I'm still not comfortable with this. We register messages in a reverse order...

This works only if thread.messages has the messages in the correct order. Which might be a good idea per se, since we could use it in some algorithms we have in ThreadUI instead of using the DOM.

But all this leads to a big refactoring of ThreadUI, which I'd like to do, but this is not the goal for this patch yet.
Therefore, how about taking this (ugly) shortcut: use the last message bubble that is in the DOM. And we'll revisit this once we use Threads as the main model.