge inconvenience to figure it out unless there are already loads of comments, but it would still be so helpful if the same way that the upvote/downvote arrows get colored in after you use them, leaving a comment also fills in that comment bubble on the card for whenever you see it going forward.