There are two other issues along the same vein.

#1. Canceling out of a message still leaves the message there. This is counter to every other game I can think of that I've played, where canceling a message completely gets rid of it, meaning I can hit ESC to clear a message then start over. Instead, I instinctively hit ESC and then have to re-activate the chat, backspace through all of it, and begin my new message. I find this counter-intuitive.

#2. Anything that causes focus to leave the chat window resets the chat channel to the current default. I can't tell you how many times I've said something in the wrong channel because I resumed typing something and sent it out without verifying that the chat channel didn't revert. If you're not going to clear the message, you shouldn't be clearing the channel. If you do resolve #1, I wouldn't worry about #2 after that.