Why do you doubt that it is on purpose? The combo box is quite clearly a static field that is not being resized. Whether Khaled was aware of the length limitation or not, it was designed to work this way. My guess is, he
is aware that there are long line limitations to such a design, but decided not to implement resizability for simplicity's sake. He likely knows about this because this is not the first non-resizable UI element in mIRC (see below), and there seems to be a conscious decision to keep these UI elements simple until someone asks for more functionality.
You want extra behaviour for mIRC to resize the combo box, similar to Change #2 in 7.22 which added resizing to Channel Central (see
versions.txt). Note that change #2 was a
feature suggestion, not a bug report. It was not a bug that the channel central could previously not be resized, it was just a design choice-- a poor one in retrospect, but a choice nonetheless.