I’m not even entirely sure how to explain it, so I included a video. You’ll notice that, as I’m scrolling, the width of each individual post gets narrower (and back to the full width) as it passes through the visible area of the screen. I know things can get wonky, but what strikes me as even odder is that when it happens, the avatar stays in the same spot instead of moving with the post.
I just noticed it this morning and it’s happened both on my laptop at home with firefox and my desktop at work with both firefox and Chrome. So, it might be the theme (Vincent) or something else, but it’s not likely to be anything on my side. Something I did notice is that when I tried it with Chrome, it didn’t happen until I was logged in (I rarely use Chrome, so I wasn’t already logged in with that browser). I suppose that suggests that the problem is the theme.