Pro tip for @SlackHQ -- if you're not a heavy keyboard user, you may not already know this:
Press the up arrow while in the message box (with nothing entered) and it takes you into editing mode for the last message you sent.
#NowYouKnow #keyboard
You can also set the arrow to move focus to the message list, and not edit, by going to `Preferences > Accessibility > Press up arrow in the message field to:` and choose either option.