Turn off mouse-over marking as read?

Recently, on the web, the behavior has changed when I mouse over the text of a story that causes it to be marked as read. I don’t remember this being true until fairly recently. I haven’t found a preference to turn this behavior off, is there something I can do?

Yep, just hit the little orange arrow that follows your mouse on the left of the viewport.

Huh, I must have clicked that by accident. Thanks!