The red line is called Line Marker.
Quote from the mIRC Help file (/help Line Marker):
"Whenever you close or minimize a window, the line marker marks the spot where new messages come in to a channel or query window. This allows you to scroll back and easily see which messages you've missed.
You can press Control+L to scroll back to view the line marker. The line marker is only updated after you've scrolled back to view it."
End quote.
CTRL-L toggles this feature on or off.

LOL :-) The number of years I have used mIRC and I never understood what this red line did, wicked thanks for the information :-D

Actually that's understandable .. the line marker is a fairly recent development. Well, not that recent, but still recent comparatively speaking. :tongue:

I remember when it used to pop up all the time and I was like 'wtf?!?', now that I know it has a meaning I might use it. It actually seems like a good function LOL :-)

Yeah..That red line seems to separate all you sentences into paragraphs. that's how it works as a 'red marker' to see where u missed from the start when u minimize ur mirc.