Or, you could just simulate the back button when hitting the "X" button (or the Esc key). I think this is what Facebook actually does. I think it would also be easier to implement.

Is this a duplicate of T64266: MultimediaViewer should not leave so many history entries when closed or T68217: MultimediaViewer should not update the browser's URL?

Kind of. However note that the information published in T64266 seems to be outdated: it's said that Facebook nukes the history entries when exiting the lightbox, however this doesn't seem to be the case anymore.

Right now, Facebook just simulates pressing the back button (so, if you hit the "forward" button after closing the lightbox, you will open it again). I think this behavior is reasonable, and I think Mediawiki could implement it with very little modification over the current behavior.

So, my report should actually be renamed to: «Opening an image preview *should* edit the browser "back-button history" but, when trying to close the preview, the back button should be automatically pressed instead», and that is not a duplicate (since T64266 suggests nuking history entries, and T68217 suggests not updating the URL, both of these things are not what this report is about).

This is a slight variation on T64266#667336 E. I would still merge the task.

That task is blocked on lack of consensus; you can find a user script there for implementing a somewhat similar behavior.