Indeed, I get a channel popup if I click the border line between the channel and nicklist, just right of the channel scrollbar. I even get this channel popup on all border lines, like the one just above the channel editbox, even where it separates the nicklist from the editbox. (mIRC 6.16 here)
To make it easier go to Options -> Display -> Options --> Visual styles, there you can set border color and size. Set size to 5 and it's a lot easier to get this 'bug'.
