In private conversations—I do not recall it happening outside of them but I reckon it would be the same—once a conversation is long enough and you scroll down to a certain point, the background image just disappears. I haven't felt like testing it extensively but I can say it occurs once I get about 35 thousands of pixels down (about 50 screens high in my case, as all the tabs I keep open in my browser take some vertical space), give or take some. It's not critical in any way; in fact, it is pretty much irrelevant since I doubt anyone outside of me would be insane enough to create messages long enough to get there (unless they tried to test it and just held “Enter” for a while), but I think you might want to know about it just for the sake of knowing that it happens as this is the feedback section. PS. I broke the forum, do I get a cookie for that?