Problem is, I can't replicate the behaviour, but I figured nothing to lose by documenting it—I was composing a reply, and was going for Command I for italics (thanks for that shortcut, BTW) and fat-fingered the I (thought I'd hit Enter or a cursor key, but it won't do it again), and my posting window did a weird rolly uppy thing I couldn't recover from.

See how the text I was entering now looks like it's on the page itself, and the posting window has shrunk?

Hopefully this is a useless report with the upcoming shiny new editor.