redesign messages window to make it easier to make sure that you're in the right tab
I have sent messages that turned out to be embarrassing, since I have accidentally sent them to the wrong person, because I had both people open in the messaging window, and I have mistaken the tabs. I would suggest that the name of the contact you're writing should be displayed at the bottom of the screen, right below the text box for writing messages, so it can be seen more easily while writing, especially while in a hurry.