In any case, I think it *should* show the last line in the active editbox regardless where mIRC's running that command. I don't think mIRC is handling that second editbox correctly in this case. I think the / should be done in the active editbox as well.
The whole point of "/editbox -n" is to be able to simulate commands executed in an editbox other than the active one. What good is that if the command operates on the active editbox instead of the editbox in which the command was executed?
