Whether it activates when scrolling down seems to be dependent on how fast I scroll down. If I scroll past the avatar quickly, it doesn’t have time to get sticky. If I don’t, then it will pop in. Scrolling up, it always appears, but @codinghorror has made it pretty clear to me that Discourse is not designed for scrolling up. So it would seem odd to design for that use case.
That said, I find it way too distracting and disabled it with a single line of CSS, like I mentioned in the thread that @GreysonCarlisle mentioned. I don’t hold my breath that there will be an option to disable it.
I currently have a small number of fixes to the themes here that I use, which I’ve not released. I make links a brighter blue and underline them so they stand out more. I remove the space between a link and the little bubble that shows how many times that link has been clicked. I fix the small sans-serif MathJax text to match the regular text. And now I stop the sliding avatars. (I’ve also considered fixing the bold to be the exact same size as the regular text, instead of slightly bigger/smaller.)
Would releasing these things be useful to anyone besides me?