When you load a forum thread you get the first few results, as you scroll down it loads new ones AND unloads old ones.
The lazy loading is probably good for massive threads where there are a lot of images and content that has to be processed by the browser (embeds, pictures etc). But I don’t understand the unloading.
The problem for me is you can’t CTRL + F to search the thread once it is loaded. I have found myself trying to do this many times recently and struggling. The resorting to using the scrollbar to quickly skim through the post to realise the page size is changing throwing my view up and down the thread. 
Perhaps on desktop you could just do away with lazy loading posts, and just lazy load the embeds, images when they are in view or do away with unloading entirely.
If this is a problem for anyone else please give this post an updoot.