The element it's anchoring to is the <ul> #webMessengerRecentMessages.

Cc'ing bmaurer (let me know if you have any insights here or perhaps it is already on your radar).

I'll play with it some more and see if there's a good heuristic we can add, but I suspect this will have to be fixed in FB.

I think this might have to do with shared links in the thread. This only reprod for me when I sent my test user a link https://www.google.com, after the preview loads it broke scroll anchoring.

Chat windows are really hard places to get scroll anchoring right. One thing that would be really helpful is to make sure that if the window is scrolled to the bottom it always stays scrolled to the bottom. It's an area where we'd probably want to explore new apis so that sites like ours could express our needs. 

I think the best fix on the Facebook side would be to put "overflow-anchor: none" on the #webMessengerRecentMessages element.

The problematic scroll adjustments occur when content is added to this element, causing it to grow upward due to an absolute-positioned container with "bottom: 0".  We could add a special case for this in the scroll anchoring algorithm but I'd prefer not to do that as it is pretty specific to the details of Facebook's infinite scroller implementation.

This no longer repros on Facebook's new messages page. :)