I don't even know what the bug is that you and drum figured out. I just use the default editbox. My feature request was to prevent accidental saying of pasted text because if it contains a linechange at the end, mIRC's default action to that line is /say.
Go to Options > Display > Options, and check the "editbox lines" setting to "automatic". Then paste stuff in an editbox and you'll see what we're talking about. My guess is you'll like this behavior better than the confirmation message. Not that we don't agree that the confirmation message could use some work, but honestly it's kind of obsolete anyway.
