improvement: scroll to new messages smoothly

[WofWca]

TODO:

• Test it and see if it gets stuck often enough
  for it to be critical. I have seen it happen a few times.
  Maybe for 1 out of 100 messages.
• Think about putting behind an experimental setting?
  Ok, I have an idea. Let's merge it, and if it's not good yet, then put it behind the setting.
• Add CHANGELOG entry.

[nicodh]

Test it and see if it gets stuck often enough
for it to be critical. I have seen it happen a few times.

Can you describe how to test and provoke "critical" behaviour?

Maybe for 1 out of 100 messages.
So this means for testing: it needs that many repetitions to run into an failure? What kind of failure would that be?

[WofWca]

Can you describe how to test and provoke "critical" behaviour?

I mean the behavior we observed in #4186, #3763, #3753. By "critical" I mean whether the occasional "scroll up" outweighs the niceness of the smooth scroll, and so whether we want to merge this, without making it an experimental setting.

So this means for testing: it needs that many repetitions to run into an failure?

Yes, it results in a "scrolled up" chat once every ~100 smooth scrolls.