I'm evaluating using this for my BlueSky client, in which I have infinitely scrolling lists of social media posts. There are a few issues that can not be resolved with the current API:

• Prepending items: this is required for bi-directional infinite scrolling. Ie. if the user scrolls to the bottom of a posts list, we load older items and append them to the list. This works fine. If the user scrolls to the top of the posts list, we load newer items and prepend them to the virtualized items list. Ideally, the scroll position relative to what is visible on screen would be retained as is the case with scroll anchoring. However, currently, the scrollTop of the scroll container stays the same, effectively pushing down the visible items outside the viewport. Trying to manually adjust the scrollTop of the scroll containter after awaiting LitVirtualizer.layoutComplete does not work sadly, as the layout keeps reflowing after that. For social media feeds, one can make due with having a "jump to top" button to jump to the newest item instead of allowing to scroll up. For a chat application, where the newest items are at the bottom and a user wants to scroll upwards through old messages, scroll anchoring is a must have.
• Frequent re-rendering of items that are visible: I have not yet figured out why this happens, but my assumption was that any item tha is within "range" (so visible, or slightly outside the viewport) does not have to be rerendered. However, in my testing, I see that some items get re-rendered often, even if they are in range. Some even get connected/disconnected from the DOM or entirely recreated. This is not great, as it will re-trigger CSS animations, kill state, and so on. Figured this one out: in my Lit templates, I had some DOM nodes embedded instead of templates. This would trigger constant re-rendering.
• Dynamic content support: posts may contain videos. Those are first represented via thumbnail <img> elements (with a fixed size so they can be properly virtualized without needing constant reflows). Upon clicking the thumbnail, the <img> is swapped out with a <video> or <iframe>, and the video playback is started. When the item is scrolled outside the "range", its DOM representation is disconnected from the DOM, which obviously means videoplayback stops. I suppose I could reparent the <video>/<iframe> to someplace else in the DOM if that happens, so playback continues (if that works at all), but have otherwise not found a solution.
• scrollIntoView() is not enough: a common feature in social media apps is timeline position restauration. E.g. the user scrolls through their home feed, then closes the tab and does something else. Later, they come back and reload the page. Their previous position in the timeline should be restored exactly how it looked when they left. With scrollIntoView(), that's currently not possible exactly, as I only have the option of scrolling to "item boundaries" and not pixel positions like with scrollTop. Sadly, using scrollTop on the scroll container can also not be used, as the virtualizer doesn't quote scroll to the saved scrollTop.

Other than that it has been extremely easy to swap out my old virtualization "solution" with lit-virtualizer and I was surprised that it just worked (minus the issues above). Great stuff!