if you disconnect from a provider and leave the chat window open you cannot re-use it after reconnection. This is a bit odd, since the window could be reused. Is there any chance to go into that direction? The behaviour of most clients I know is in that way.
If not, input to the chat window should not be possible at all. I wrote a longer message into the old window, pressed enter only to find out that I had to open a new one and transfer the content there...