I've been using this feature for ages, but it got much worse about six months ago when the last message in a thread became automatically "focused". Now when I scroll up in a thread and select some text, the message I'm selecting text from doesn't automatically get focus. I have to click one extra time to get focus on the message I'm selecting. Even after half a year, I still forget the extra click in about 20 emails per day, making the reply go to the last message in the thread, with the entire thing quoted. To recover, I have to discard my new message, scroll back to the thing I wanted to reply to, and repeat the process. I filed a bug report in the first week, but no response and the bug remains.
