This depends on the mouse driver and how it interacts with the custom text display control in mIRC. It turns out that different mouse drivers behave differently, using their own logic to determine which windows to send wheel messages to. The result is that some mouse drivers behave oddly in mIRC windows.
The only solution I could find on my own system was to uninstall the logitech mouse driver and to use the standard microsoft intellimouse driver with my logitech mouse.
