The previous behavior (don't autoscroll if the mouse is over the chat
area) was not intuitive and caused problems for people in chat only
mode, which led to a lot of people assuming that it was a glitch.
This change introduces the following behavior:
* Hovering over chat no longer affects autoscroll.
* Scrolling up in chat turns off autoscroll.
* Scrolling to the bottom of the chatbox resumes autoscroll.
* If a new message is added while autoscroll is off, a "New Messages
Below" indicator is added to the bottom of the chatbox.
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