As we have now upgraded the main XenForo community, this forum is now being retired. It will remain open in a mostly read only state. Any further feedback, suggestions or bug reports should be posted on the main forum. We will still process the existing bug reports from this forum.
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The RM will certainly be ready as we will be upgrading https://xenforo.com/community a short while before the public betas are released Though it will also be the first time it has been used in production so it really depends on how stable it is between the upgrade at XF.com and the public beta releases.
My feedback on the sticky-scrolly avatar is that I think it's distracting when you scroll. There is little chance anyone will start reading a post and forget who started it, or care, and that is offset by everyone using the forum having the page respond strangely when they scroll. I really like the idea of it, but I think in practice it's too annoying. Please make it an option to turn it off, if you do release it as a feature.
Sticky feature is very useful on large posts, as @Kier has previously stated and I completely agree. So detecting post height and comparing it to the viewport height seems like a fine solution for that. IntersectionObserver can help executing that job only on visible posts (though it needs some testing whether it is more effective to calculate on scroll or on page load).
I agree with you it is very useful on long posts. It's when every single post makes the UI go "clunk, clunk, clunk" as you scroll down it's distracting. If it was just on rare, long posts, that would be great.