Comment 19 for bug 270689

checking for an existing chat window shouldn't be too hard I think.

But however, assume the following:
 * you are chatting with some other party
 * the other party disconnects, and you keep the chat window or tab open
 * the other party reappears after some hours, and starts a new chat
 * you would miss the chat state notification, because the chat window is still open

so also would not be perfect.