improve scrolling behavior

[tarasglek]

https://duckduckgo.com/?q=DuckDuckGo+AI+Chat&ia=chat&duckai=1 is a good example of how to scroll

1. On question being asked, scroll such that question is top of view
2. as message streams in, don't do any further scrolling.

This is user-friendly and efficient

[humphd]

This mirrors what ChatGPT does, so despite the fact that it's a pretty big change, it's likely the one most people will expect (some of our our users might not love it). The crap we have to do to support our current scrolling is terrible, and it still doesn't work perfectly.

One thing we'll need to make this work is proper bottom-padding/margin (not sure which) so that we can scroll beyond the end of the current size of the new message. Otherwise, we'll only be able to scroll to the offset height of the message when it's created, which is not enough.