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.

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