Very odd one...
If I reply to a thread with a quote, as I start typping, it will let me get about 10-15 letters in, then start putting any additional text in the line above.
I'll try to give an example, let say I was replying to this:
"Goats are blue".
suddenly this happens.
I start typing and
Anything added to the line then gets put in the line above.
If I press enter a few times, the problem goes away.